Chapter 38Metals and alloys: an introductionWei Sha School of Planning, Architecture and Civil Engineering, Queens University Belfast, Belfast, UKThis chapter introduces metals and alloys as a type of construction material, includingdiscussions of their background and use in construction, their mechanicalandphysical properties, microstructural factors that influence properties, and recycling andsustainability issues associated with metals and alloys. It references chapters in theMetals and Alloys Section of the Manualof Construction Materials and gives a briefoverview of the coverage in order to point readers towards those chapters. Thechapter serves as a starting point to a practitioner researching the properties andusefulness of metals and alloys to construction applications.Metals and alloys as constructionmaterialsIntermsof volumeorweight, metalsandalloysascon-struction materials are well behind concrete. However,their strength, ductility, toughness, surface properties,weldability,electricalandthermalconductivity,andmanyotherpropertiesuniquetothisgroupof materials, makemetalsandalloysindispensiblematerialsforconstruction.These materials are usedinhouses andother buildings,structural steelworkincludingbuildings, bridges, oshorestructures and piling, road structures, and does not requirethe extensive machinery necessary for completing anymajor construction project, for example cranes, trucksand concrete mixers.A variety of metals are used. The predominant type is, ofcourse, theferrousmetals, i.e. ironandsteel. Withinthistype, therearecastiron, wroughtiron, carbonsteelsandalloy steels, dierentiated mainly by their composition.Amongthem, carbonsteels formthe basis of structuralsteelworkas well as concrete reinforcingbars, andthusareusedinthelargestquantityamongalltypesofmetals.Although metals were used extensively from ancienttimes, as indicatedbyhistoryterminologies suchas theBronze and Iron Age, the scale of their use was dramaticallyincreased with the modern steelmaking technology.Althoughthedevelopmentofsteelmakingtechnologyhasstabilised in the last few decades, the processing techniquesfor steels as well as for iron have continued to develop andimprove to great eect, including thermomechanicalprocessing, heat treatment, and surface engineering.Amongthenon-ferrous metals, aluminium, zinc, lead,copperandtinaretraditionallywidelyused. Lesswidelyused are nickel and chromium. There are also new metallicmaterialsforconstruction,forexampletitaniumforclad-dingpurposes. Althoughfar fromwidelyused, titaniumoersanattractivealternative,duemainlytoitsaestheticfeatures and corrosion resistance. The development ofnon-ferrous metals andalloys has beenmorerapidthanthatofironandsteel,althoughthisismainlytocatchupwith rather than replace the ferrous metals.Microstructure and properties ofmetals and alloysMicrostructural defects and theirrelations to strength and ductilityThe properties of metals and alloys which are unique to thisgroupofmaterials, asopposedtosayceramicsandpoly-mers are due totheir microstructure, starting fromthesmall, atomic scale of metallic bonds, uptotheir grainand phase structures, usually in the micrometre scale,hence the termmicrostructure. Inmetals, there are freeelectrons shared by many atoms, as against the ionicbondor covalent bondinceramics andpolymers. Suchfree electrons are the causeof the metals electrical andthermal conductivity. This unique type of bonding alsodetermines thehighstrengthandductilityof metals andalloys.Inmaterialsscience, materialsarebroadlydividedintostructural materialsand functional materials. The applica-tionof therst groupisbasedonthematerial strength,while for the second group it is based on electrical and elec-tronic, magneticandoptical properties. Themost widelyused properties of metals and alloys for constructionpurposesaretheirstrengthandductility,fortheso-calledstructural materials. Inphysical metallurgyandmaterialsscience, the strengthandductility are explainedmainlywithtwomicrostructural terms, namelygrainstructuresand dislocation structures.Microscopically, with the exception of amorphousmaterials not normally usedinconstruction, a piece ofmetal ispackedwithsmall crystals, althoughtheexteriorof themetal does not normallyshowfeatures of crystalthat wenormallyassociatewithgeological minerals. TheICE Manual of Construction Materials # 2009 Institution of Civil Engineers www.icemanuals.com 465ice | manualsdoi: 10.1680/mocm.35973.0465CONTENTSMetals and alloys asconstruction materials 465Microstructure and propertiesof metals and alloys 465Sustainability and recycling 467Section contents and authors 467Reference 468reasonwhywecannot seethecrystals (or evensigns ofthem) withthe nakedeye is because theyare verytiny,usuallyaroundthescaleof 106m(onemicrometre) buttheycouldbemanipulatedbyspecial treatment todownto109m(onenanometre). Recent researchhasresultedin the development of nanostructures in bulk metallicmaterials (Bhadeshia, 2005).These tiny crystals are packed densely with no gapsbetween them, and are only separated by grain boundaries.The grain boundaries have signicant eects on metal prop-erties such as strength, ductility, atom diusion andconductivity.For theperfect crystal structuretodeformplastically,extremely large forces are required, in order for theatomic planes toslide against eachother. Imagine eventheforcerequiredtopull fromoneendandslidealargecarpet onaoor, inone single movement. The frictionforcebetweenthecarpetandtheoorishuge.Theslidingof atomic planes has similar diculties, only a milliontimesharderbecauseofthetightbondingbetweenatomsnext to each other in adjacent atomic planes. In realmetals, however, thereisacommontypeofdefectcalleddislocations (Figure 1) that completely change themechanism of plastic deformation.Dislocations appear in large numbers in metals andalloys, some formed naturally during cooling after meltingduring the alloy manufacture, andsome formedduringdeformationprocessingsuchas rollingandforging, andsomeevenformedduringtheplasticdeformationprocesswhichis facilitatedbythemintherst place. Withdis-locations(butnottoomanybecausethentheywilltangleupandbe hardtomove), plastic deformationbecomeseasierbecause,insteadofmovingthewholecrystalplanesinonego, it is mucheasier topusheachdislocationtomove. This is very similar to moving carpets by rstformingakink, andthenpushingthekinkfromoneendof thecarpet toanother. Wormsmoveinthesameway,too.Thedeformationmechanismthroughthemovementofdislocationsisthereasonwhymetalshavegoodductility.Grainboundaries,ontheotherhand,limitthemovementof dislocations because it is not always easy for dislocationsto traverse across them. By controlling the quantity and sizeof the grain boundaries and dislocations, virtually un-limited range of property combinations can be achieved.A grain boundary caused by the discontinuity of crystalsisa2Ddefect, andadislocationcausedbyatomicplanemisalignment is a1Ddefect. Inadditionto1Dand2Ddefects, thereare0Ddefects, theso-calledpoint defects,usuallyintheformofvacancies,i.e.latticespots,butnotoccupiedbyanyatom. Thevacanciesdeterminetherateofdiusionbecausemetal atomsdiusebyjumpingintoanadjacent vacancy. There are 3Ddefects, too, intheformof inclusionandprecipitationfor example, whichweakens or strengthens themetallic materials dependingon their characteristics. As it transpires, atomic andmicro-scaledefectsinmetalsarefarmoreimportantanduseful than the perfect crystal structure.Thephysicalmetallurgyandmaterialsscienceofmetalsandalloysaredescribedinmanymetallurgytextbooks.APPPQQQMNM MNNBurgers vectorBurgersvectorFigure 1 The lattice arrangements of edge (top right) and screw (bottom right) dislocations compared to perfect crystal structures466 www.icemanuals.com ICE Manual of Construction Materials # 2009 Institution of Civil EngineersMetals and alloyscomprehensive source of information for metals is the ASMHandbook series published by ASM International. For freeonline information, the Wikipedia (http://en.wikipedia.org)containsmanyrelevantarticles,althoughtheymaynotbeas authoritative.Corrosion and fire resistanceThemostcommonlyusedmetals,i.e.structuralsteels,areoneorderofmagnitudestrongerthanconcrete, andevenmuchstrongerthanotherconstructionmaterialssuchastimber andbitumen. However, someother properties ofmetalsandalloysalsocontribute,orsometimeslimittheiruse in the construction industry.Onesuchpropertyiscorrosion.Mostmetalsreactwithoxygen, i.e. they are oxidisedinair. Whenmoisture ispresent, e.g. underwateror inahighhumidityenviron-ment, thecombinedeectofwaterandoxygencancausecorrosion. This is amajor problemfor structural steels,andahighproportionof constructioncost of structuralsteelwork is on corrosion prevention. However, othermetals, suchas aluminium, titaniumandstainless steel,have means of overcoming the corrosion problem, byforming a thin and stable protective layer on their surface.The prevention or minimisation of corrosion does notalwaysrelyon usingnon-metalswhichdo nothavecorro-sionproblems themselves, suchas polymer-basedpaints.One of the most eective anti-corrosionmeasures is touse zinc and, toalesser extent, aluminium, twoof themost activemetals. Thereisawidevarietyof protectiontechniquesbasedonthegalvanisingactionbetweenzinc/aluminiumandsteels, because zinc/aluminium, whenincontactwithsteels,willcorroderstandinfacteliminatethe corrosion of steels. Such techniques include zinc(alloy) galvanising, metal spraying and painting containingzincor aluminium. Metals canbeusedtoprotect othermetals.Another consideration, usuallytothe disadvantageofusing metals in construction, in particular structuralsteels, is related to the properties of metals at hightemperatures, for example ina re. The reasonfor theusually relatively poor metal performance at elevatedtemperatures is related to its metallic bond. The highthermal conductivity is as important as the loss ofstrengthitself,because itpermitsheattoreach andspreadover metal parts quickly. This is a particular issue forstructural steelwork, the issue of its re resistance.Althoughtheconcretestrengthwouldhavedroppedevenmore heavilyat agiventemperature thansteel, the lowthermal conductivity and the bulkiness of the concretemeanthataconcretestructuredoesnotusuallyhaveanyreresistanceproblem, simplybecausetheheatdoesnotreachthe insideofconcrete.Onthe otherhand, structuralsteelwork,withsomeexceptionsof,forexample,opencarparkstructures, will always needre protection, whichadds asignicant amount of cost. Inrecent years, therehavebeenlargeadvancesinreengineeringresearchanddevelopment, resultinginsome reductionof re protec-tion cost in structural steelwork.Sustainability and recyclingMetals and alloys are champions in the race towardsmaximumsustainability and recycling. In principle, allmetals and alloys can be recycled, usually up towards100%, althoughinpractice, this may be achievedonlywith considerable cost.The fundamental reason for the excellent recyclability ofthisclassofmaterialsis,intheauthorsview,thatmetalsoriginallyappear inearthnot intheir pureformbut incompound formwith, for instance, oxygen, i.e. oxides.Theseareores.Metalsneedtobeextractedfromtheores,i.e. separatedfromthe compoundedor mixedelements,whichisalongandcostlyprocess. Afterametal isused,if it haschangedfromitspurestate, it maybecontami-nated, or mixedor compoundedwithother materials, itmay be corroded or rusted and it may be oxidised.However,inprinciple,thegoalofreturningsuchused,ornon-puremetal toitspureform(i.e. fromoldtonew)isfundamentally no dierent from extracting the metalfromore.Theprocessmaybeverydierenthowever,butit isinmanycasescheaper. This, plusthepressurefromthevanishingresourcesthatwearefacing, maketheideaof recycling metals very attractive.Onemajorexampleistheuseoftheelectricarcfurnaceforsteel making. Thisprocessusesscrapsteel asitsrawmaterial, and produces high-quality alloy steels, forexample stainless steel, a product used heavily for construc-tion. The process has been in use for more than 100 years,long, longbeforethewordsustainability was invented.This was because metallurgists andsteel users knewtherecyclability of steels, and the value of it. This same examplealso illustrates the cost factor mentioned above, because anelectricarcfurnaceoperateswithahighconsumptionofelectricity, andthe cost of runningit is higher thantheother route of steel making, the basic oxygen furnace,whichdoesnotusescrapsteel asthemainsourceofrawmaterial. Despite the cost factor, a higher and higher frac-tion of metals and alloys is being recycled, due to environ-mental regulations and sustainability issues and concerns.Section contents and authorsThissectionoftheICEManualofConstructionMaterialsgives detailed information on the class of materials ofmetals andalloys. Before dealingwithindividual types,the section starts with describing the nature and behaviourof alloys, which essentially covers basic metallurgy, neededfor understanding and appreciating the properties of metalsICE Manual of Construction Materials # 2009 Institution of Civil Engineers www.icemanuals.com 467Metals and alloys: an introductionand alloys. This chapter is not about specic types ofmetals, it intends togive the basic metallurgical theorythat is applicable to all metals and alloys.The chapters that followdescribe the most commontypesofmetalsandalloysusedinconstruction,includingferrousmetals, aluminium, copperandzinc. Inaddition,anup-and-comingmetal notpreviouslyusedinconstruc-tion,titanium,isdiscussedinChapter44.Thefocusisonthepropertiesandusesofthesemetals,sothatthereadercanapplytheknowledgestraight tothepractical useofthemin the design and maintenance of structures andconstruction projects.Traditionallyandfortheforeseeablefuture,intermsoftonnage,structuralsteelsaredominantamongmetalsandalloys used in construction, so the nal chapter con-centrates onthe applicationanddesignissues of struc-tural steels. Theaimof this chapter is for thereader togainanadvancedknowledge of these materials, andbeabletoapplythisknowledgedirectlyincivil engineeringconstruction.I amextremelyproudtosaythat wehavegot atop,distinguishedteamof Britishauthors for these chaptersand topics, each a world authority in his own eld. Chapter39,Thenatureandbehaviourofalloys,iswrittenbyDrJosephRobson, aSeniorLecturerinPhysical Metallurgyat the University of Manchester. Among other professionalachievementsandcredentials, heisanassociateeditorofMaterials Characterization, an international journalpublishedbytheInternational MetallographicSocietyonmaterials structure and behaviour.As Chapter 40 is concerned with dierent types of metals,wehaveseveralauthors,writingabouteachtypeofmetalandits alloys. The chapters onferrous metals andzincarewrittenbyArthurLyons. Histextbook, MaterialsforArchitects &Builders, is widely usedby universities forteaching of their Construction Materials module, includingthat run at my own university, coordinated by myself.However, fortheManual, DrLyonsisabletoelaborateto a much more advanced level compared with theundergraduatetextbook. Thechaptersonaluminiumandcopper are writtenbyrespective topspecialists inthesemetals, namely David Harris of AluminiumAdvocatesandPeterWebster of theCopper Development Associa-tion. Neil Lowrie, a technologist at NAMTEC, hascontributed the chapter on titanium(NAMTECis theNational Metals Technology Centre in the UK).The last chapter of the Metals andAlloys SectioniswrittenbyMarkLawson, amostfamiliarnameinstruc-tural steels in the UKand beyond, for his inuentialworkat theSteel ConstructionInstitute(SCI). Technicalpublications onstructural steel designproducedbySCIunder his direction are widely used by consulting engineersas well as in the relevant professional and higher educationsectors. SCI also has a strong inuence in British Standardsand now Eurocodes for structural steelwork.Each of these authors is a fellowor member of therelevant professional bodies in the UK.I am therefore condent that this section of the Manual ofConstruction Materials forms a most authoritative guide ofmetals and alloys for construction practitioners.ReferenceBhadeshiaH.K.D.H.BulkNanocrystallineSteels.Ironmakingand Steelmaking, 2005, 32, 405410.468 www.icemanuals.com ICE Manual of Construction Materials # 2009 Institution of Civil EngineersMetals and alloysChapter 39The nature and behaviour of alloysJoseph Robson School of Materials, University of Manchester, UKThe performance of metal alloys is understood by studying the relationshipsbetween composition, processing, structure and properties. Knowledge and controlof structure on the microscopic scale is critical in optimising macroscopicperformance, producing the versatile range of alloys available today.IntroductionMetal alloysformoneofthemostwidelyusedclassesofmaterials in engineering applications. Metals are rarelyusedintheirpureelemental form, butarecombinedintomixtures of elements known as alloys. Alloying is the delib-erate addition of extra elements to a pure metal to improvesome aspect of its properties. In additionto deliberatealloying elements, all commercial metals also containimpurity elements that are uneconomical to removeduringrenement andprocessing. Alloyingcanproducelargeimprovementsinproperties.Forexample,steelsandaluminium alloyscan have strength levelsthat are over anorderofmagnitudegreaterthanthebasemetal (ironandaluminium respectively) in pure form.Theengineeringproperties of metals, suchas strength,toughness andcorrosionresistance, arecontrolledbythestructure of the metal onthe nano- (109m) andmicro-(106m) structural scale. In particular, it is through manip-ulation of alloy structure at the micro-scale (the microstruc-ture) that required properties are obtained. The metallurgisttherefore has toconsider the relationships betweenalloycomposition, processing, microstructure and properties.This chapter will introduce the most important of theserelationships for commonly used engineering alloys.Microstructure of metals and alloysStructuralalloys areall crystallineat an atomic scale;thatis, their atoms are arrangedina regular, orderedway.Theindividual crystals (whichareusuallyreferredtoasgrains)aretypicallylessthanamillimetreinsize, sothatbulkmetals areanaggregateof manymillionsof grains(i.e.polycrystalline).The arrangement of atoms in a crystalline material can bedenedby its unit cell. The unit cell is the minimumvolume of material that fully characterises the crystal struc-tureandsymmetry. Thestructureofany(perfect)crystalcan be generated by repetition of its unit cell in three dimen-sions. The metals that formthe basis of the commonengineering alloys have crystal structures that can be char-acterised by one of three unit cells. Figure 1 shows how theatoms are arranged in each of these cells.From an engineering perspective, the crystal structure ofalloysisimportant,sinceitiscriticalindeterminingmanyofthemacroscopicproperties.Forexample,aluminiumiseasilydeformedat roomtemperaturewithout fracturing,whereas magnesiumwill crack if deformed under thesame conditions. This can be largely attributed to thehexagonal crystal structureofmagnesiumcomparedwiththeface-centredcubiccrystal structureof aluminium, asdiscussed later.Phase transformationsInmanyalloys, several dierent crystal structures (allo-tropes)arepossible.Themoststablecrystalstructurewillthendependontheexternalconditions(suchastempera-tureandpressure). Alloyingadditions canalsoinuencethe relative stability of the dierent allotropes. If the condi-tionsarechangedsothatthemoststablecrystalstructurechanges,theatomswillattempttorearrangefromtheoldto the new stable crystal structure. The dierent crystallineformsthatanalloycanexistinarereferredtoasdierentphases, andsothechangefromonecrystal structuretoanother is known as a phase transformation. Phase transfor-mationsareexploitedintheprocessingofalloystoobtainthe requiredmicrostructure. The best-knownexample isICE Manual of Construction Materials # 2009 Institution of Civil Engineers www.icemanuals.com 469ice | manualsdoi: 10.1680/mocm.35973.0469CONTENTSIntroduction 469Microstructure of metals andalloys 469Phase transformations 469Defects in metals and alloys 470Plastic deformation ofmetals and alloys 471Strengthening of metalsand alloys 472Other mechanical properties 473Processing and formingof metals 474Physical properties ofmetals and alloys 474Corrosion of metals 475Alloy selection 476Key points 476References 476Further reading 477the phase transformation that occurs in iron and steel(steels are based on the ironcarbon system, with additionalalloyingelements). Atroomtemperature, themoststablecrystal structure for iron is body-centred cubic (BCC).This phase is called ferrite. When heated above 9108C(1183K),theface-centredcubic(FCC)structurebecomesmore stable, and a phase transformation occurs. TheFCC iron phase is called austenite.Thepresenceofadditionalelements,asinsteel,addstothe complexity of the phase transformations that canoccur.Steels are oftenprocessed withthe ironin the high-temperature(austenite)formbeforebeingcooledtoroomtemperature, where the ferrite phase is stable. Austenitehas ahigher solubilityfor carbonthanferrite (approxi-mately 100 times more carbon can be dissolved in austenitethanferrite,byweight).Thismeansthatforirontoformstable ferrite on cooling, carbon must berejected from thetransformingaustenite. This carbonforms anewphase,which in simple (plain carbon) steels has the chemicalformulaeFe3C, andiscalledcementite. If otheralloyingelementsarepresent,theycanalsoformnewphaseswiththe excess carbon, known as carbides.If cooling is very rapid (such as obtained if steel isquenched into water) then there is usually insucienttime for the movement of atoms that is requiredtore-arrange the austenite crystal structure to the two newcrystal structuresof ferriteandcementite. Inthiscase, aseries of dierent phase transformations is possible,forming phases that give dierent microstructures andproperties. The ability to exploit the phase transformationsfromaustenitetogivearangeof dierentstructuresisagreat advantage of steels. Apart fromiron/steel, otherimportant industrial metals that transform from one crystalstructure at high temperature to another at lower tempera-tureincludetitaniumandzirconium. Thestablestructurefor both of these is hexagonal close packed (HCP) atroom temperature, but BCC at high temperature.Another type of phase transformation that is widelyexploited in engineering alloys is particle precipitation.Particleprecipitationreferstotheformationof (usually)micron- or sub-micron-sizedparticles of asecondphaseintheinitial (matrix) phase. Precipitationoccursbecausethe solubility of alloying elements in the matrix phasedecreases with decreasing temperature. When the solubilityis exceeded, there will be adrivingforce for precipitateformation. Precipitation involves alloying elements leavingthe matrix phase and forming particles of a dierent phasewith distinct composition and structure. This process takestime, however, and precipitation can be suppressed bycoolingveryrapidly.Inthiscase,excessalloyingelementscanbetrappedinthematrixphase. Theseelements willthenformprecipitateparticlesovertime,andheatingcanaccelerate this process by enhancing atomic movement(diusion). This methodcanbe usedtoproduce a neanduniformdistributionof precipitateparticlesthat canprovide a large strengthening eect (precipitation strength-ening) (AluMATTER, 2007). Precipitation strengthening isexploited in many aluminium alloys, steels, and other alloysystems.Defects in metals and alloysThecrystalsthat aregeneratedbystackingtogetherunitcells, as described in the previous section, are perfect;there are noirregularities insuchastructure, andeachpart of the crystal is identical to every other part. In reality,real crystals are not perfect but contain defects. Thesedefectsplayacrucial roleincontrollingthepropertiesofmetals and alloys.Two types of defect are of particular importance in metalsand alloys: vacancies and dislocations. Avacancy is a gap inthe crystal structure, where there is no atomwhere one wouldbeexpected(Figure2(a)). Vacancies areimportant intheprocessofatomic diusion(the movementof atomswithinthecrystalstructure).Anyoneoftheatomsthatsurroundthe vacancy can move by jumping into the vacant site, leavinga vacant site behind (the atom and vacancy swap positions).Another atomcanthenjumpintothe newly vacant site andso60(a) (b) (c)Figure 1 Unit cells showing the arrangement of atoms in (a) face-centred cubic, (b) body-centred cubic and (c) hexagonal close-packed crystalstructures. The atoms outlined in bold define one of the planes of closest atomic packing in each structure470 www.icemanuals.com ICE Manual of Construction Materials # 2009 Institution of Civil EngineersMetals and alloyson. Avacancy is one type of point defect since it is localised toa single point in the crystal structure.Dislocationsareanexampleof alinedefect, creatingadisruptiontothecrystallatticearoundalineratherthanasinglepoint. Figure2(b)showsanexampleofonetypeofdislocation, an edge dislocation. An edge dislocation iscausedbythepresenceof anextraportionof aplaneofatoms that terminates within the crystal (commonly referredtoasanextrahalfplane).Theatomsthatformtheextrahalf plane are circled in blue in Figure 2(b). Other dislocationtypes, such as screw and mixed dislocations are also possible(Hulland Bacon,2001). Dislocations playakeyrole in thedeformation of metals and alloys; deformation, usuallyoccurs primarily by dislocation motion.Plastic deformation of metals andalloysIt is possible to prepare and deform large single crystals ofmetals. For example, consider a single crystal specimenpulledintension. At rst, deformationoccurs elasticallyand,iftheloadisremoved,thespecimenwillreturntoitsoriginal dimensions. However, above a certain appliedstress(theyieldstress)thespecimenstartstopermanently(plastically)deform.Itisfoundthatwhenthesurfaceofasuitablechosencrystal isexaminedaftersuchatest, itischaracterisedbyaseriesofbands,closeto458totheaxisalongwhichthecrystalwaspulled,andthesebandsformstepsatthesurfaceofthecrystal(Figure3(a)showstheseslip bands in a single crystal of cadmium after deformation).Furthermore, it is found that the planes dened by the slipbands (slip planes) correspond to the planes of closestatomic packing in the crystalstructure. For certain crystalstructures andorientations, it may be that none of theplanes of closest atomic packinglie close to458 totheaxisalongwhichthetensileloadisapplied.Insuchcases,itisoftenfoundthatfailurewilloccurbybrittlefracture,rather than progressive plastic deformation.Severalimportantconclusionscanbedrawnfromtheseobservations.First,deformationoccursbyshearingofthecrystalalong well-dened slipplanes. Thisis trueeven ifapurelytensileorcompressiveloadisapplied,sincesuchaloadwill generateashearstress (whichisamaximumat458to theaxis alongwhichthetensileload isapplied thetensile axis). Second, deformationoccurs onlyoncertainwell-denedplanes inthecrystal structureandincertainwell-dened directions. These directions and planes areknownas slipdirections andslipplanes respectively. It isfoundthattheslipplanesusuallycorrespondtotheplanesonwhichtheatomsinthecrystalaremostcloselypackedtogether, with the slip directions corresponding to the direc-tions along which the atoms are in closest contact.Figure 3(b) shows four unit cells of the FCC crystal struc-ture. The slipdirections andslipplanes for this crystalstructurearemarked. It canbeseenthat therearefourpossible slip planes, each containing three possible slip(b) (a)Figure 2 Schematic showing (a) a vacancy in a crystal structure (b) anedge dislocation: the atoms outlined in bold form the extra half plane,and the line where this ends inside the crystal defines the dislocation line100 mm(a) (b) (c)Figure 3 (a) A cadmium single crystal after deformation showing slip bands at the surface. (b) The close-packed planes and directions in the FCC crystalstructure that define the 12 slip systems. (c) Schematic showing how an applied tensile stressproduces a shear stress on a slip plane inclined closeto 458 to the tensile axisICE Manual of Construction Materials # 2009 Institution of Civil Engineers www.icemanuals.com 471The nature and behaviour of alloysdirections. This gives 12combinations (4 3), andeachcombinationisknownasaslipsystem. Therefore, metalswithFCCcrystal structures have atotal of 12possibleslipsystems.MetalswithBCCcrystalstructuresalsohaveat least 12 possible slipsystems, whereas metals withHCPcrystal structures mayhaveas fewas threeslipsystems.Thisisofpractical importance, sincetheeasewithwhichametal canbe deformedor shapedwithout crackingisstrongly dependent on the number of available slip systemsthat can accommodate deformation.Plastic deformation will initiate on a slip plane in a parti-cular slipdirectionwhentheshear stress onthat plane,resolvedintheslipdirection,exceedsacriticalvalue.Foracrystal pulledintension(asshowninFigure3(c)), theresolvedshear stresswill beamaximum on theslipplanesthatareorientedcloseto458tothetensileaxis.Deforma-tion will therefore take place by shearing on these planes.How, onanatomicscale, doesthisshearingoccur? Itmightbeimaginedthatalloftheatomsononeslipplaneslideoverall of theatomsontheslipplanebelow, sincethis would produce the observed deformation at the crystalsurface. However, when a calculation is made of thetheoreticalcriticalshearstressrequiredforthisprocess,itis foundtobeseveral orders of magnitudegreater thanthe critical shear stress measured, suggesting anothermechanism must be operating.In practice, it is much easier to produce slip by introducingandpropagating a dislocationintothe crystal structure,rather thansliding one whole atomplane over another.Then,ratherthanhavingtomovealltheatomsintheslipplane at the same time, disrupting the crystal structureacrossthewholecrystal, onlyalocal regionofthecrystalstructureisdisruptedatanyonetime.Ananimationillus-trating this process can be found at (DoITPoMS, 2007).Themovementofonedislocationanditsdestructionatthesurfaceof thecrystal produceadeformationstepoflessthan0.5 nmforatypicalmetal,fromwhichitisclearthatmovementofmanymillionsofdislocationsisneededtoprovideeasilyseenmacroscopicdeformation. Someofthese dislocations will already exist in the crystal(since, asdiscussed, noreal crystal isperfect).Othersaregeneratedduringdeformation(seeHull andBacon, 2001for moredetails).The knowledge that deformation in metals occurs on anatomiclevel bymovement of dislocations has importantpractical consequences, sinceitsuggeststhatincreasingametals resistance toplastic deformation(i.e. increasingstrength) requires inhibition of dislocation movement.The mechanisms used to do this are discussed next.Strengthening of metals and alloysIn many applications, yield strength is a critical mechanicalproperty, since this is the maximumstress to which amaterial can be subjected without permanent plastic defor-mation. There are several ways that the yield strength of ametal canbe increased, bothby alloying additions andprocessing. Fundamentally, eachof thesemethodsworksbymakingdislocationmovement more dicult, therebyincreasing the resolved shear stress that is required toinitiate plastic deformation.Grain size strengtheningSo far, the deformation of a single crystal has beenconsidered. In practice, bulk metals are nearly alwayspolycrystallineandconsist of aggregatesof manycrystals(grains) indierent orientations. Theboundaries betweengrains provide a barrier to dislocation motion. Theseboundaries therefore provide a strengthening eect and, themore grainboundary area there is per unit volume, thegreater the strengthening eect. This means that ne-grainedmaterials will be stronger than the same coarse-grainedmaterial, because a ne-grainedmaterial will have moregrainboundary area toimpede dislocationmotion. Therelationshipbetweenthegrainsizeofanalloyanditsyieldstrengthis describedby the HallPetch equation, whichstates that the yield strength is inversely proportional to thesquare root of the grain size (Callister, 2006).Areductioningrainsizecanbeachievedinpracticebythermomechanical processingof the alloy(e.g. rollingittoproduce sheet,or extrudingittoproduce rodsor bars).Itisawidelyusedstrengtheningmechanismsinceitdoesnot rely on expensive alloying elements and, in addition, agrainsizereductionalsoimprovesthetoughnessofmanyalloys (in contrast with the other strengthening mechanismsdiscussedhere, where the increase instrengthis usuallyaccompanied by a decrease in toughness).Solid solution strengtheningSomealloyingelements canbedissolvedintothecrystalstructure of the matrix phase forming a solid solution.Alloyingelementscanbeaccommodatedintothecrystalof theparent element inoneof twoways. If theatomicradius of the alloyingelement is muchsmaller thantheparentatomicradius,thenthealloyingelementatomscansit in the gaps (interstices) between the parent atoms.Suchsmallatomsarecommonlyreferredtoasinterstitialswhen in solution. Carbon atoms occupy the interstitialsites whendissolvedinironandthis plays akeyroleinthe metallurgy of steels.Moreoften, solublealloyingelementswill notbesmallenoughtobeaccommodatedwithingapsbetweenparentatoms, in which case the alloying addition will be dissolvedin the parent phase by replacing parent atoms in the crystalstructure. Suchelementsarereferredtoasbeingsubstitu-tional when in solution.Bothinterstitial andsubstitutional additionswill createdistortions (strains) in the crystal structure due to the472 www.icemanuals.com ICE Manual of Construction Materials # 2009 Institution of Civil EngineersMetals and alloysmismatchinsizebetweentheparentandalloyingelementatomic radius. Dislocations also result in a local distortion(strain) because the atoms around the dislocation are not intheir ideal positions. Thestrains duetodislocations willinteractwiththestrains surroundingsolute atoms becausestrainsofoppositesignswilltrytocanceleachotherout.This creates an attraction between the dislocation andsolute atomthat must be brokenfor the dislocationtomove, increasing strength. The eectiveness of soluteelements in increasing yield strength increases with anincreaseinthemistbetweenthesoluteandparentatomsandalsowiththeamount of thesoluteelement that canbe dissolved into the matrix crystal structure.Strain hardeningMost metals have the useful propertythat theybecomestronger as they are plastically deformed. This eect isknown as strain or work hardening, and it is often exploitedin the strengthening of metals and alloys. The origins of thestrain hardening eect can be traced to interactions betweenthe dislocations that are generated during plastic deforma-tion. As discussed, these dislocations locally distort thecrystal structureandarethussurroundedbystrainelds.Asthenumberof dislocationsincreasesduringdeforma-tion, andthe dislocations move, their spacingdecreasesuntil their strain elds start to interact with each other.Onaverage, theinteractionbetweenthestraineldsofneighbouring dislocations leads to repulsive forces betweenthedislocations,withtheresultthatanadditionalappliedstressisrequiredtoovercomethisrepulsion. Thisresultsin an increase in strength. The eectiveness of strain hard-ening will depend on the amount of deformation impartedandalso thetype ofalloy(some alloysshow muchgreaterstrainhardeningthanothers). Likegrainsizerenement,strainhardeningis awidelyusedmethodtostrengthenlow-cost alloys, sinceit doesnot dependontheadditionof expensive alloying elements.Precipitation strengtheningThe most potent strengthening mechanism that is exploitedin the highest-strength alloys is precipitation strengthening(also known as age hardening). This strengtheningmechanisminvolvesformingne(sub-micron)particlesofasecondcrystallographicphasethatareembeddedinthematrix crystal. These particles are usually formed by a suit-ableprecipitationheattreatment,asdiscussedpreviously.The precipitate particles increase strength by acting asbarrierstodislocationmotion.Itisfoundthatthereisanoptimumsizeandspacingof particlesthatgivesthebeststrengtheningeect (theoptimumparticlesizeis usuallyonly a fewnanometres). If the particles are too small,dislocationsareabletocut throughthem, whereasiftheparticlesarelargeandwidelyspaceddislocationscanpassbetween the particles by bending (AluMATTER, 2007,StrengtheningMechanismsModule). Thecorrect particlesize is obtained by carefully controlling the alloy composi-tion and heat treatment.Other mechanical propertiesThediscussionsofarhasfocusedontheuseof alloyingelementsandprocessingtoincreasetheyieldstrengthofalloys.Whilestrengthisusuallyacriticaldesignproperty,there are other mechanical properties that must be consid-ered, and may indeed be of overriding importance in someapplications.StiffnessStiness (usually characterised by the Youngs modulus) isthe ability of a material to resist elastic deformation. Sincemost components are designed to operate within the elasticdeformation regime (i.e. without plastic deformation) thenstiness is usually a key property. However, althoughdierent metals havewidelyvaryingstiness values (e.g.the Youngs modulus of tungsten is 400 GPa, that ofmagnesiumis 45 GPa), processingandalloyingelementsaddedat typical levelsdonot usuallygreatlychangethestinessof theparent metal. Onlyinalloys that containseveral phases, which have markedly dierent stinessvalues, is it possible to obtain signicant variations instiness by varying the proportion of the two phases.Fracture toughnessFracture toughness characterises the ability of a material toresist the propagation of cracks. Tiny cracks and defects arepresentinallcommonlyusedmaterials.Thesecrackswillleadtothe concentrationof stress at the cracktip; thecritical stress concentration required to propagate thecrackcharacterisesthefracturetoughnessofthematerial.Materials with low fracture toughness, such as manyceramics, will tend to fail by rapid growth of intrinsiccracksbeforereaching the yieldstressrequired for generalplastic deformation. Most alloys have a higher fracturetoughness than this, and will yield plastically beforefracture.Alloyingandprocessingoftenhavealargeinuenceonfracturetoughnessofmetals. Thegeneral trendobservedis that anymechanismthat increases the strengthof analloyconcomitantlyreducesitstoughness.Thisisbecauseoncetheyieldstress is exceededat thecracktip, plasticdeformationis abletoredistributethestress andreducethe stress concentration. The exception to this trend isstrengthening by grain size renement, which usuallyincreases bothstrengthandfracturetoughness. Fracturetoughness is also degraded by the presence of a low-tough-nessbrittlesecondphase, whichcanbothactassitesforinitial crackformationandas easy pathways for crackgrowth.ICE Manual of Construction Materials # 2009 Institution of Civil Engineers www.icemanuals.com 473The nature and behaviour of alloysFatigueFatigueoccurswhenamaterial issubjecttoauctuatingstress,andcanleadtofailureafteraperiodoftimeevenifthe maximum stress level experienced is considerably belowthestress requiredfor global plasticdeformation(i.e. theyieldstress).Fatigue iscausedbytheinitiation andgrowthof acrackat apoint of highstress concentration, whichmaybeduetoadesignfeature(e.g. asharpcorner) oramicrostructural feature (e.g. large, hard particle).Microstructure will inuence both the initiation andpropagationof fatiguecracks inalloys. Local defects inthe microstructure such as voids or particles (e.g.inclusionparticles formedbyimpurityelements) canactasstressconcentratorsandsitesofcrackinitiation. Sincefatigue is ofteninitiatedat surface scratches or defects,changingthesurfacepropertiescanhaveamarkedeectonfatigueresistance. Alloyingadditionsandmicrostruc-turewillalsoinuencecrackgrowth.Solutestrengtheningelements tend to increase fatigue properties in parallelwiththeireectonyieldstrength. Secondphaseparticlesand grain size have a more complex relationship withfatigue properties that depends both on alloy and thefatigue conditions, e.g. whether low-stress amplitudefatigueor high-stress amplitudefatigue(Callister, 2006).Ingeneral, alloysthatrelyonprecipitatestoobtaintheirstrength generally have lower fatigue properties in relationtotheir yieldstrengththanalloys strengthenedbyothermechanisms.CreepCreepdescribes theprocess bywhichalloys deformper-manentlywhensubject toastress belowtheyieldstresswhenheldat elevatedtemperature. Creepinmetalsonlybecomes signicant at temperatures greater than about0.4 Tm,whereTmistheabsolutemeltingtemperature(K).It follows that metals with lower melting points (e.g.aluminium Tm933 K, lead Tm600 K) are more suscep-tibletocreepthanhighermeltingpointmetals(e.g.nickelTm1728 K, titanium Tm1941 K). Creep resistancealso depends on microstructural features. Grain boundariesaccelerate creep deformation, so a ne grain size is undesir-able for maximumcreepresistance. Indeed, inthe mostdemanding applications, such as high-performance jetengines, an entire component (e.g. a turbine blade) ismade fromasingle crystal. Solidsolutionelements andprecipitate particles can also enhance creep resistance,provided they are stable at elevated temperature.Processing and forming of metalsThe starting point for nearly all fabrication processes usingalloysismoltenmetal.Alloyingismosteasilyachievedbyco-meltingthebasemetalandalloyingadditions.Impuri-ties canalsobe removed(e.g. byadditions whichformsolidparticlesthatsinktothebottomoroattothetopof the melt, binding impurity elements).Themoltenmetal isthensolidied. Inthecaseofcastcomponents, solidicationtakesplaceinamouldthat isclose to the nal shape of the product, so that once solidi-cationiscompletelittlefurtherfabricationisundertaken(althoughheattreatment ofthecastingiscommon). Themajorityof metals, however, are predominantlyusedtomakewroughtproducts, whichrequiresthermomechanicalprocessing (TMP).In the fabrication of wrought products,casting is the rst step in a process chain that includes defor-mationofthe alloy.Deformationisinitially carriedoutatelevated temperature, since this reduces the strength ofthe alloy, making it easier to work. Final deformationstepsmaybecarriedout cold. Typical processesusedtomake wrought components include rolling (to producesheet, plateandbeams), extrusion(toproducerods andbars),forging(toproducecomponentswithsimpleshapesbut excellent properties), anddrawing(toproducewire)(Dieter, 1989). These processes are shown in Figure 4.Wrought products generally have better and morereliablepropertiesthanthoseofas-castcomponents.Thisis because it is very dicult to produce defect-free castingswith the optimummicrostructure (e.g. ne grain size,uniform distribution of strengthening particles). TMPallowsgreatercontrol overthenal microstructureandareductionof defects suchas pores, leadingtothebetterproperties exhibited by wrought products.ThemicrostructuralchangesthatoccurduringTMParecomplex, since bothdeformationandheat inuence keymicrostructural features. Thesize, shape, andorientationof eachgrain(crystal) is changedby deformation. Newgrains may form, consuming the old grains (a processknown as recrystallisation). Alloying elements can formprecipitate particles, either during cooling fromelevatedtemperature deformation, or during a separate precipitationheat treatment. Defects from casting, such as pores and largebrittle particles, can be largely eliminated by TMP.Physical properties of metals andalloysIn some applications, alloys are chosen based on theirphysical rather than mechanical properties. The key(a) (b) (c)Figure 4 Thermomechanical processing methods used for theproduction of wrought alloy components: (a) rolling; (b) extrusion; and(c) forging474 www.icemanuals.com ICE Manual of Construction Materials # 2009 Institution of Civil EngineersMetals and alloysphysical properties of relevancetometals andalloys arediscussed briey below.Electrical propertiesMost metals areverygoodconductors of electricityduetotheir delocalisedelectrons that respondreadilytotheapplicationof anelectric eld. Microstructural featuresthat act as barriers to electron motion decrease the conduc-tivityof metals. Suchfeatures includealloyingadditionsdissolvedin solidsolutionand defects such asdislocationsandvacancies. Theconductivityof alloys alsodecreaseswith increasing temperature due to increased thermal vibra-tion of the atoms and an increase in defect density, both ofwhich increase electron scattering.Thermal propertiesThethermal conductivity ofmetals occurslargely throughfree electrons, and thermal conductivity is therefore closelylinkedtoelectrical conductivityandis inuencedbythesame microstructural features. Other thermal propertiesofalloyssuchasheatcapacityandcoecientofthermalexpansion(CTE) varysignicantlyfrommetal tometal,but are relatively insensitive to microstructural changesfor a given alloy composition.Magnetic propertiesMetals may either show a permanent magnetic moment (bemagnetic) or show magnetisation only in the presence of anapplied electromagnetic eld (be non-magnetic). Non-magnetic alloys may be divided into two types: diamagneticandparamagnetic.Diamagneticmetalsreactweaklytoanapplied eld, with the weak induced magnetic momentbeinginadirectionthat opposes theappliedeld. Dia-magnetismis so weak that it rarely has any practicalconsequences. Diamagneticmetalsincludecopper, silver,and zinc. Paramagnetic metals respond to an externaleldinawaythatenhancestheeld(ratherthanopposesit, as with diamagnetic metals). This eect, althoughstronger than that exhibited by diamagnetic metals, isweak compared with the magnetism exhibited by magneticmetals.Aluminium,titaniumandchromiumareexamplesof metals that show paramagnetic behaviour.Magnetism is most important in metals that show a largeand permanent magnetisation, which is known as ferromag-netism. Iron(inits BCCferriteform), cobalt andnickelshowferromagneticbehaviour. Thestrengthof themag-netisation depends on temperature. As temperatureincreases,thermalvibrationofatomsresultsinadecreasein magnetisation until a critical temperature is reached(the Curie temperature), above which there is no permanentmagnetisation (and paramagnetic behaviour is shown).Alloyingcanbeusedtooptimisemagneticpropertiesfora given application. Steels containing chromium and tung-sten, Cu-Ni-Fe alloys, and Al-Ni-Co alloys are all magneticmaterialsthat showahighresistancetodemagnetisationthanks tofavourablemicrostructures, makingthemsuit-able for applications where permanent magnetism isrequired.Optical propertiesBulkmetalsareall opaquethroughout thewholevisiblelightspectrum; lightradiationfallingonametal iseitherabsorbed or reected. Reectivity for most metals isbetween0.9and0.95oftheincidentlightenergy. Metalsthat appear bright and silvery do so because they arereective over the whole light spectrum. Metals thatappearcoloured(suchascopper)dosobecausethereisabias inthe visible light photons theyreect whenwhitelight is incident, andphotons of certainwavelengths aremissing from the reected spectrum.Corrosion of metalsIn choosing an alloy for a given application, it is critical toconsidertheenvironmentinwhichthealloyistobeused.All metalsandalloyscanbesubjecttocorrosion. Inthisprocess, metal interacts withits environment leadingtodegradation of the metal.Corrosion of metals is most commonly an electro-chemical process that involves bothachemical reactionand a transfer of electrons. Corrosion occurs by theelemental metal undergoing a reactionthat results inaloss of electrons, creating metal ions, which are thentakenintosolution.Thechemicaldrivingforceforcorro-sionvarieswidelyfrommetal tometal andalsodependsonalloying. Another critical factor is whether themetalcanformaprotective oxide coatingwhichresults initspassivation(reductioninchemical reactivity). Aluminium,for example, is a reactive metal but forms a protectiveoxidelminmanyenvironments, givingahighlevel ofcorrosionresistance.Stainlesssteelshaveahighcorrosionresistanceundermanyconditionsduetothepresenceofchromium, which reacts to forma protective surfacelayer. Care must be taken with metals that rely on passiva-tionfor corrosionresistance, since achangeinenviron-mental conditions can result in a breakdown of theprotective lm, and a very large increase in corrosion rates.Alloying and microstructure both inuence corrosionperformance. Grain boundaries and second phase particlescan act as sites for preferential attack, leading tointergranular and pitting corrosion respectively. Thesimultaneous action of corrosion and an applied stresscanresult ingreatlyacceleratedrates of failure, due tostress corrosion cracking.Particularcarehastobetakenwhenmetalsofdierentreactivity are used together in a component. Couplingtogether of two dierent metals can lead to galvanic corro-sion, accelerating the dissolution of the more reactive metal.ICE Manual of Construction Materials # 2009 Institution of Civil Engineers www.icemanuals.com 475The nature and behaviour of alloysThis problem can be avoided by ensuring the two metals arenot inelectrical contact (e.g. byusinganon-conductiveinterlayer).Alloy selectionMetal alloys areextremelyversatilematerials, oeringawide range of properties depending on alloy type, composi-tion and microstructure. Steel is by far the most dominantalloyintechnologicalusetoday,withmorethan40timesmore steel producedthanaluminium, whichis the nextmost widely used metal.The dominance of steel can largely be attributed tothe huge range of dierent microstructures that canbeobtainedthroughchangesincompositionandprocessingbyexploitingthe wide range of solidstate phase trans-formations that arepossible. Steel canalsobeproducedwith good properties at a cost that can compete eectivelywith other materials. It should be noted that there are hugeranges of steels of dierent compositions for dierentapplications.Examplesofsteeltypesincludeplain-carbonsteel, high-strength low-alloy steel, stainless steel, toolsteeland many others.Othermetalsareusuallyusedprimarilyinapplicationswhere there is a special requirement that makes themmoresuitablethansteel.Forapplicationswhereweightisimportant suchasinaerospaceorothertransport appli-cations, aluminiumor magnesiumare often preferred.Aluminiumhas density that is approximately one-thirdthat of iron, givingaluminiumalloys specic properties(e.g. specic yield strengthyield strength/density) thatcan exceed those of steels. Aluminiumalloys are alsohighly formable, and many of themare much morecorrosionresistant thanplain-carbon steels.Magnesium islighter still, havingadensitythat is approximatelytwo-thirds that of aluminium. However, asaconsequenceoftheir HCPcrystal structure, magnesiumalloys are moredicult toform. Magnesiumalloys canalsosuer fromhigh levels of corrosion if not properly protected.High-temperature applications require alloys that areresistant to creep and thermal fatigue. Nickel-based super-alloys areusedinaerospacegasturbines(jetengines)toproduce components for the hottest part of the engine,wherecreepresistanceisparamount. Sinceweightisalsoimportant inaerospace applications, titaniumalloys arewidely used where temperatures are lower, since thesealloys combine good thermal resistance with lower densitythannickel alloys. Titaniumalloys areoftenalsohighlycorrosionresistantandforthisreasonareusedwidelyinthechemical industryandalsoinbiomedical applications(such as for hip replacements and other implants).Otheralloyshavepropertiesthatmakethemsuitedforapplicationsthatareusuallydominatedbyoneparticularrequirement. For example, copper andcopper alloys areoftenusedwhereelectrical conductivityisimportant, andtheyarealsousedinarchitectural applications(e.g. asaroong material) because of their high corrosion resistanceand aesthetic properties (forming a green protective surfacelmwhenweathered).Copperisalsousedwithsignicantadditions of other metals toformbronzes andbrasses;bronzeismostcommonlyusedtorefertoalloysbasedonthecoppertinsystem, whereas brasses arebasedonthecopperzincsystem. Thecopperzincsysteminparticularcanbeusedtoproduceawiderangeof useful alloysbyvaryingthezinccontent,whichcan beasmuchas50%insome brasses. The microstructure of these alloys caneither be single phase, with the zinc as substitutionalatomsintheFCCcopperphase, atwophasemixtureofcopper-rich FCCand zinc-rich BCCphases, or at thehighest zinc levels single BCCphase (containing bothcopper and zinc). The wide range of possible micro-structures explains the versatility of this alloy system.Other metals that are used as engineering materialsinclude leadandtinalloys. Bothhave a relatively lowmeltingpoint andaresoft andweak. Theadvantagesofthese attributes are that these metals are easy to cast(which requires melting and resolidication) and form.Metals used in signicant quantities in special applicationsinclude zirconium alloys that are widely used in the nuclearindustry for in-reactor components because of their combi-nationof goodcorrosionperformance andlowneutroncapture characteristics.Key pointsMetalalloys areaversatile classofmaterials;the additionofthecorrectalloyingelementstoapuremetal canleadto large property improvements.The properties of alloys depend critically on their micro-structure,whichinturnisstronglydependentonboththealloy composition and processing route.The deformationof alloys is controlledonanatomicscale by the movement of crystal defects known as disloca-tions.Alloystrengtheningmechanismsworkbyinhibitingthe movement of dislocations.In addition to mechanical properties, physical propertiesand corrosion resistance are often key to determining whichalloy is best for a given application.ReferencesAluMATTER, 2007, http://aluminium.matter.org.uk.CallisterW.D.MaterialsScienceandEngineering:AnIntroduc-tion, New York: Wiley, 2006.Dieter G. E. Mechanical Metallurgy, McGraw-Hill Education,New York, 1989.DoITPoMS, 2007 http://www.doitpoms.ac.uk/.Hull D. and Bacon D. J. Introduction to Dislocations, Butterworth-Heinemann, Oxford, 2001.476 www.icemanuals.com ICE Manual of Construction Materials # 2009 Institution of Civil EngineersMetals and alloysFurther readingBhadeshia H. K. D. H. and Honeycombe R. W. K. Steels:Microstructure and Properties (3rd Edition), Butterworth-Heinemann, Oxford, 2006.HumphreysF.J.andHatherlyM.RecrystallizationandRelatedAnnealing Phenomena (2nd Edition), Pergamon, Oxford, 2004.PolmearI. LightAlloys(3rdEdition), Butterworth-Heinemann,Oxford, 1995.Porter D. A. and Easterling K. E. Phase Transformations in Metalsand Alloys, CRC Press Inc., London, 1992.Steel University http://www.steeluniversity.org.ICE Manual of Construction Materials # 2009 Institution of Civil Engineers www.icemanuals.com 477The nature and behaviour of alloysChapter 40Ferrous metalsArthur Lyons Leicester School of Architecture, De Montfort University, Leicester, UKFerrous metals describes steels, cast and wrought iron, and alloy steels in relation tomanufacture, composition, physical properties and applications. Paint and metalliccoatings for steel are described in relation to corrosion protection. The sustainability ofsteelas a construction material is discussed.IntroductionSteels are dened as the alloys between iron and carbon, butaredistinguishedfromcastironwithitsveryhighcarboncontent and wrought iron which is virtually free ofcarbon. Inadditiontovarying the carboncontent, therange of steels is extended by the addition of furthermetallic andnon-metallic alloying elements andvariousheat treatments whichmodifythe mechanical propertiesandcrystalstructures.Figure1showstherangeofferrousmetals,andthebroadeectofcarboncontentonthekeyphysical properties of steels is illustrated in Figure 2.Steelsarecategorisedbycarboncontent,butthebroaddescriptiveterminologyisappliedexiblytothefollowingranges of carbon content:Ultra low carbon steel 3053.2 mm4.8 mm5.0 mmBow of web Greater of d/150 or 3 mmHorizontality of flangeB < 110B > 7001.5 mm2% of b 56.5 mmGreater of B/100 or 3 mmVerticality of web atsupport Greater of d/300 or 3 mmSquareness of cut endnot prepared for bearing(plan or elevation)D/300 Squareness of cut endprepared for bearing (planor elevation)D/1000 Overalllength 2 mm 3 mmStraightness along lengthof beamGreater of L/1000or 3 mmGreater of L/1000 or3mmNotes:L is member length; d is web depth; D is section depth; B is the section width (all inmm)Table 20 Typical geometric tolerances for steel sections (adapted fromBCSA National Structural Steelwork Specification)Level of adjacent beams 5mmPosition of floor beams at columns 10mmLevel at each end of same beam 5 mmPosition of beam from wall 25mmLevel of foundations 0 to 3 mmPosition of holding-down bolts 20mmMaximum gap between bearing surface of column end(depth D)D/1000 1 mmTable 21 Acceptable tolerances in general steel construction (adaptedfrom BCSA National Structural Steelwork Specification)ICE Manual of Construction Materials # 2009 Institution of Civil Engineers www.icemanuals.com 567Structural steelnVertical movement of low-pitch portal frames or roofs.nDeection of beams supporting internal compartment walls.nVisualdeectionsorinstallationofraisedoorsinlong-spanbeams.nHorizontal movement of tall structures.nUse of isolating pads for deection-sensitive equipment.nMovement of supports to cranes and travelling machinery.The designer should agree sensible deectionlimits withspecialistsuppliersofthecladdingandliftsetc., andwiththe steel fabricator. The normal limit ondeections forbeamssubjecttoimposedloadisbeamspan/360,inorderthatdeectionsarenotnoticeableandthatpartitionsarenot subject tocracking. Stricter deectionlimits mayberequiredinmanyoftheabovecases. Alimitofspan/500isoftenspeciedforedgebeamssupportingbrittleformsofcladding.Itmaybenecessarytopre-camberlong-spanbeams(open>12 m) inordertooset permanent deec-tionswhichmaybevisuallyunacceptable.Theamountofpre-camber is normally set at half the anticipatedtotaldeection, but not less than a practical minimum of 25 mm.ReferencesBaddooN.R.,BurganB.andOgdenR.G.ArchitectsGuidetoStainlessSteel,SCIPublicationP179,TheSteelConstructionInstitute, 1999, Ascot.Baddoo N. R. Castings in Construction, SCI Publication P172. TheSteel Construction Institute, 1996, Ascot.British Constructional Steelwork Association/The Steel Construc-tion Institute. Joints in Steel Construction: Moment Connections,SCI Publication P207. 1995, Ascot.British Constructional Steelwork Association. National StructuralSteelwork Specication for Building Construction (5rd Edition).British Constructional Steelwork Association, 2007, AscotBCSA/The Steel Construction Institute. Joints in Steel Construc-tion: Simple Connections, SCI Publication P212, 2002, Ascot.BritishStandardsInstitution.BS4190:ISOMetricBlackHexa-gonal Bolts, Screws and Nuts. Specication, 2001, BSI.British Standards Institution. BS4395: High Strength Friction GripBolts and Associated Nuts and Washers forStructural Engineer-ing. MetricSeries. Part1:General Grades, 1969. Part2:HighGrade Bolts and Nuts and General Grade Washers, 1969, BSI.British Standards Institution. BS476. Fire Tests on BuildingMaterialsandStructures.Part20.MethodsfortheDetermina-tion of the Fire Resistance of Elements of Construction (GeneralPrinciples). Part 21: Methods for the Determination of the FireResistance of Elements of Construction, 1987, BSI.British Standards Institution. BS5400: Steel, Concrete andComposite Bridges. Part 3: Code of Practice for Design ofSteel Bridges, 2000, BSI. Part 6: Specicationfor MaterialsandWorkmanship:Steel,1999,BSI.Part10:CodeofPracticefor Fatigue, 1980, BSI.British Standards Institution. BS5950-1: Structural Use of Steel-workinBuildings. Part1:CodeofPracticeforDesign-Rolledand Welded Sections, 2000, BSI.British Standards Institution. BS5950-3: Structural Use of Steel-workinBuildings. Part 3.1: Code of Practice for DesignofSimple and Continuous Composite Beams, 1990, BSI.British Standards Institution. BS5950-8: Structural Use of Steel-workinBuildings. Part8:CodeofPracticeforFireResistantDesign, 1990, BSI.British Standards Institution. BS7079: Part A1 (Also BSEN ISO8501-1):General IntroductiontoStandardsforPreparationofSteel Substrates before Application of Paints and RelatedProducts. Visual Assessment of Surface Cleanliness. Part 1:Rust Grades and Preparation Grades of Uncoated SteelSubstrates and of Steel Substrates after Overall Removal ofPrevious Coatings, 2007, BSI.British Standards Institution. BS7608: Code of Practice forFatigue Design and Assessment of Steel Structures, 1993, BSI.British Standards Institution. BS7974: Application of Fire SafetyEngineering Principles to the Design of Buildings Code ofPractice, 2001, BSI.British Standards Institution. BS8110: Structural Use of ConcretePart1:CodeofPracticeforDesignandConstruction,1997,BSI.British Standards Institution. BSEN10025-2: Hot RolledProducts for Structural Steel. Technical Delivery Conditions forNon-Alloy Structural Steels, 2004, BSI.British Standards Institution. BSEN10025-5: Hot RolledProducts for Structural Steel. Technical Delivery Conditions forStructural SteelswithImprovedAtmosphericCorrosionResis-tance, 2004, BSI.British Standards Institution. BSEN10088-2: Stainless Steel. Part2:Technical DeliveryConditionsforSheet, PlateandStripofCorrosionResistingSteel. Steels for General Purposes, 2005,BSI.British Standards Institution. BSEN10088-5: Stainless Steel. Part5: Technical Delivery Conditions for Bars, Rods, Wire, Sectionsand Bright Products of Corrosion-Resisting Steels for Construc-tion Purposes, 2009, BSI.BritishStandardsInstitution. BSEN1011-2: WeldingRecom-mendations for Welding of Metallic Materials. Part 2: ArcWeldingofFerriticSteels, 2001, BSI. Part3:ArcWeldingofStainless Steels, 2003, BSI.British Standards Institution. BSEN10169-2: ContinuouslyOrganic Coated (Coil Coated) Steel Flat Products. Part 2:Products for Exterior Building Applications, 2006, BSI.BritishStandardsInstitution.BSEN10210:HotFinishedStruc-turalHollowSectionsofNon-AlloyandFineGrainSteelsPart1. Technical Delivery Conditions, 2006, BSI.British Standards Institution. BSEN10219: Cold Formed WeldedStructural Hollow Sections of Non-Alloy and Fine Grain Steels Technical Delivery Conditions, 2006, BSI.BritishStandards Institution. BSEN10240: Coatings for SteelTubes: Specication for Hot-dip Galvanised Coatings, 1998.BritishStandards Institution. BSEN10346: ContinuouslyHot-Dip Coated Steel Flat Products Technical Delivery Instructions,2009,BSI(supersedesBS EN10147,10292,10326,10327,and10336).BritishStandardsInstitution. BSEN1090-2: Executionof SteelStructures and Aluminium Structures. Part 2: Technical Require-ments for the Execution of Steel Structures, 2008, BSI.568 www.icemanuals.com ICE Manual of Construction Materials # 2009 Institution of Civil EngineersMetals and alloysBritish Standards Institution. BSEN1365: Fire Resistance ofLoad-bearing Elements. Part 1: Walls. Part 2: Floors andRoofs. Part 3: Beams. Part 4: Columns, 1999, BSI.British Standards Institution. BSEN14399-1. High StrengthStructural Bolting Assemblies for Pre-Loading, 2005, BSI. Part1: General Requirements. Part 2: Suitability Tests. Part 3:SystemHR: Hexagonal Bolt and Nut Assemblies. Part 4:System HV: Hexagonal Bolt and Nut Assemblies.British Standards Institution. BSEN15048: Non-preloadedStructural Bolting Assemblies, 2007, BSI. Part 1: GeneralRequirements. Part 2: Suitability Tests.British Standards Institution. BSEN1992-1-1 Eurocode 2: DesignofConcreteStructures. Part1.1:General RulesandRulesforBuildings, 2004, BSI.British Standards Institution. BSEN1993-1-1: Eurocode 3, Designof Steel Structures. Part 1-1: General Rules and Rules for Build-ings, 2006, BSI.British Standards Institution. BSEN1993-1-10: Eurocode 3,Designof Steel Structures. Part 10: Material Toughness andThrough Thickness Properties, 2005, BSI.British Standards Institution. BSEN1993-1-2: Eurocode 3, Designof Steel Structures. Part 1.2: Structural Fire Design, 1994, BSI.British Standards Institution. BSEN1993-1-4: Eurocode 3, Designof Steel Structures. Part 1.4: Supplementary Rules for StainlessSteels, 2006, BSI.BritishStandardsInstitution.BSEN1993-2:Eurocode3,Designof Steel Structures. Part 2: Steel Bridges, 2006, BSI.British Standards Institution. BSEN1994-1-2. Eurocode 4, Designof Composite Steel and Concrete Structures. Part 1.2: StructuralFire Design, 1994, BSI.BritishStandardsInstitution. BSEN22063: MetallicandOtherInorganicCoatingsThermalSpraying Zinc,Aluminiumandtheir Alloys, 1994, BSI.British Standards Institution. BSEN IS0 3506: Mechanical Prop-ertiesofCorrosion-ResistantStainlessSteel Fasteners. Part1:Bolts, Screws and Studs, 1998, BSI.British Standards Institution. BSEN ISO12944: Paints andVarnishes. Corrosion Protection of Steel Structures by ProtectivePaint Systems, 2007, BSI. (replaces BS 729). Part 2: Classica-tionof Environments. Part 4: Types of Surface andSurfacePreparation. Part 5: Protective Paint Systems.British Standards Institution. BSEN ISO1461: Hot DipGalvanizedCoatings onFabricatedIronandSteel Articles Specications and Test Methods, 1999, BSI.British Standards Institution. BSEN ISO14713: ProtectionAgainst Corrosionof IronandSteel inStructuresZincandAluminium Coatings Guidelines, 1999, BSI.British Standards Institution. BSEN ISO 4014: Hexagonal HeadBolts. Product Grades A and B, 2001, BSI.BritishStandards Institution. BSEN10240: Coatings for SteelTubes: Specicationfor Hot DipGalvanizedCoatings, 1998,BSI.CorusConstructionandIndustrial. AdvanceSectionsBrochure.Corus, Scunthorpe, UK, www. corusconstruction.com.Corus Construction and Industrial: A Corrosion Protection Guidefor Steelwork Exposed to Atmospheric Environments, 2004.Corus Construction and Industrial: A Corrosion Protection Guidefor Steelwork in Building Interiors and Perimeter Walls, 2004.CorusTubes. CorusCelsius355Technical Guide, CorusTubes,Corby, UK.Corus Tubes. DesignGuide for SHSConcrete-FilledColumns,Corus Tubes, Corby, UK.Corus. Reinstatement of Fire Damaged Steel and Iron Framed Struc-tures. Corus, Swinden Technology Centre, Rotherham, UK.Cosgrove T. Tension Control Bolts in Friction Group Connections.SCIPublication324,TheSteelConstructionInstitute, Ascot,2006.Craddock, P. GuidetoSiteWelding, SCIPublication161. TheSteel Construction Institute, Ascot, 2002.Euroinox and The Steel Construction Institute. Structural Designof Stainless Steel, 2006.FireProtectionforStructural Steel inBuildings, AssociationofSpecialist Fire Protection Contractors and Manufacturers,SCI Publication P13.Gorgolewski M., Grubb P. J. and Lawson R. M. Building DesignUsing Cold Formed Steel Sections; Light Steel Framing in Resi-dential Construction, SCI Publication 301, 2003, The SteelConstruction Institute, Ascot.Ham S. J. et al., Structural Fire Safety: A Handbook for ArchitectsandEngineers,SCIPublicationP-197,TheSteelConstructionInstitute, 1999.Joints in Simple Construction, Vol. 1 Design Methods, Vol. 2 Prac-tical Applications, BritishConstructional SteelworkAssocia-tion/TheSteel ConstructionInstitute. SCIPublicationsP205and P206.LawsonR. M.BuildingDesignUsingModules,SCIPublication348, 2007, The Steel Construction Institute, Ascot.Lawson R. M., Grubb P. J., Prewer J. andTrebilcockP. J. ModularConstructionusingLightSteel Framing:AnArchitectsGuide,SCI Publication 272, 1999, The Steel Construction Institute.Newman G. M. and Sims W. I. Fire Resistance of Concrete-lledTubes to Eurocode 4, SCI Publication 259. The Steel Construc-tion Institute, 2000, Ascot.Slimdek Design Manual. Available from Corus. www.corusconstruction.com.SteelworkDesign GuidetoBS 5950,Vol.4 Chapter5, TheSteelConstruction Institute SCI Publication P070.TheAssociationofSpecialistFireProtection.FireProtectionofStructural Steel inBuildings, 3rdedition. ASFP, 2004: www.asfp.org.uk.The Prevention of Corrosion on Structural Steelwork, Corus(former British Steel) Publication.The Steel Construction Institute. Steelwork Design Guide toBS5950Part1:2000.SectionPropertiesandMemberCapaci-ties, SCI Publication P202, 7th Edition, 2007.Trebilcock P. J. and Lawson R. M. Architectural Design in Steel.2004, Taylor & Francis.TrebilcockP.J.andLawsonR.M.BuildingsDesignUsingColdFormedSteel Sections:AnArchitectsGuide, SCIPublication130, 1993, The Steel Construction Institute, Ascot.WayA.G.andSalterP.R.IntroductiontoSteelworkDesigntoBS5950-1: 2000. The Steel Construction Institute, 2003, Ascot.YandzioE., DowlingJ. J. andNewmanG. M. Structural FireDesign:O-SiteAppliedThinFilmIntumescentCoatings,SCIPublicationP160(2ndedition). TheSteel ConstructionInsti-tute, 2004, Ascot.ICE Manual of Construction Materials # 2009 Institution of Civil Engineers www.icemanuals.com 569Structural steelChapter 46Polymeric materials: an introductionVasileios Koutsos Institute for Materials and Processes, University of Edinburgh, UKThe polymers are a diverse group of engineering materials. They are the maincomponents of plastics, rubbers, resins, adhesives and paints. These materials havedistinctive microstuctures built from macromolecular chains and networks of carbonand other light elements.IntroductionMost polymer materials are of organic composition, that istheycontain carbon,and are composedof large molecules(macromolecules)eachbuiltofmanyatoms.Theyincludematerialssuchaspolyethylene,poly(vinylchloride), poly-amide and epoxy resins. A list of polymers used in engineer-ing along withtheir standard abbreviations is presented inTables 1 and 2. These abbreviations are helpful to engineerssince they simplify the complicated and unfamiliar chemicalnamesof polymers. Foramoredetaileddescriptionanddiscussionof themolecularstructures, classicationsandnomenclature of polymers there is an abundance ofspecialisedliterature(forexample,Hall,1989,PainterandColeman 1997, Osswald and Menges, 2003).Polymers are natures materials of choice: proteins, DNAand polysaccharides are macromolecules. Natural polymersgenerally have well-dened, precise and highly complexmolecularstructures.Inengineering, mostofthepolymersused have simpler structures and are produced synthetically.Definitions, structure and typesThe molecular structure of polymers is responsible formany of the intriguing physical properties which liebehindtheirvariousapplications.Polymersarecomposedofverylargemolecules(macromolecules)whichconsistofsmaller units, calledmonomers, tightlybondedtogetherwith(strong) covalent bonds, as shownschematicallyinFigure 1 for the case of a linear polymer chain. The chemicalformula is of the type (A)n where Arepresents themonomerandtheintegernumbern, calledthedegreeofpolymerisationor polymerisationindex, is the number ofmonomers composing the chain. The length of the polymerchain (and the molar mass) is proportional to n.Polymer architecture at the molecular scale can be ratherdiverse. InFigure 2three possible molecule architecturesaredepicted, leavingouttheatomicscalechemical detailsand representing them with lines. This is a useful representa-tionof the molecular conformations of polymers inspace andis used extensively in demonstrating their microstructure.A linear polymer consists of a long linear chain of mono-mers. Abranched polymer comprises a long backbonechain with several short side-chain branches covalentlyattached. Cross-linkedpolymers have monomers of onelongorshortchaincovalentlybondedwithmonomersofanother short or long chain. Cross-linking results in athree-dimensional molecularnetwork; thewholepolymeris a giant macromolecule.Another useful classication of polymers is based on thechemical typeofthemonomers(Figure3): homopolymersconsist of monomers of thesame type; copolymers havedierent repeatingunits. Furthermore, dependingonthearrangement of the types of monomers in the polymerchain, we have the followingclassication: the dierentrepeating units are distributed randomly (randomcopo-lymer) or therearealternatingsequencesof thedierentmonomers (alternating copolymers) in block copolymerslong sequences of one monomer type are followed by longsequencesofanother type; andgraftcopolymers consistofachainmadefromonetypeof monomerwithbranchesof another type.Elastomers(alsocalledrubbers) arelightlycross-linkednetworks while thermosets are densely cross-linkednetworks. Thermosets soften mildly and ultimately degradeuponheating,whilethermoplastics,whichdonotcontaincross-links, melt uponheatingandtheycanbereshapedrepeatedly. These thermomechanical dierences betweenpolymers,owingtothesignicantlydierentorganisationatthemolecularscale,haveimportantconsequencesbothintheir processingandusage. Rubbers arecharacterisedbythepropertyofhighelasticity, i.e. elasticbehaviourathighstresses andstrains. Polymers canbe dilutedinavariety of solvents (usually organic but there are a few poly-mers calledpolyelectrolytes whichare water soluble). Asucientlydensepolymersolutioncanbecross-linkedtoform a polymer gel which is a soft solid.Polymermaterialsmayalsobeformedbyblendingtwoor more polymers intophysical mixtures. For example,theratherpoorimpact strengthof polystyreneisgreatlyimproved by incorporating small particles of an elastomer.This material, high impact polystyrene (HIPS), was an earlyexample of a polymer hybridor alloy (Figure 4). Suchpolymeralloysshowadistincttwo-phasemicrostructure,often with chemical grafting at the interface. AnotherICE Manual of Construction Materials # 2009 Institution of Civil Engineers www.icemanuals.com 571ice | manualsdoi: 10.1680/mocm.35973.0571CONTENTSIntroduction 571Definitions, structure andtypes 571Specific examples andchemical structure 573The polymer solid state 575References 577Bibliography Further reading 577importantrubber-toughenedcommoditypolymer, acrylo-nitrile-butadiene-styrene (ABS), has particles of styrene-butadiene rubber dispersed in a glassy styrene-acrylonitrile(SAN) matrix. Other well-established polymer hybrids (alsocalled blends) include polyphenylene oxide/polystyrene(PPO/PS) and poly(methyl methacrylate)/poly(vinylchloride) (PMMA/PVC).Manypropertiesofpolymericmaterialsdependonthemicroscopic arrangement of their molecules. Polymerscan have an amorphous (disordered) or semicrystallinePolymer ISOabbreviationOther customarynamesHomopolmersPolyethylene PE PolythenePolypropylene PPPolystyrene PSPolybutylene PBPoly(methylmethacrylate) PMMA Acrylic, perspex,plexiglasPolytetrafluoroethylene PTFE TeflonPoly(vinylfluoride) PVFPoly(vinylidene fluoride) PVDFPoly(vinylchloride) PVC VinylPoly(vinylidene chloride) PVDCPoly(vinylacetate) PVACPoly(vinylbutyral) PVBPoly(ethylene terephthalate) PETPPolyetheretherketone PEEKPolyacrylonitrile PANPolyethersulphone PESUPolycarbonate PCPoly(butylene terephthalate) PBTPPolyoxymethylene POM Acetal, polyacetalPolyamide PA NylonPolyacrylamide Poly(phenylene oxide) PPOPoly(phenylene sulphide) PPSEpoxy EP EpoxidePolyurethane PURNatural rubber NRPolyisoprene rubber (synthetic) IRPolychloroprene rubber CR NeopreneSilicone polymers SI PolysiloxanesCopolymers, hybrids and alloysAcrylonitrile-butadiene-styrene ABSMelamine-formaldehyde MFPhenol-formaldehyde PFUrea-formaldehyde UFUnsaturated polyester UPStyrene-acrylonitrile SANTable 1 Some engineering polymersPolymer ISOabbreviationOther customarynamesM groupChloropolyethylene rubber CMChlorosulfonylpolyethylene rubber CSMEthylene-propylene-diene rubber EPDMO groupEpichlorohydrin rubber CO Epoxide rubbersQ groupFluorosilicone rubbers FVMQSilicone rubber MQR groupAcrylonitrile-butadiene rubber NBR Nitrile rubber, Buna-NButadiene rubber BRChloroprene rubber CR Neoprene,polychloropreneIsobutene-isoprene rubber IIR ButylrubberIsoprene rubber IR PolyisopreneAcrylonitrile-isoprene rubber NIR Nitrile rubberNatural rubber NRStyrene-butadiene rubber SBRT groupPolysulphide rubbers OT, EOTU groupPolyester urethane AU PolyurethanePolyether urethane EU PolyurethaneTable 2 Some engineering polymers: elastomersAAis a monomer unitrepresents a covalent bondAAAAAAAAAAAAAFigure 1 A polymer chain(b) (c) (a)Figure 2 Types of molecular architectures: (a) linear chain; (b) branchedmolecule; (c) cross-linked network; molecules are linked through covalentbonds, the network extends over the whole sample forming a giantmacromolecule572 www.icemanuals.com ICE Manual of Construction Materials # 2009 Institution of Civil EngineersPolymers(partiallycrystalline,partiallyordered)structure(Figure5).Amorphous polymers lack order andare arrangedinarandom manner, while semicrystalline polymers are partiallyorganised in regular crystalline structures.Polymers are often mixed with inorganic particles(usuallyintheformofcontinuousbres,suchasglassorparticulates such as mica, talc and clay) in order tomodify and improve (mainly but not exclusively) theirmechanicalproperties.Reinforcementusingorganicbres(for example, kevlar (poly(paraphenylene terephthalamide)or aramid) or carbon bres) is also possible. Such compositematerials are fully described elsewhere.Thereisanotherwayofclassifyingpolymersaccordingto their application areas: (1) plastics (for structuralcomponents, packaging); (2) elastomers (for dampingorhighfriction); (3) bres (for reinforcement); (4) coatings(for protectionof materials surfaces); and(5) adhesives(for joining of structural components).Specific examples and chemicalstructurePolyethylene (PE) has the simplest molecular structurewhichisshowninFigure6.Thisstructurecorrespondstoa chemical formula of the form (CH2CH2)n. Themonomerunitisshownwithintheparenthesisandrevealsthat PEis produced fromethylene gas, CH2CH2, bybreakingthe double covalent bonds andconnectingthegas molecules consecutivelyat highpressure. For PE, ncantakevaluesfromafewhundredtohundredsofthou-sands. It has to be noted that the occurrence of sidechains or branching (Figure 2(b)) is inevitable if PEisproduced by way of the high-pressure polymerisationrouteresultinginlow-densitypolyethylene(LDPE). Thishas important consequences for the polymer microstructureandphysical properties. It canbeavoidedbyemployingcatalysts and low-pressure synthesis; in this case, thebranching is minimal and high-density polyethylene(HDPE) is attained. Higher density allows a closer packingof PE chains at the molecular scale which promotes crystal-lisation and higher stiness and strength. Some typicalapplicationsofLDPEareinpackagingandinsulationfor(a)(e)(b)(c)(d)Figure 3 (a) Homopolymer; (b) random copolymer; (c) alternatingcopolymer; (d) block copolymer; (e) graft copolymerFigure 4 Microstructure of high-impact polystyrene: rubber particleswithin a polystyrene matrix (reprinted from T. Kuboki et al.,Macromolecules #2002 American Chemical Society)(a) (b)Figure 5 (a) Amorphous polymer (observe the entanglements among thepolymer chains) and (b) simplified model of a semicrystalline polymer(observe the crystalline and amorphous domains)Figure 6 Molecular structure of linear polyethylene depicting the carbonatoms of the chain backbone covalently bonded in a linear fashion. Eachcarbon atom is also covalently linked to two hydrogen atomsICE Manual of Construction Materials # 2009 Institution of Civil Engineers www.icemanuals.com 573Polymeric materials: an introductionelectrical cables, while HDPE is used for heavy-usecontainers such as tanks, pipes and structural panels.The molecular chain structures of many commercialpolymers are shown in Figures 7, 8 and 9. Poly(vinylchloride) (PVC) andpolypropylene (PP) have molecularstructures similartothat of PE, basedonCCchainswiththeimportant dierencethatonehydrogenatominthemonomerissubstitutedbytheelementchlorine,Cl,inPVC,and by the methyl group, CH3, in PP. In the caseofpolytetrauoroethylene (PTFE) all hydrogen atoms arereplacedbytheelement uorine, F. PPisusedwidelyinwater andgas pipes, PVCinpipe, ductingandwindowframes while PTFEs low surface energy nds many appli-cations in low friction coecient applications such as bear-ings. Polystyrene (PS) has the same basic backbone but onehydrogenissubstitutedbyabulkyaromatic(orbenzene)ring(C6H6);itisaglassy,brittlepolymerwithlowtough-nessandrelativelylowsofteningtemperatureandbecauseof its easy processing its uses are widespread as acommodity plastic, for example in packaging applications.Incellular form, it is animportant insulationmaterial.Poly(methyl methacrylate) (PMMA) is another polymerwith a simple carbon backbone containing in its monomera methyl CH3 and a methacrylate group COOCH3; it is aglassy,transparent,toughmaterialwithhighresistancetooutdoor weathering; it is used extensively as a replacementfor glass in constructions. It is important to note that if themolecular structure is characterisedby a highorder byplacingthesubstituentgroupsoratomsatthesameplaceinall monomerunits, micro/mesoscalestructuresofhighordersuchascrystallinedomainsarepromoted. Thishasimportant consequences for various physical propertieswhichareimportantinapplications. Togiveanexample,if themethyl substituent inPPisattachedinanorderedfashion, isotactic PP is produced with high degree ofcrystallinity. Thisisthemost useful formof PP. Ontheotherhand,PSisusedinitsdisorderedatacticcongura-tion and it is amorphous.Polyoxymethylene (POM) and related polymers incorpo-rate oxygen within the polymer chain backbone; they havehigh strength and stiness. Polycarbonate (PC) has a morecomplicated molecular structure containing aromatic ringsalongthebackbone; it isalsoverytoughglassypolymerwithgoodmechanical properties at arangeof tempera-tures, good dimensional stability, and resistance to burning,environmental and chemical conditions. It is used widely intransparent roof panels. Poly(ethylene terephthalate)(PETP) andpoly(butyleneterephthalate) (PBTP) includetheester linkOCOinthebackbone; theyhavegoodinsulatingandpermeationproperties andre resistance,andareusedintheformofthinsheetsforinsulationandsealinginbuildings.Polyamides(PA)(nylons)containtheamide link NHCO within the backbone; they arewidelyusedpolymersinvariousengineeringapplicationsfrom bearings and gears to ropes and pipes.Polymers such as polybutadiene, polyisoprene andpolychloroprene are liquids withhighviscosityat roomtemperature (the viscosity increases with the degree of poly-merisation) and they become solids characterised byrubberybehaviourifthepolymerchainsarelightlycross-linked(cured), that is, connectedcovalentlybychemicalreaction(addingsulphurwhichiscalledvulcanisation)orbyothermeans(UVradiation, electronbeamprocessingor simply by heating). If the number of cross-links ishigh, athermoset material forms, characterisedbyrela-tivelyhighstinessandstrengthinadditiontotoughness.Thermosets are usually formed by cross-linking shortchainsorevendirectlythemonomerunitswhichcontainatleastthreebindingsites.Epoxyresins(EP)andphenol-formaldehyderesins(PF)arethermosets, andareusedinmany structural applications reinforcedby mainly glassbres. Formaldehyderesinsareusedextensivelyaswoodadhesives.Copolymersincorporatetwoormoredierenttypesofmonomers withinthe samemacromolecule. Inthis way,one can create new materials with the desired combinationof properties. Styrene-butadiene-rubber (SBR) combinesthe(exibleandmobile)liquid-likecharacterofpolybuta-dieneandthe(rigid)glassybehaviourofstyreneatroomtemperature toproduce physical cross-links betweenthePEPSPPFigure 7 Some major hydrocarbon polymersNR BRFigure 8 Some major hydrocarbon polymers: rubbers574 www.icemanuals.com ICE Manual of Construction Materials # 2009 Institution of Civil EngineersPolymerscopolymer chains by segregation of the two types of mono-mers. In this way, we produce a material which is rubber atroom temperature but it can melt at high temperature andsolidify at roomtemperature reversibly and repeatedly.Thus, unvulcanised SBR is a thermoplastic elastomer(TPE).Ethylene monomers canbe combinedwithpropylenemonomer units to produce poly(ethylene-propylene).Dependingonthe composition, the resultingcopolymercan behave from modied polyethylene to modiedpolypropylene. Asimpler andinexpensive waytotailorthedesiredpropertiesistophysicallyblendtwoormoretypes of polymers. The resulting microstructure andconsequentlythephysical properties dependcruciallyontheabilityofthepolymerstoblendhomogeneouslyatthemolecularscale. High-impactpolystyrene(HIPS)consistsof a PS matrix and dispersed polybutadiene rubber particles(Figure 4), since PS and polybutadiene are immiscible.PPO-PS and PETP-PBTP are examples of miscible blends.Polyacrylamide is a water-soluble polymer. At a solutionofonly3%inwateritcanbechemicallycross-linkedandformapolymergel whichhassomeintriguingproperties.It can behave like a viscous uid during shearing/pumpingand like a soft solid at rest. It has some uses as a chemicalgrout for soils.The polymer solid stateAbove a certain temperature all polymers soften and, in thecase of thermoplastics, are able to ow (they become a melt).Atthisraisedtemperature(forcommonplasticsusuallyinthe range of 1002508C) the polymers have liquid-likeorder,i.e.theyaredisorderedandinthemeltstate.Asthetemperature drops, their density increases, i.e. specicvolumedecreases(Figure10). Dependingonthepolymer,there are two possibilities: (1) polymers with irregularmolecular structure (atactic PS, atactic PP, PMMA) solidifykeepingtheirdisorderedmicrostructureandformingastibut brittle amorphous solid called polymer glass; the specicvolume-temperatureslope(Figure10)changesinacontin-uous fashionat thespecictemperatureof thetransitionandforthisreasonitiscalledglasstransitiontemperature,Tg; (2) polymers which have a regular structure at themolecular scale(PE, PEO, isotacticPP, isotacticPS, PA,PTFE, PETP) crystallise (partially) forming a semi-crystallinematerial inanabrupt manner (Figure10) at acharacteristic temperature called crystallisation temperature,Tc. Forpolymers(unlikecrystalsof small molecules), thecrystallisationtemperature might dier fromthe meltingtemperature, Tm, byseveral degrees. UsuallyTc< Tm, aneectcalledundercooling.Furthermore,bothtemperaturesdepend on therateofcooling/heating.Thisis becauselongpolymer chains, unlike small molecules, have decreasedexibility and are easily trapped in kinetically arrestedstates.Itisdicult(orevenimpossible)forthemtoattaintheir absolute thermodynamic equilibriumstate of fullcrystallinity. Highdegrees of crystallinitycanbeattainedinthecaseof veryslowcoolingrates. If theyarecooledabruptlyenough, theyfreezeinthefullydisorderedstateforming a polymer glass.PVCPTFEPVDCPOMPANPMMAFigure 9 Some halopolymers and heterochainsCrystalMeltTgTcTemperatureSpecific volumeGlassFigure 10 Cooling down from the melt state to glass or to crystal. Notethe relatively smooth transition to glass compared to the abruptcrystallisationICE Manual of Construction Materials # 2009 Institution of Civil Engineers www.icemanuals.com 575Polymeric materials: an introductionAtypicalcrystallinityofLDPEisintheorderof50%,while HDPEcanbe upto90%crystalline. The crystalgrains (crystallites)inpolymerstakearoughlysphericalshape (Figure 11) and they are called spherulites. Howeverithastobenotedthat,unlikegrainsinmetals,spherulitescontain both crystalline and amorphous domains organisedin a radial lamellar structures (Figure 12).The crystallites formandgrowwithinthe melt stateduring cooling down and their morphology dependsstronglyonprocessinghistorysuchastherateofcooling.Themorphology,andconsequentlytheprocessing,aectssignicantlythephysical properties of thenal material.Within spherulites, a typical polymer chain belongs toboth crystalline and amorphous domains connectingecientlythe whole structure. Furthermore, inpartiallycrystalline polymers, the crystallites play the role ofphysical cross-links withinanamorphous matrix. If thetemperature of use is below Tmand above Tg, the materialis tough with a leather-like character.Thecrystallinestateischaracterised byhigherdensities,and improved mechanical properties. Semi-crystalline poly-mers capable of a high degree of crystallinity (PE, PP, PA)areusedfortheproductionofbreswhichcontainhighlyorientedcrystallinedomains inthedirectionof thebreaxis by cold drawing. The axial elastic modulus andstrengthof these bres are signicantly improvedsincethestressisopposedbythestrongprimarybondsofthemacromolecular backbone. Themechanical properties ofPVCpipesarealsoenhancedinasimilarway. Aftertheextrusionprocessthepipesareradiallyexpandedinordertoenhancetheorientationofpolymerchainsinthehoopdirection.Thermoset polymers andgels lackorganisationat allscales and are inherently amorphous. However, elastomersarecapableofhighstrainbehaviourwhichinduceschainorientationandalignment; aprocess whichcanresult inFigure 11 (Left) polarised and (right) non-polarised optical micrograph of atactic/isotactic PP blend spherulites (reprinted from E. M. Woo et al., Polymer#2007 Elsevier)Crystalline domain Amorphous domainChain foldInterlamellar linkFigure 12 Spherulite organisation576 www.icemanuals.com ICE Manual of Construction Materials # 2009 Institution of Civil EngineersPolymerstemporarycrystal formationwithconsequences ontheirphysical properties. Thusmanyelastomersbecomestierat high extensions.ReferencesHall C. Polymer Materials an Introduction for Technologists andScientists, 2nd edition, 1989, New York, Halsted.Kuboki T., JarP.-Y. B., Takashi K. andShinmuraT. Macro-molecules, 2002, 35, 35843591.Osswald T. A. and Menges G. Materials Science of Polymers forEngineers, 2003, 2nd edition, Munich: Hanser.Painter P. C. andColemanM. M. Fundamentals of PolymerScience, 1997, 2nd edition, Lancaster, PA: Technomic.Woo E. M., Cheng K. Y., Chen Y.-F. and Su C. C. Polymer, 2007,48, 57535766.Bibliography Further readingBirleyA. W., HaworthB. andBatchelorJ. PhysicsofPlastics:Processing, Properties and Materials Engineering, 1992,Munich: Hanser Gardner.CallisterW.D.MaterialsScienceandEngineering:AnIntroduc-tion, 2007, 7th edition, New York: Wiley.Ebewele R. O. Polymer Science and Technology, 2000, BocaRaton, FL: CRC Press.Ehrenstein G. W. Polymeric Materials: Structure, Properties,Applications, 2001, Munich: Hanser.Fried J. Polymer Science and Technology, 2003, 2nd edition, UpperSaddle River, NJ: Prentice Hall.HaddadY. M. Viscoelasticity of Engineering Materials, 1995,London: Chapman and Hall.ISO1043. Plastics Symbols andAbbreviatedTerms Part 1:Basic Polymers and Their Special Characteristics, 2001.ISO1629. Rubbers and Latices Nomenclature, 1995.ISO18064. Thermoplastic Elastomers Nomenclature andAbbreviated Terms, 2005.McCrumN.