Manufacturing Engineering Technology in SI Units, 6th EditionPART I: Fundamental of Materials Their Behavior and Manufacturing Properties Presentation slide for courses, classes, lectures et al. Copyright 2010 Pearson Education South Asia Pte Ltd PART I: Fundamental of Materials Their Behavior and Manufacturing Properties
Materials were selected because they possess desiredproperties and characteristics for the intended functions Copyright 2010 Pearson Education South Asia Pte Ltd PART I: Fundamental of Materials Their Behavior and Manufacturing Properties
Task of engineers becomes very challenging when ever- increasing variety of materials are now available Copyright 2010 Pearson Education South Asia Pte Ltd PART I: Fundamental of Materials Their Behavior and Manufacturing Properties
Material behavior, properties and characteristics will helpthe engineer understand their relevance to themanufacturing processes Copyright 2010 Pearson Education South Asia Pte Ltd Manufacturing Engineering Technology in SI Units, 6th EditionChapter 1: The Structure of Metals
Presentation slide for courses, classes, lectures et al. Copyright 2010 Pearson Education South Asia Pte Ltd Copyright 2010 Pearson Education South Asia Pte Ltd 5 Chapter Outline Introduction Types of Atomic Bonds
The Crystal Structure of Metals Deformation and Strength of Single Crystals Grains and Grain Boundaries Plastic Deformation of Polycrystalline Metals Recovery, Recrystallization, and Grain Growth Cold, Warm, and Hot Working Copyright 2010 Pearson Education South Asia Pte Ltd Introduction Different metals behave differently under differentsituations We would need to study the atomic structure of metals,which is the arrangement of the atoms within the metals This allows us to predict and evaluate properties, thuswe can make appropriate selections of metals Copyright 2010 Pearson Education South Asia Pte Ltd Types of Atomic Bonds All matter is made up of atoms containing a nucleus ofprotons and neutrons and surrounding orbits ofelectrons Atom is called an ion Excess of electrons results in a negatively chargedatom, called an anion Too few electrons results in a positively charged atom,called a cation Multiple atoms combine to form molecules Molecules held together by bonds through electroninteraction Copyright 2010 Pearson Education South Asia Pte Ltd Types of Atomic Bonds Basic types of atomic attraction are primary or strongbonds Ionic Bond When one or more electrons are transferred from onematerial to another, a strong attractive force developsbetween the two ions Example is sodium (Na) and chlorine (Cl) in table salt Copyright 2010 Pearson Education South Asia Pte Ltd Types of Atomic Bonds Covalent Bond
Electrons in outer orbits are shared by atoms to formmolecules Examples are water (H2O) and nitrogen gas (N2) Metallic bonds Metals have few electrons in their outer orbits, thuscannot complete the outer shell of other self-matedatoms Copyright 2010 Pearson Education South Asia Pte Ltd The Crystal Structure of Metals
Metals solidify from a molten state and the atomsarrange themselves into crystals Atomic arrangement is called crystal structure orcrystalline structure Smallest group of atoms showing the lattice structureis known as a unit cell 3 basic atomic arrangements in metals: Body-centered cubic (bcc) Face-centered cubic (fcc) Hexagonal close-packed (hcp) Copyright 2010 Pearson Education South Asia Pte Ltd The Crystal Structure of Metals
Distance between the atoms is on the order of 0.1 nm Models shown are known as hard-ball or hard-spheremodels Single crystal with many unit cells Hard-ball model Unit cell BCC Structure Copyright 2010 Pearson Education South Asia Pte Ltd The Crystal Structure of Metals
FCC Structure Single crystal with many unit cells Hard-ball model Unit cell HCP Structure Single crystal with many unit cells Unit cell Copyright 2010 Pearson Education South Asia Pte Ltd The Crystal Structure of Metals
HCP crystals have the most densely packedconfigurations, followed by fcc and bcc Arrangements can be modified by adding atoms ofother metals known as alloying BCC Structure Single crystal with many unit cells unit cell Copyright 2010 Pearson Education South Asia Pte Ltd Deformation and Strength of Single Crystals
When a single crystal is subjected to an external force,it returns to its original shape when the force isremoved (elastic deformation) When force increased, the crystal does not return to itsoriginal shape when the force is removed (plasticdeformation or permanent deformation) 2 basic mechanisms: Slipping Twinning Copyright 2010 Pearson Education South Asia Pte Ltd Deformation and Strength of Single Crystals
Slipping is where one plane of atoms slide over anadjacent plane (slip plane) under a shear stress A single crystal exhibits different properties when testedin different directions is called anisotropy Twinning is where a portion of the crystal forms amirror image of itself across the plane of twinning Slip System Combination of a slip plane and its direction of slip isknown as a slip system Metals with 5 or more slip systems are ductile Copyright 2010 Pearson Education South Asia Pte Ltd Deformation and Strength of Single Crystals
Slip System Bcc crystal has 48 slip systems, high b/a ratio, high shear stress, good strength and moderate ductility Fcc crystal has 12 slip systems, low b/a ratio, low shear stress, moderate strength and good ductility Hcp crystal has 3 slip systems, low slip, brittle at room temperature Copyright 2010 Pearson Education South Asia Pte Ltd Deformation and Strength of Single Crystals: Imperfections in the Crystal Structure of Metals
Actual strength of metals is one to two orders ofmagnitude lower than theoretical calculations Discrepancy is due to defects and imperfections inthe crystal structure They are categorized as: Point defects Linear defects Planar imperfections Volume imperfections Copyright 2010 Pearson Education South Asia Pte Ltd Deformation and Strength of Single Crystals:Imperfections in the Crystal Structure of Metals
Types of defects in a single-crystal lattice Types of dislocations in a single crystal Edge dislocation Screw dislocation Copyright 2010 Pearson Education South Asia Pte Ltd Deformation and Strength of Single Crystals: Imperfections in the Crystal Structure of Metals
Dislocations Dislocations are defects in the orderly arrangement of ametals atomic structure 2 types of dislocations: edge and screw Edge dislocation is the progress of an earthworm Screw dislocations are due to atomic planes forming aspiral ramp Copyright 2010 Pearson Education South Asia Pte Ltd Deformation and Strength of Single Crystals:Work Hardening (Strain Hardening)
Dislocations can: Become entangled and interfere with each other Be impeded by barriers Increase in the strength and the hardness of the metalis known as work hardening or strain hardening Work hardening is used extensively for strengthening inmetalworking processes at ambient temperatures Copyright 2010 Pearson Education South Asia Pte Ltd Grains and Grain Boundaries
When molten metal solidify, crystals begin to form atvarious locations and have random orientations These crystals then grows into a crystalline structure orgrain Number and size of the grains depends on the rate atwhich nucleation takes place Surfaces that separate individual grains are calledgrain boundaries Copyright 2010 Pearson Education South Asia Pte Ltd Grains and Grain Boundaries: Grain Size
Grain size influences the mechanical properties ofmetals Grain size number, n, is related by N = number of grains Copyright 2010 Pearson Education South Asia Pte Ltd Grains and Grain Boundaries: Grain Size
Example 1.1 Assume that the ball of a ballpoint pen is 1 mm in diameterand has an ASTM grain size of 10. Calculate the numberof grains in the ball. Solution The volume of the 1-mm-diameter ball is Total number of grains is Copyright 2010 Pearson Education South Asia Pte Ltd Grains and Grain Boundaries: Influence of Grain Boundaries
Grain boundaries influence the strength and ductility ofmetals as they interfere with the movement ofdislocations Depend on temperature, deformation rate, and the typeand amount of impurities present along the grainboundaries Creep is the elongation under stress over time, usuallyat elevated temperatures Results from grain-boundary sliding Copyright 2010 Pearson Education South Asia Pte Ltd Grains and Grain Boundaries: Influence of Grain Boundaries
Grain-boundary embrittlement: When exposed to certain low-melting-point metals Liquid-metal embrittlement: Embrittling element is in a liquid state Solid-metal embrittlement: Embrittlement occur at temperatures below the meltingpoint of the embrittling element Hot shortness is caused by local melting of aconstituent or of an impurity in the grain boundary at atemperature below the melting point of the metal itself Copyright 2010 Pearson Education South Asia Pte Ltd Plastic Deformation of Polycrystalline Metals
When a polycrystalline metal with uniform equiaxedgrains is subjected to plastic deformation at roomtemperature, the grains become deformed andelongated During plastic deformation, the grain boundariesremain intact and mass continuity is maintained Increase in strength depends on the degree ofdeformation (strain) Copyright 2010 Pearson Education South Asia Pte Ltd Plastic Deformation of Polycrystalline Metals
Anisotropy (Texture) A result of plastic deformation Grains have elongated in one direction and contractedin the other Metal has become anisotropic, where properties in thevertical direction are different from those in thehorizontal direction Anisotropy influences both mechanical and physical properties of metals Copyright 2010 Pearson Education South Asia Pte Ltd Plastic Deformation of Polycrystalline Metals
Preferred Orientation Also called crystallographic anisotropy When metal is subjected to tension, the sliding blocksrotate toward the direction of the tensile force Slip planes and bands tend to align themselves with thegeneral direction of deformation Mechanical Fibering Results from the alignment of inclusions (stringers),impurities, and voids in the metal during deformation Impurities will weaken the grain boundaries andbecome less ductile when tested in the verticaldirection Copyright 2010 Pearson Education South Asia Pte Ltd Recovery, Recrystallization, and Grain Growth
Properties of the metal can be recovered by heating themetal to a specific temperature range for a given periodof time A process called annealing Copyright 2010 Pearson Education South Asia Pte Ltd Recovery, Recrystallization, and Grain Growth
3 events take place consecutively during the heatingprocess: Recovery:Occurs below recrystallization temperature, stressesin the highly deformed regions are relieved Recrystallization: Within a certain temperature range, new equiaxed andstrain-free grains are formed to replace older grains Grain growth: Grains begin to grow in size and exceed the originalgrain size when temperature is raised further Copyright 2010 Pearson Education South Asia Pte Ltd Cold, Warm, and Hot Working
Cold working is plastic deformation that is carried outat room temperature Hot working is when deformation occurs above therecrystallization temperature Warm working is carried out at intermediatetemperatures Copyright 2010 Pearson Education South Asia Pte Ltd