Metal matrix composite materials

L. D. BLACKBURN, F. R. BONANNO, H. M. BURTE,J. A. HERZOG and VV. F. STUHRKE

COMPOSITF materials, especially metal matrix com-posite materials are possible substitutes for metalsin use today. However, because they can possess

extremely high strengths, moduli and excellent hightemperature properties they may be employed mostprofitably in applications which do not presently exist,since no present material has the required capabilities.In addition, their present high cost and very early stageof development make them unlikely candidates for mate-rials substitution in the context of this meeting. Thispaper is oriented more towards the development ofcomposite materials for aerospace structures.

composite, and prediction of composite properties fromthe properties of the components and the geometry ofthe microstructure. Comprehensive reviews of theliterature on filament-metal matrix composites haverecently appeared."

The simple concepts of reinforcement in unidirectionalcomposites have been fairly well established, Thus,the work of McDanels, Jech, and Weeton3 in Fig. Ishows that the modulus of a copper matrix-continuoustungsten wire composite can he predicted by a linear ruleof mixtures.

Ultimately composite materials, especially metal E , = E,V,1 E,,,( l-V,)matrix composites may become vital materials in theindustrial growth of a nation. The employment ofcomposite technology may permit the large scale substitu-tion of aluminium for the more scarce metal copper inhigh voltage electrical transmission. The improvedstrength and strength retention at temperature of axiallyreinforced aluminium composites would permit significantincreases in the power carrying capability of the alu-minum due to the high permissible operating tempera-tures. In addition, other significant savings would befound in the ability of the composite to support its weightover longer distances due to higher tensile strength andimproved creep resistance. This could reduce the numberof support structures by more than a factor of two.This brief example is only one of the potential ways inwhich these new metal matrix composite materials canultimately provide a better life for us all.

Glass filament-polymeric matrix composites alreadyhave become familiar materials in uses ranging fromlight-weight luggage to high performance solid propel-lant rocket motor cases. D uring the last decade therehave been a few developments-oriented attempts to in-corporate glass or metal wire filaments in metal matrices,but these did not generate any significant practicalenthusiasm. Some work with such filament-matrixcombinations, particularly during the last few years,has been very valuable from the viewpoint of modelsystem information to stimulate and verify theoreticalmicromechanics. The latter involves analysis of stressand strain distribution between components of the

Lt. L. 1). Blackurn, Major F. R. Bonanno, Mr H. M. Burte,Mr J. A. Herzog and Lt. W. F. Stuhrke, AF Materials Labo-ratory, Wright .Patterson AFB, Ohio, USA.

Where E is modulus, V is volume fraction of a compo-nent, and the subscripts c, f, and m refer to composite,filament and matrix, respectively. In a composite con-taining discontinuous filaments, shear within the matrix isnecessary to transfer the load to and between the fila-ments. The work of Sutton and Chortle," for example,shown in Fig. 2 for the model system of AI,O, whiskersin silver demonstrates that reinforcement can occur.The shear strength of the matrix and of the bonddeveloped between the filament and the matrix in thesespecimens is sufficient to permit most of the strengthof the filaments to be developed in the composite. Again,a simple linear rule of mixtures provides a relativelygood fit of these data. Some preliminary work from ourown laboratory, such as on the boron filament-metalmatrix composites shown in Fig. 3 also demonstratesthat significant reinforcement can occur. For 7.5 volumeper cent filament the strength and modulus of this speci-men were increased by 17 per cent and 27 per cent respec-tively over that of the unreinforced matrix.

Interest in metal matrix composites received its mostsignificant stimulus from the emergence of a variety ofnew, high modulus, high strength, low density and oftenrefractory filamentary materials. Table I shows theproperties of a few of these and of some typical glass andmetal filaments. To a first approximation, the distributionof stress between the components of a composite will bein proportion to their moduli of elasticity. Thus, sincethe moduli of some of these filaments are significantlygreater than those of potential metal matrices such asaluminum, titanium, or even steel and nickel, they canact as efficient reinforcements, Since, in addition, their

43

Blackburn et. al.: Metal matrix composite inalerlals

0 20 4U 6o &U 100

I

1 Dvnarnic modules of elasticity of tungsten, copper, andcopper reinforced with tungsten wires (McDANELS ETAL (4) )

3 Macrostructure of alcunituuin-boron composite )chich

exhibits fiber reinforcement x 25

densities are low, improved strength to weight ratios arepotentially attainable in metal matrix composites withthese filaments. The availability of it variety of these ascontinuous filaments, and the promise of more such ashigh modulus, high strength carbon filaments, has led to amajor acceleration of activity and interest in metal matrixcomposites.

TABLE I Filament properties

400

W

in0

0

200

0 IC ?p 5.0 40

VOLUME PERCENT WHISKERS (uAI2031

2 Tensile strength of silver-alumina whisker composites ( Suttonand Chorne (7) )

MaterialTensilestrength,

Elasticmodulus,

Density,lbs. in.J

Strength/Density,

psi psi in.

Beryllium 180 45 > 106 (1066 2'73 x 108Molybdenum 320 48 „ 0.369 0.88 „Steel 600 29 „ 0-283 2.12

Tungsten 580 59 „ 06,97 0'83F-Glass 350 l0 .. 4092 3.81

Silica 500 l0 (1079 6.35Boron 400 55 „ 0083 4'82

Carbon 200 25 0060 2'17

Silicon carbide 400 70 „ 0.142 2.79

Alumina whiskers 2000 75 „ 0.143 24.50Graphite whiskers 350 45 „ 0 060 5 84Silicon carbidewhiskers 1500 123 .. 0-115 13007irconia whiskers 126 26 ., 0.200 0.63

The current activity in this field in our own laboratoryand in the United States in general can be divided intothe following major categories

44

BIarkburer et. a!. r Metal matrix eontposile materials

( a) Development and characterization of filaments.

(b) Analysis of the micromechanics of composites

(c)

composite preparation and use.

i.e., of the detailed nature of the stress and straindistribution among the components of a compositeas a function of external loading.

Study of filament-matrix interactions, includingthe nature of the interfaces and the reactivity orstability of the components (chemical reaction,dissolution, etc.) towards each other during both

(d) Preparationbehaviourcomposites.

of and characterization of theof a wide variety of filament-matrix

(e) Comparison of composite properties with therequirements of potential uses.

(f) Development of test and analysis methods bywhich suitable materials properties may be cor-related with and used to predict componentperformance.

Analysis of possible future uses for metal matrixcomposites will depend upon properties and behaviourwhich are attainable. These. in turn, can be varied veryextensively as a result of the highly anisotropic contri-bution of the filaments to overall properties and theopportunity to control the geometry of filament locationwith respect to the external loading imposed on itstructural component.

We shall outline first the possible directions of develop-nient interest for metal matrix composites followed by ananalysis of the status of theory for, and experimentalverification of their inicromechanics. We shall thendiscuss and illustrate several topics of current experi-mental interest and present techniques and results in thecharacterization of some of the new filaments and theinteractions Which occur in metal matrices.

Potential for metal matrix composites

Analysis of the potential for future development and useof any new materials must be done within a frameworkof competition both with other possible materials andmaterials development routes, as well as with the varietyof different ways in which a designer may use materialsto perform it function. Initially, the properties of nmate-rials are compared. Eventually, of course, the cost-effectiveness of competing combinations of materials,fabrication processes and designs should (lictate actualselection and use.

To correctly assess future potential, comparisonshould be with appropriate existing and possible futurecompetition. Thus, it metal matrix composite for use at600"F should be compared to titanium alloys, notaluminium, even if the matrix is aluminium. Further, thecomparison should consider not only the materials them-selves but also the most efficient ways of using materials ;i.e., not only a sheet-stringer structure, but also a honey-

Al 20.1 WHISKERS ( FRCM REFERENCE 22)

s CLASS )(FROM REFERENCE 2I)

E OIASS ,

bUKUN )! (FMOM REFkRENCL 24)

SiC

M. WINS ( FROM NEFERENCE 25)

Tt MPI HAILRF •F

4 / filament sire m t!r as a fjuuetion of temperature

comb structure of titanium when this would be moreefficient.

Two points of view will now be delined for a discussionof future potential. One extreme considers metal bondedfilaments. the other considers reinforced metals. Actualfuture development will. of course, involve considerationsbetween these extremes, but knowledge of the behaviourof metal matrix composites is not yet sufficient to warranta much more refined analysis. Consider first the metalbonded filament approach which focuses attention onthe extreme strength or modulus to density ratios availa-ble in filaments, and attempts to devise ways of efficientlyusing these in lightweight structures. This might beconsidered a direct outgrowth of existing polymerbonded glass filament composite technology. The poten-tial for metal bonded filaments is best evaluated byconsidering the reasons, it' any, for a metal matrix ratherthan a polymer.

The most obvious potential advantage of a metalmatrix is in resistance to long time exposure to manysevere environments. Temperature will he used to illu-strate ; advantages in resistance to chemical attack anderosion, either singly or in combination s\ith eleva-ted temperature, are also probable. Fig. 4 shows thestrength of some currently available filaments as afunction of temperature. Very attractive strength re-tention suggests combination with appropriate matricesto provide high temperature composites. Specific com-parisons will depend on the nature of the application,the loading, the time at temperature and other factors,but it is reasonable to assume that metal matrices mayprovide significant competition to polymer matrices attemperatures above about 250` F. Other possibleadvantages of metal matrices can be derived by con-sidering some of the factors which currently preventusing the unidirectional strength to weight ratio offilaments as a direct indication of the weight of the

45

Blackburn et. al . : Metal matrix composite materials

TABLE' 11 Weight comparison of rocket motor cases (12)

MaterialStrength, Density, Strength Casepsi lb./in' Density, weight,

in. lb.

Steel Alloy 200x 103 0283 0.71 x 10, 536

Titanium Alloy 160 „ 0-160 1 -04) „ 321

E. Glass Filament 350 „ 0.092 3.81 „ 301

S 994-Glass Filament 437 „ 0088 4-97 254

structural elements made from them and polymermatrices. Table lI is derived from a recent study'into the optimization of polymer matrix-glass filamentwound solid propellant rocket motor cases. For aspecific application the weight of optimized filamentwound cases is compared to that for cases fabricatedfrom metals with the design allowables shown. Notethat the enormous increase in strength-to-density ratiobetween the "homogeneous" metals and the filamentsis not reflected in a comparable decrease in theweights of final cases. Some of the reasons for thisare as follows :

(a) The composite is, of course, not 100% filament.These particular examples contained 18'8% weight% epoxy. Although the polymer matrix performsa necessary function in permitting the strength ofthe filaments to be utilized, it is generally assumedthat it does not itself provide a significant additionto either the strength or elastic modulus of thecomposite.

(b) The filaments are assumed to be effective onlyalong their axes. If the application involves amultiaxial stress field, additional filaments mustbe added to bear the load.

c) Additional reinforcement must be added to thefilament wound structures to cope with stiffnessrequirements and for cutouts, attachments, stressconcentrations, i.e. at any point where stressestransverse to the filaments or shear loads occuror where the continuity of the filaments is dis-turbed or both.

In addition, a metal can offer very significantly greatertensile, shear and hearing strength as well as greaterdeformation prior to fracture than current polymermatrices. It can itself provide meaningful contributionto the elastic modulus of the composite in all directions.Also, if the mechanism by which the composite fracturesinvolves a statistical accumulation of individual fiberbreaks, a metal matrix may delay the onset of finalcatastrophic failure and thus increase the strength.

The reinforced metal approach focusses attention onthe metallic matrix and the possibility of usefully im-proving its strength by the addition of' filaments. In theextreme the addition of filaments might be consideredto be comparable to any other strengthening mechanismsuch as cold 'working or precipitation hardening. This

extreme would offer very real advantages. Existingtechnology for the reliable and effective design, fabrica-tion and use of metals would be applicable. and newmaterials could find early acceptance and use. However,such an uncomplicated possibility is unlikely, andattention must he directed towards determining thenature and extent of differences between fiber reinforcedand conventionally strengthened metals.

Finally, some comments relative to the use of variousclasses of filaments with metal matrices are appropriate.If appropriate strength or modulus to density ratios ofthe filaments alone are compared, metal wires are notattractive relative to glass, boron, silicon carbide, etc.filaments. High strength beryllium wires would be asingular exception. Thus, there is as yet little basis forthe use of metal wires in the metal bonded filamentapproach. For reinforced metals, the ahilit', of a metalwire to show plastic yielding, as contrasted to the essen-tially elastic behaviour of most other filaments, mightbe of some value in permitting deformation processingof the composite without unacceptable breakdown ofthe filaments, or in contributing to fatigue and impactresistance. Glass filaments do not have higher modulithan potential matrix metals such as aluminium or tita-nium. Ho\Never, if the metal tnatri\ is allowed to yieldplastically, its effective modulus to use in determining themicrostress distribution within the composite will by thetangent modulus and significant reinforcement by glassfilaments can be demonstrated. The glass filaments mayalso influence crack propagation resistance.' Whetherthis implies advantages for glass. relative to the use ofhigher modulus filaments, is not yet clear.

High modulus filaments are currently of prime interestfor use in reinforcing metals. Continuous filamentssuch as boron or silicon carbide will he applicable toeither of the development approaches discussed above.Short filaments such as whiskers will initially he of in-terest primarily in a reinforcement approach. Highervolume percentages of such discontinuous filamentsmight also eventually he approached in a variety of wayssuch as forming :t whisker-metal matrix wire for usein filament winding.

Microinechanics of metal matrix composites

Most of the past theoretical and experimental analysisof the mechanical behaviour of composites has beendeveloped within the framework of polymer matrices.Some of these results are sufficiently general to apply tometal matrices in the low range of stress where bothfilament and matrix remain elastic. Beyond this range ofstress it fundamental difference appears between metalsand polymers. As the load increases, the elastic limit ofthe metal will he exceeded at local areas of' high stressconcentration and plastic flow will occur, accommodatingthe stress distribution at these locations. Further aspectsof metal composite behaviour %vere aptly indicated byMcDanels, Jech, and Wecton'` in their delineationof the stages of deformation as: (a1 elastic deformationof both filament and matrix, b elastic deformationof filament and plastic deformation of matrix, (c) plasticdeformation of both filament and matrix, and (d) fracture.

46

Blackburn er. al.: Aleta! n+atrix composite materials

When one considers structural applications, there are avariety of properties which must he known before anydesigner will attempt to use any material. Tsai." hasshown that in order to characterize the mechanical be-haviour of a composite material one must know thelongitudinal strength, modulus and Poisson's ratio, thetransverse strength. modulus and Poisson's ratio, andthe shear strength and modulus. Relationships betweenthe elastic constants of the composite and those of theindividual constituents have been developed by severalworkers.-" In general. the ability to predict longitu-dinal strength and modulus under uniaxial tensile loadsis fairly well advanced. Further work is needed to developthe ability to predict more accurately the transverse andshear strengths and moduli, and Poisson's ratio. All ofthese will include studies of the effects of such micro-mechanical variables as filament size, packing density,interfacial bond strength, and the materials propertiesof both constituents.

An exact analysis of the micromechanics of a com-posite containing discontinuous filaments such as whis-kers or chopped fibres does not exist. !Much of whatis known has been summarized in recent reviews.'-',' In-dications are that the flow of stress or transfer of loadin such a composite is along shear paths from the cir-cumferential surface of one Iilamment through the matrixto another filament. It is obvious that this mechanismis strongly influenced by the fibre-matrix interface.

The status of knowledge relating to creep and fatiguehas been succintly indicated by Kelly and Davies' :"As yet there are no satisfactory analytical treatmentswhich allow prediction of the creep and fatigue behaviourof fibre reinforced composites." This applies equallywell to other structural design limiting parameters suchas stress rupture, fracture toughness, ductility and yieldstrength. Some probing experimental efforts, notablythose of Forsyth et al,'" Baker and Cratchley," andParikh,17,1" have been initiated in some of these areas.The effort of Forsyth and co-workers is aimed atimproving creep, fatigue and crack propagation behaviourof aluminium rather than strengthening the metal.Results to date have shown that incorporation of stain-less steel wires in aluminium sheets led to improvedcreep resistance and fatigue life.

The role of the interface in the mechanical behaviour ofcomposites has yet to be treated satisfactorily but it hasnot been overlooked. Baker and Cratchley" in someearly observations of silica reinforced aluminium underfatigue loading have indicated differences in behaviourbetween composites having weak and strong boundaries.Specimens which had strong interfaces showed bettertensile behaviour than those with weak interfaces. Theinverse was true for fatigue specimens, i.e. the weakinterfaces gave better behaviour.

Recently Parikh'7 has been investigating the influenceof critical transfer length, fiber diameter and interfiberspacing on the strength, deformation and fracturecharacteristics of metal wire-metal matrix composites.As shown in Figure 5, his preliminary results revealthat the ratio of the critical transfer length, as me-asured by pull-out tests, to the interfiber spacing affectsboth the tensile strength and the type of farcture.

300

IUu

^'. tacit.

1.0C r i t i c a l Tr_9n f ennui

Int.rflb . r pacing

20

5 EIteci of critical transfer length and interfiber spacing onJ;actnre and ttipe of Jractare in steel wire-lead solder compo-sites (Parikh 17)

A ductile or fiber "pull-out" type of fracture ischaracterized by failure in the matrix progressing alongthe composite. Brittle fracture is characterized by grosscontinuity in the filament and then proceeding acrossfailure of the filaments, and hence the composite, intension. Ductile fracture is accompanied by a highvalue of the work of fracture, and thus toughness ;while the brittle fracture involves low fracture energy.Here one observes again preliminary indications of theimportant role of the filament-matrix interface, sinceone way of controlling the critical transfer length andthus the type of fracture is by controlling the natureof the interface.

Filament characterization

Some of the methods we have developed for examin-ing the filaments and measuring their mechanical pro-perties will now be described. Observation of the fiberunder the light microscope should be directed notonly at the nature of the surfaces, Figure 6, but alsoto search for any fractures or fissures. Figure 7. Me-tallographic studies of cross sections and of sectionsparallel to the filament provide additional information.Our observations on boron filaments, grown by vapourdeposition of boron on a tungsten wire substrate, maybe selected as typical illustration of this procedureand of' features for which to look.

Optical surface observation of the as-received fila-ment reveals both a "corn cob" structure, Figure 6(al, and often contamination of the surface by oil films,grease and other substances, Figure 8. Further exa-mination of the boron filament often reveals extendedstraight dark lines on the surface, Figure 9. Whensuch a filament is cross sectioned, these can be iden-tified as radially oriented cracks or fissures, Figure7. Longitudinally sectioned specimens show the facesor walls of these fissures ; since they do not exhibit

of of fibers

47

Blnekhurn et_ cif. : Alf-t( / trrarrix c )mpo+ire nrurerio/s

(u) Borotz Jilanu'ut(Afauu /ircturer A) f 0110

(h) Rorun Jiltrnneut (AlunuJuetm-er B) x 1 000

(c) Boron filarneni coating --SiCx1000

(d) Boron filament coating +Ti

(e) Boron Jilatnctzr coarinc- f Tr;,',

If) Si/icon rrul» r%o Jilauuvri

X 1000

x 101)o

(e) Con enrional gla filament niih ^urfarePrortr,rting oil film x I H10

(h) (. intentional class J . Ianiint cIIarz strlarexi 10)

6. Various filaments with and wick. ut surface coutings

48

Blackburn el . rd. ' Mcle / mal ix r•or1PosiIt runler icrls

7 C'rn;s sectioned filament with radial fisrtires

8 curfaee contamination with oil or grease filet; on boronfilata nt x 1500

9 Longitudinal fissure on the surface

10 Vie ii of longit udinal forced cruel, in a born filament withconch oidal fracture lines x 300

any conchoidal fracture lines as commonly seen onforcefully split filaments , Figure 10 , the fissures pro-bably occurred during vapour deposition of theboron.

In some instances these fissures cannot be observed35o by optical scanning since they are overgrown and thus

self-sealed at the surface, Figure 11. If the fissures. areopen to the surface they may acquire contaminants bycondensation of vapours or capillary adsorption of liquids.This is illustrated by the globules of liquid seen onthe face of the fissure (after the filament was split)in Figure 12. Such contamination may interfere witheffective filament-matrix bonding.

Since it is the mechanical properties of the filamentswhich initially generate interest in their use in com-posite. these properties should he carefully measured.The difficult case of a fine, short whisker and the-mea-surement of its tensile strength and its modulus ofelasticity will be considered. Testing of such a smallspecimen requires observation of several essential factorswhich are of vital importance for reliable data. Theseare : the centricity of load application, i.e., eliminationof moments the exact measurement of elongations for

11 View of the face of a fissure in a boron fdcunent X 800

49

Blackh,nrt et. al.: Meta/ n,atri.r composite maleriais

12 View of the face of 'a fissure in the boron filament-one sideself-sealed at the surface

the determination of elastic behaviour , the measurementof the cross sectional area. Equally important is theaccuracy of measurement of' load on the specimen dur-ing the test . For the sake of convenience , with fineshort fibers or whiskers elongation is often measuredas separation between the grips. However , this con-venient approach often results in considerable error dueto deformation which can occur in the cement in the grips.Therefore . it is often desirable to use some type of gageon the filament itself, and we have been cementing gagemarks on the fiber specimen using microntanipulators,Figure 13. The gage marks are actually other whiskersand are cemented perpendicular to the fiber to be testedat a specific distance from each other . This distance ismeasured immediately before the tensile test of the

(n) Exploded riesr

(b) Asvemhlerl rieu ' suit/t mire leaps and whisker

14 1lo „ntiug block for whisker %pecin)ens

specimen on a microscope. Wire loops are alignedbetween pins, one fixed and the other removab)Ie andclamped in a mounting block, Figure 14, and then thewhisker specimen is placed on and cemented to thecenter of these loops. This approach with prealignedmounting blocks increases centricity of the small speci-mens in the tensile machine and eases the problemsof handling and the danger of accidental damage untilthe test can he started. The images of the art- projected ona screen with the aid of an optical s}stem consistingof a light bulb with a condenser lens and a highprecision projection lens, Figure 15. These screens are

GAGE NIAt , S " .E'. •.

I)M.P

CuhDf NSOR ,`^ A IRE tUOP

_LPt1ti MAIt o wi•,DOLEtu ' r.6 ixF

MV ("ArItk tJ- UI'u

-1l,RUlJ%U GEAn

OF SCREEN

IU1 A: 'I AGNIt ICAI[ON1 :00300

13 Gage marks on whisker cemented to a wire loop x 15 15 Schematic diagram of the tensile tester

5U

Blackburn et. el.: .Metal nrafrt.s composite inaterlals

PLEXIGLAS CUBE

TAP OR CEMENL

WHISKER

OIL IMMERSION FRACTURED

( CEDAR 0I1.1 l I ti CROSS SECI IUN

MICROS CCJPE____

0BJECIIVE

16 Arrangement of the fracttoed whisker far tneasuremnent of thearea under the microscope ( Schematic)

2001

IeoL

I(0L

F Ii AC T UPI

140E

120 F

I00L

NIa eo

$0E

404

1000 2000 3000 4000 5000 X 10-'

STRAIN IN/IN

17 .Stress-strain curve of iron whisker as obtained on the whiskertensile tester ( Cross section at fracture = 3'3µt used tocalculate stress)

18 One half of a split and curled boron filament XI

actually two inch square pieces of ground glass mountedon dial gages of I : 1000 at a distance of 200 inchesfrom the lens. This provides an amplification factorof I : 200000'".

After specimen fracture the wire loops with thefractured part of the specimen still attached are mountedon one side of a plexiglass block with scotch tape insuch a manner that the specimen tip is protruding afew thousandths of an inch above the perpendicularface of' the block, Figure 16. Then the specimen canbe viewed from the side perpendicular to its axis.2°Similarly by reorienting the block on the microscopestage with the specimen axis parallel to the microscopeaxis, the fracture area can he seen. The area can hemeasured with a micrometer eye piece which should beprecisely calibrated for the exact optical magnificationfactor. After the strain readings are plotted and thearea is determined, the strength and the elasticity modu-lus can be calculated, Figure 17.

During handling it was observed that the boronfilaments sometimes split when they are being cut todesired lengths. This indicates the presence of highresidual stresses. In one instance, a foot long specimenof a boron filament split during cutting and the twohalves curled up in nearly perfect cirices, Figure 18.Once the splitting started. it progressed slowly butcontinuously until both parts were completely sepa-rated.

51

Blackburn ri. al.: )tfrial maircx ccnnpo.cite nrc,ierials

20 (1/,., sf, ,,o,, of ;n'iallu ,.r,irvcl bans/ in a Ti-6.41-4V-Boronojogw,iii' (CourFe)y NA.-i i.-ID) x 100

(a) Boron Jihers emherh(ed in cr nic1: r'1 ruairi r X25()

Filament -ma(rix; interactions

Two types of bond at the fiber matrix interface shouldhe distinguished : mechanical and metallurgical. Amechanical bond is illustrated in Figure 19 by a nickel-boron composite fabricated h` a molecular formingprocess by the General Technologies Corporation.Figure 19 (a) is a transverse section of the compositeshowing the filaments in a nickel matrix. Figure 19 (b)shows the characteristic ''corn cob' stucture of the surfaceof a boron filament. Figure 19 (c) is a longitudinalsection of the composite showing the surface of thenickel matrix from which a filament hid been extractedleaving a replica of its surface. ,A metallurgical bondillustrated in Figure 20 by a Ti-6A 1-4V boron filament

(b) Surface of boron filament

M

(e) S'cnjuce of' nickel matrix +howhitz replication of horunIur/m x x:10

19 1/hrstration of invehanical bond in a nickel-boron composite(Courtesy General Technologies Corporation) 21 Al-B Contposire (Courtesy NAA L.' D) X 500

52

Blackburn et. al.: Metal matrix compo site materials

22 Ti-8 .41-fl - iMo-B composite after exposure at 1650 F for 69hours

composite fabricated by the North American Avia-tionlLos Angeles Division. The compound titaniumdiboride forms at the interface between the boron fila-ment and the alloy.

The follo%,ing series of examples taken from workdone at the AFML will illustrate briefly the types ofreaction which may result between boron and variousmetal matrices.

In an aluminium-boron composite fabricated by dif-fusion bonding there is no reaction evident at theinterface, Figure 21. Simple compound formation wasillustrated in Figure 20 while the growth of this phaseafter heat treatment is shown in Figure 22. Complexcompound formation consisting of several phases isillustrated by a nickel-boron composite after heat

24 Fe-B Composite after exposure at 1650 ' F for 1 hour

treatment, Figure 23. A sunburst-like diffusion patternis illustrated by the iron-boron filament interaction.Figure 24. An 18% Ni maraging steel-boron interactionis illustrated in Figure 25. It is entirely different thanthat of iron and thus qualitatively shows the effectof matrix alloying elements on the interaction behaviour.Eutectic formation and the resultant cast structure isillustrated in Figure 26 for a copper-boron com-posite.

Another type of interaction which may occur withcomposites containing boron filaments is the formationof a glass, as illustrated by an aluminium-boron com-posite, Figure 27, which was fabricated by powdermetallurgy techniques.

The illustrations certainly indicate that a variety of

25 Maraging steel (Fe-18Ni)-boron composite after exposure at23 Ni-B composite after exposure at 1380 ' F for 1 hour )( 500 1650-F for 1 hour x 500

53

Blat•hhurn t't. W. Metal matrix composite ntalerials

26 Cu-B Composite after synthesis x 500 (a) Refieered f rlrt

interactions may occur. Control of the interface willbe vital, but the nature of this control is not yetestablished. Some of the more obvious possibilitiesare fiber coating, matrix alloying and the developmentof compatible fibre-matrix combinations. Regardlessof' the method or methods chosen, control of theinterface is inherent for control of the mechanicalproperties.

Concluding remarks

Filament-metal metrix composite materials show signi-ficant potential for future development and use. However,much theoretical analysis and experimental verificationof both micro mechanics of' composites and their be-haviour under a variety of use conditions, and on thebroad subject of filament-matrix interactions must yetbe done to indicate the possible as well as the mostfeasible and attractive directions to follow. The acqu-isition of reliable experimental data, on reproduciblespecimens with well characterized structures, warrantspriority attention.

The overall area of composite materials will demandunprecedented cooperation between activities such asstructure-property relationships, laboratory preparation,production processing, materials testing and structuraldesign and evaluation. In the extreme, a material andcomponent might be "synthesized" to final shape for itspecific application. New use concepts will be requiredto cope with the highly anisotropic contribution of thefilaments to overall properties and to derive maximumbenefit from the opportunity to control location geometrywith respect to external loading. An iterative approachwill be necessary, incorporating a significant amount of'what we have called "vertical interdisciplinary" com-petence.21

(h) Trancioritierl Irt;ht

27 .41-B Composite showing Matsu phure

References

), 840

x ?s0

I. A. Kelly and G. L. Davies, ,%fct. RC). it), No. 37, I (1265),

2. 1). Cratchle , lfet. Rrr. 10, No. 37, 79 (1965).

3. D. L. McD:uu'Is, R. W. .Tech and J. W. Weelon, NationalAeronautics and Spacr Ad'nini,traIion Technical NoteD-1881. October 1963.

4. W. 11. Sutton and J. Chronc'. •1S\l Seminar on Fiber Contt-positc Materials, Ocroher 1964. 1(1 lie pi hltshad_

5. F. .I. D.urn,, R. Molhu and B. 1-. ('pester 'ImprovedFilament-W,unut Cunsu'urti_>n fur Cylindrical Pressure Vessels-'ML-TDR-64-43. Vol. I, Air Force Technical Report, March1964 (AD 436 272)-

54

6. A. A. Baker and D. Cratchley, Appl. Mat. Research 3, 215(1964).S. W. Tsai. National Aeronautics and space AdministrationContractor Report 71, July 1964.

tt. S. W. Tsai, National Aeronautics and Space AdministrationContractor Report 224, April 1965.

9. R. Hill, J. Mech. Phys. Solids 12, 199 (1964).

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H. J. M. Whitney and M. B. Riley, Air Force Materials Labo-ratory Technical Report 65-238, June 1965.

12. B. Paul, Trans. Met. Soc. AIME 218, 6 (1960).

"i. B. W. Shaffer, Trans. S. P. E. 4, 267 (1964).

14. J. C. Ekvall, ASME Paper No. 61-AV-56.

I5. Z. Hashin and B. W. Rosen, J. App!. Mech. 31, 223 (1964).

lb. J. P. E. Forsyth, R. W. George and D. A. Ryder, App!,Mat. Research 3, 223 (1964).

17. N. M. Parikh, ASM Seminar on Fiber Composite Materials,October 1964. To be published.

Ili. N. M. Parikh, Illinois Institute of Technology Research

Discussions

Mr L. J. Balasundaram , (N.M.L.) : Is there any relation-ship between the mode of distribution of filaments inthe matrix and the strength of the reinforced material,and if so how do you control it ?

How does the filament reinforced materials comparewith dispersion hardened materials ?

Lt W. F. Stuhrke, (Author) : In reply to your firstquestion we were very interested in trying many approa-ches to control the distribution of these filaments inthe metal matrix because obviously distribution is whatdetermines the properties of the particular direction,the material being extremely anisotropic. We are tryinga number of different methods ranging from diffusion

Blackburn et. al .: Metal trmtr 'x composite materials

Institute First Bimonthly Report, Ccntract No. NOw-6-50271-f, April 1965.

19. J. A. Herzog, "Festigkeitsuntersuchungen An EindimensionalGewachseneni Kristallinem Eisen (Whiskern)", Metal!. 17,No. 1, 1963.

20. R. L. Mehan, W. H. Sutton and J. A. Herzog, "Measure-ment of the Strength of Whiskers and Their Role in Re-inforcing Experimental Composites", to be published in J.AIAA, 1965.

21. H. M. Burte, "Trends in the Pursuit of Materials Scienceand Technology", Aerospace Engineering 21, No. 65 93 (1962).

22. S. S. Brenner, "Mechanical Behavior of Sapphire Whiskersat Elevated Temperatures". J. App!. Physics 33, No. 1, 33-9,1962.

23. R. C. Tornashot, "AF-994-A Superior Glass Fiber Reinforce-ment for Structural Composites", ASD-TDR-63-8I, Air ForceTechnical Report, March 1963 (AD 404 667).

24. Unpublished data from General Technologies Corporation,

25. R. W. Yancey, "Fabrication of Electronic Tube Compo-nents", Fabrication of Molybdemon, 205-213. American Societyfor Metals, Cleveland, Ohio, 1959.

technique, random alignment technique, etc.Now about dispersion hardening there is a signifi-

cant difference between these two materials. On dis-persion hardening, size factor is somewhat smaller thanone is interested in dislocation motion where as inthese materials when a filament is 5/1000 of an inchwhich is many tens of thousands of an augstromand speaks about stress interaction rather thandislocation interactions. We don't feel this results inany strengthening. We feel that there is really no suchrelationship. At the moment we stopped with circularfilaments but we would like to try square ones ; hollowones ; as a matter of fact the technology of distributionis not much advanced.