ELSEVIER

Abstract

Journal ofMaterials Processing Technology 63 (1997) 339-353

The processing of metal matrix composites - an overview

Brian Ralph·, H.C. Yuen· and W.B. Lee'"Department of Materials Engineering, Brunei University,

Uxbridge, West London, UBS 3PH, U.K.'Department of Manufacturing Engineering,

Hong Kong Polytechnic University, Kowloon, Hong Kong.

Joumalof

MaterialsProcessingTechnology

This overview begins by considering the mature situation for polymer matrix composites (PMCs). After a further short section devotedto ceramic matrix composites (CMCs), the main text is devoted to the variety of routes available for processing metallic matrixcomposites (MMCs). These are divided into those where the main steps are performed in the solid state and those where the processroute involves a stage where the matrix is molten. Some discussion is also given to the thermomechanical processing of MMCs and oftheir properties.

KeywordsMetal matrix composites. Processing. Microstructure - property relationships. Polymer matrix composites. Advanced fibre reinforced

composites. Ceramic matrix composites. Internal oxidation. Sintered aluminium powder. Particulates. Short and long fibres. Mechanicalalloying. Diffusion bonding. Molten metal mixing. InfIltration. Dispersion. Spraying. In-situ processing. Solidification processing.Texture.

1. Introduction1.1 Historical perspective

We tend to think of the latter half of the twentieth century asthe "Composite" age. In some ways this is realistic and gives usa feeling of continuity from former "material-based" ages such asthe Stone, Bronze and Iron ages. Certainly the last 50 years havebeen associated with some remarkable developments in compositematerials; some 0 f which will be alluded to in various degrees 0 fdetail below.

However, in another sense, man's use of composite materialshas a very much longer history than just the last 50 years. Manynatural materials may be considered as of composite type; theclassic example being wood [e.g. 1]. Further, mankind relativelyearly on in its development discovered and employed one of thecentral principles of composite materials, that is enhancing oneor more properties by mixing materials in various ways. Thus avery elementary example of a ceramic matrix composite (CMC)would be mud mixed with straw; still a very widely used materialin the construction of houses. The incorporation of the strawimproves the strength, toughness and the thermal insulationproperties of this very basic composite. In principle at least, thedegree of reinforcement (volume fraction of straw) and the levelof alignment of the straw stalks (and their lengths) may beadjusted so that not only the properties but their anisotropy maybe optimised differently in various parts of the structure. In

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terms of exploiting modem engineering composites. this remainsa central principle.

Modern engineering composites may be said to have "designedmicrostructures" in that the dispersed phases/reinforcement tendto have sizes in the micrometre range (that is from lOs ofnanometres to 100s of micrometres). Wood and straw (e.g.adobe) tends to rely on rather coarser dispersions of the phases(say at a scale centred around the millimetre level). In another,relatively modem, engineering composite - reinforced concrete ­the scale is larger still (here the scale is perhaps centred at the 100millimetre level). Reinforced concrete may be looked at in atleast two different ways. It can be thought of as a ceramic matrixcomposite (CMC) toughened/reinforced with steel bar/mesh.Equally the civil engineer tends to think of it as a basically steelstructure with the function of the concrete to stop Euler bucklingof the steel elements and to protect the steel from corrosion.Many problems with structures made from reinforced concrete inrelatively recent times arise because of inadequate protection ofthe steel reinforcing bar by the concrete against corrosion.

Ingress of salt-laden solutions into highway bridges has been ofparticular concern ("concrete cancer") requiring very expensivepatch and repair processes. Now that the problem has beencorrectly identified solutions have emerged including modifyingthe concrete chemistry and/or precoating the steel reinforcing barwith protective (often epoxy) layer.

340 B. Raifet al./Journal ofMaterials Processing Technology 63 (1997) 339-353

1.2 Modern engineering composites1.2.1. Polymer matrix composites

The major thrust of this overview is to consider critically theprocessing routes for metal matrix composites (MMCs).However, before going into some detail on MMCs, it is perhapspertinent to look at recent developments in composites moregenerally.

The obvious initial subdivision of modern composites is byway of the nature of the matrix, i.e. polymer, metal or ceramic.Of these three, clearly polymer matrix composites (PMCs) arethe most extensively developed and applied [e.g. 2-4].Reinforced polymer composites fInd very wide applicationindeed, from uses in aerospace structures (control surfaces, etc.,in aeroplanes, for the rotor assembly in helicopters) to veryextensive utilization in sports equipment (from shafts for golfclubs, handles of rackets to marine applications and racing cars).Here whilst it is still possible to buy small yachts, etc., made inwood, steel or aluminium this market sector is dominated byglass reinforced polyester (GRP) and other polymer/fibrecombinations. In addition many smaller domestic structures(garden ponds, water tanks, etc.), are marketed in reinforcedplastic and reinforced plastics fInd increasing use in theautomobile sector. In addition, some specialist structures such aswindmills are now conventionally fabricated from GRP orvariants thereof. In essence where wood would have been thenatural choice of material in the past (i.e. boats, windmills, etc.),GRP and other PMCs have tended to become the preferredreplacement materials. In one sense this is because of thesimilarity of the materials (wood and GRP are both compositesand plywood is very much like a "layered" polymer fibrecomposite). In addition, some of the work skills required in thefabrication of artefacts in wood are not needed to the same extentfor PMCs. Perhaps of more significance is the reproducibility ofproperties in the case of PMCs compared to wood.

PMCs may involve thermoplastic or thermosetting matricesand the reinforcement may be in the form of relatively equiaxparticles, short fIbres or long fIbres. In the case of smallerengineering components processing by injection moulding of athermoplastic matrix reinforced with particulate or short fibredispersions is common; the major attributes here are the speed ofprocessing and the increased stiffness which the dispersoid givesto the polymer matrix. This fIeld is relatively mature, andadvanced gating systems permit the management of the volumefraction and alignment of dispersed fibres in various parts of amoulding by shear within the moulding of the matrix [e.g. 5]. Inother developments reactive processing of polymer blendes(alloys) can be used to create in-situ reinforcement [e.g. 6,7,].

The reinforcing phases used in PMCs may be polymeric orceramic. Glass fibres (continuous or chopped strand) are used toreinforce thermosetting polyester matrices and the resultant GRPforms the basic material used in the marine industry and formany domestic applications. When there is a need for higherperformance, advanced fIbre reinforced composites (AFRCs)come into play with epoxy matrices reinforced with carbon oramide (Kevlar) fIbres. AFRCs offer improved specificmechanical properties (that is property divided by density) andare of particular importance in aerospace applications, where

weight is a prime factor. However, often AFRCs are also foundwithin sports equipment, sometimes because of the improvedperformance offered but often because of the dictates of fashion!

What is particularly appealing about PMCs as a class ofmaterials is that many of the relationships between the designedmicrostructure and the properties which result are relatively wellunderstood [e.g. 2-4, 8]. For instance, the variation in elasticmodulus with content of reinforcement is usually found to followa simple law of mixtures. In other cases, the relationship betweenthe microstructural parameters and a specific property may bemuch more complex. For instance, explaining the tougheningeffect of glass fIbres in a polyester matrix requires considerationof the details of the fracture process in both phases (which areboth brittle). Here essentially the analysis of fractographs leadsto the conclusion that much of the toughening effect (energyabsorption) arises because of fIbre pull out. This type ofthinking/analysis has become quite sophisticated particularly in thecase of failure under compressive loads [e.g. 9]. In turn it is thenpossible to use this approach to redesign the microstructure ofcomposites to resist more complex loading situations such asfatigue. In general, this requires the use of hybrid microstructures[e.g. 10, 11].

The processing of PMCS based on thermosetting polymericmatrices varies from minimalistic to highly sophisticated. At theminimalistic level the approach may be almost at the "cottageindustry" level. Taking the fabrication in GRP of small boats asan example, the glass fIbres are supplied either in the form ofcloth (woven roving) or as a chopped strand matte in variousweights per square metre. A mould of the boat to be made iscovered with a release agent and then with some barrier (gel)coating of polymer before the cloth/matte is shaped to the mouldand saturated with resin and hardner. At the most primitive levelthe "processing" consists of stippling the fIbres with theresin/hardner mixture and then using a roller to squeeze outair/gas. The quality of the moulding produced by this methoddepends greatly on the skill and experience of the moulder.

Problems (boat pox) with older GRP boats due to osmosis havetended to lead to improved barrier (gel) coatings and also morecare being exercised in the moulding process. Here, andparticularly with the use of AFRCs in the aerospace and sportsindustries, often vacuum bagging (to reduce air/gas entrapment!voidage) and heat curing in especially designed autoclaves isbecoming more important and represents the sophisticated end ofthe processing chain. Another aspect of the more sophisticatedapproach is to be able to monitor the state of cure of the matrix(in terms of cross link density, etc), and in-situ cure monitoringis emerging as an extremely powerful technique [e.g. 12]. Ingeneral, there is an interest in monitoring the state of "health" ofthe composite once it is in service and again sensing systems arearising which can condition monitor impact and fatigue damage[e.g. 13].

In a more critical structure, it may be necessary to take thedesigning in of an anisotropy of properties a stage further. Formany engineering artefacts now it is possible to analyse the stateof stress which the artefact will encounter during its lifetime usinga computer-aided design (CAD) approach. Such is the sophisti­cation of CAD systems that it is possible to give a mesh of stress

B. Ralfet al./Journal ofMaterials Processing Technology 63 (1997) 339-353 341

distributions over a structure (e.g. the mast of a windsurfer, anAFRC gas bottle). This mesh can then be used to modify theoverall design but, more importantly, in tenns of producingstructures from AFRCs, can also be used to calculate the localfibre densities and orientations needed to cope with this stresspattern. In the most sophisticated cases this can then be used asinput data to an automatic fIlament winding device which is usedto fabricate the structure.

Whether thennosetting or thennoplastic matrices are used withwhatever fonn of reinforcing, the major attraction of PMCs is theability to design/optimise a microstructure which delivers anoptimised "basket" of properties. Whilst it would be wrong toclaim that a detailed understanding of the mechanisms of allprocesses in polymer composites is known, in general, goodmodels are available which are predictive. This means that thedesign engineer has "codes" which will enable him/her tooptimise the choice of material parameters for a particular design.This is probably the main reason that PMCs have become so wellestablished within such a relatively short time.

However, one problem now beginning to emerge is how tohandle PMCs when the lifetime of the structure they have beenused in is at an end. Increasingly, the importance ofenvironmental issues comes to the fore with many agenciesstressing the importance of perfonning a life cycle assessment("cradle-to-grave") approach. Here the difficulty for PMCs, inparticular, and perhaps other composites as well, is to suggest asensible recycling process or how they may be disposed of in anenvironmentally-sensitive way. There is a tendency to think ofused PMCs as "toxic waste" and this may limit their exploitationin the future [e.g. 14J.

1.2.2. Ceramic matrix composites

Reference has been made in the Introduction to some of thetraditional ceramic matrix composites (CMCs) where the matrixis concrete or dried mud. Here we are concerned with modemengineering versions thereof which would be characterized byhaving much more finely divided microstructures.

The main reasons for exploring the possibilities of CMCs isthe hope of achieving substantial increases in toughness (that isincreasing K1c above 2MPa m'h). As with all inherently brittlematerials this means invoking processing techniques whicheliminate critical defects. Recent reviews survey these issues[e.g. 15-18].

A very wide range of matrices and reinforcements have beenexplored. For instance, there has been a wide range of studiesexploring the possibility of reinforcing AlZ0 3 as the matrix witha range of oxides, carbides, nitrides and borides. In general, theresults have been disappointing because the presence of thedispersoid has resulted in poor sinterability and the resultingporosity has degraded the mechanical properties achieved [e.g.19]. One of the more promising particular additions to AlZ0 3 hasbeen zrOz where hot pressed composites with high fracturetoughnesses (- 10MPa m'h) and high strengths [e.g. 17,20,21]have been made. Others have been able to show that these andother low defect content CMCs are able to be superplasticallydefonned to large strains [e.g. 22, 23]. Other successful

additions to AlZ0 3 matrices are the carbides TiC and SiC.Alumina matrix-TiC composites show excellent wear resistanceand cutting tools have been produced by hot pressing at 16OC1'C[17]. Composites based on AlZ0 3 reinforced with SiC arecommercially produced by hot pressing for use as microwareabsorbers [17]. Exploratory studies of many other systems havebeen made [e.g. 24J.

The main processing route for CMCs, as for ceramics ingeneral, is via powder fonning. A crucial factor here is to startwith a fme particle size and to avoid agglomerates. This will aidthe sintering/densification process and lead to lower concentrationsof critical defects. The aim during sintering is to get to the lowestpossible porosity without extensive grain growth. Because of thehardness of ceramics and CMCs, together with the possibility ofintroducing critical surface flaws, in general the aim is to use thepowder processing route as a net shaping technique and avoidfmal machining as much as possible. In some cases, a binder willbe added to give "green strength" after the pressing stage andsintering aids may also be incorporated. However, often theseadditives lead to other problems later in the processing sequence(e.g. the need for binder removal, often leading to the productionof defects in thicker sections).

The cost of processing through a powder route is high and todate CMC components produced this way tend to be small innumber and in overall size. However, the potentialities ofartefacts produced from CMCs by this route are acknowledged tobe high and a greater exploitation of CMCs in the future isgenerally anticipated. Already CMCs are being exploitedextensively as coatings to protect high temperature metallic alloys(nickel-base superalloys). These thennal barrier coatings (TBCs)are used to insulate the underlying alloy from the extremes oftemperature in the hotter parts of gas turbine engines. In general,TBCs are plasma sprayed over a bond coat which is itself anMMC [e.g. 25]. In the future, one may expect a competitionbetween engine components produced from monolithic ceramic orCMC and those produced from a high temperature alloy andcoated. Another development for internal combustion enginesuses inserts of ceramic/CMC in alloy pistons. One may expectmany other applications for CMCs where the combination of wearresistance, ability to withstand exposure to high temperature,chemical inertness and potentially high specific mechanicalproperties are of significance.

1.2.3. Metallic matrix composites

One difficulty in reviewing this topic is to decide theboundaries of what is treated as an MMC. For instance, inprinciple, wrought irons and most conventional steels could betreated as MMCs since they have a metallic matrix which isreinforced with dispersoids of oxides, sulphides, carbides, etc.Even if we take a defmition through "microstructural design"many conventional engineering alloys, such as some steelsinvolving dispersions created at a moving alpha-gamma interface,really ought to be included [e.g. 26]. However, this is still muchtoo all-embracing a defmition and, to some extent by analogy withPMCs, a cut-off is taken in the defmition which really looks at"blending" the reinforcement with the matrix. A number of otherways of limiting the scope of the defmition have been suggested[e.g. 27]. Even the "blending" approach has in PMC tenns to

342 B. Ralfet al./Journal ofMaterials Processing Technology 63 (1997) 339-353

Table 1 Main categories of MMCs (modified extensively from [29])

Composite type/process Section Examples (reinforcement/matrix) Main features

SOLID STATE _2_PROCESSINGIn-situ forming 2.1 AlP3' Si02 BeO particulate in Cu · Modest strength improvement- internal oxidation or Ag · Good electrical conductivity

Powder forming- sintered aluminium 2.2 Al20 3 particulate/AI matrix · Moderate strength and stiffness to

powder around 300°C· Low density

.------------. - - - -- - --- ---- --- --.------ -- - ------ - - -- - -. - ---- ---

- long or short fibres or 2.2 AlP3' SiC in AI alloy matrices · Good stiffness/strength to modestparticulate incorporated temperaturesby powder metallurgy · Low density

· Low thermal expansion.-----------. - - - - ---- - - - - ---- --- - ----- ------------------------

- hard metals 2.2 WC particulate in Co matrix · Well developed class of material forcutting applications

.------------. - - - - -- - .--- - .-- ---- - - - --- ------------------------

- mechanical alloying 2.2 oxide particles in superalloy matrix · High performance alloy [e.g.30, 31]· High strength at high temperature

Diffusion bonding 2.2 SiC fibres in Ti3AI etc. · Some problems over oxidation at- long fibres in high temperature [e.g. 32]

intermetallics-------------. - - - ----------------------- ------------------------

- anodised AI roll 2.3 AlP3 particulate/Al · Moderate strength and ductilitybonded and consolidated · High electrical conductivity

LIOUID STATE _3_PROCESSINGMolten metal mix 3.1 SiC or Al20 3/light alloy matrices · Modest improvements in properties

processing

Infiltration of preforms 3.2 SiC whisker, Al20 3 fibres/AI alloys · Good stiffness and strength to 200°C(may be short or long fibre C/AI and Mg alloys (Mg), 300°C (AI) and 6000C (Ti)or particulate) SiC/Ti alloys · Low density

B/AI alloys · Low thermal conductivity

Dispersion 3.3 various ceramic dispersoids into the · Some problems in controlling themelt microstructure

------------- - - - - --- - - - - -- -- - ---- - ---- -------------------Semi solid processing 3.3 Si in AI · Modest strength(rheocasting/thixoforming) · Good wear characteristics [e.g. 33]

Spraying 3.4 particulate/short or long fibres in alloy · Good stiffness and strengthmatrices, e.g. SiC or Al20 3/in AI alloy · Low density

· Low thermal expansion coefficient

In-situ processing 3.5 TiB2 particulate/AI alloy · Good strength ductility and toughness· Fatigue resistant

Solidification - processing, 3.6 TiC fibres in "(/"(' matrix · Questions over stability in thermaldirectional solidification gradients and thermal cycling [e.g.28].

B. Ralfet al./Journal ofMaterials Processing Technology 63 (1997) 339-353 343

allow for reactive processing [e.g. 6, 7] and so it is quite nonnalto think of directionally solidified eutectics within the frameworkof an MMC defmition [e.g. 28]. One possible scheme forconsidering MMCs is given in Table 1. However, two thingsshould be appreciated about this listing. Firstly, many processingroutes are included here which would not agree with a defmitionfor an MMC that the reinforcement and matrix are mixed [e.g.34]. Secondly, whilst the list may seem quite extensive, it is notexhaustive and does not attempt to distinguish between processingroutes which are already being used to produce commercialquantities of material and those which are very much at thedemonstration stage.

The following sections of this overview look at some of theprocessing routes mentioned in Table 1 in more detail. Whilstthe emphasis is directed to the processing route, some attentionis also given to microstructure/property relationships (in the samemanner as was done for PMCs earlier in this paper). The nextsection (2) looks at the main processing routes for fonningMMCs in the solid state, whilst section 3 perfonns the samefunction for those cases where the main processing step involvesthe liquid state.

In looking at the wide variety of processing routes availablefor making MMCs (Table 1) thought must be given as to thelikely applications of a particular MMC and the processing route.Some of these methods produce MMCs with relatively lowvolume fractions of dispersoids and so any property enhancementis likely to be relatively modest. What has to be rememberedhere is the cost of using some of the more esoteric long fibres[e.g. 35, 36]. For instance, one estimate of carbon/boronprotected 100 [.Lm diameter SiC long fibres suggests a cost ofaround 10k£ kg,l. Whilst these costs are likely to fall if majorapplications for such fibres arise, these high costs tend to limitthe rate at which potential applications for MMCs employingthese fibres become commercially realised. Again, theprocessing route has a considerable bearing on the type ofapplication. For instance, some MMCs may be processed to verynearly net shape by casting routes including pressure die casting[e.g. 37]. In other cases, the initial processing will producesomething which may be identified as a billet which in turn canbe further processed by conventional thennomechanical routes,such as extrusion, forging and rolling [e.g. 38]. In many cases,the fabrication route will also require a strategy for making joinsand various joining techniques, such as diffusion bonding, arenow being explored [e.g. 39].

The aim of much of the more fundamental investigationsshould be to establish mechanisms relating microstructure toproperties. Eventually, the purpose of this will be to createpredictive models which compare with those available for otherclasses of materials, and some efforts along these lines are nowbeing made [e.g. 40). Two particular complications may arise inthe case of MMCs. The first one arises because many of thereinforcing phases (e.g. SIC) are thennodynamically unstable inthe chosen matrix (e.g. AI) [e.g. 29,41). Table 2 lists a numberof examples of the type of interactions encountered. This haslead to a whole range of fibres with protective coatings. Thesecond complication arises because of the differences in thennalexpansion coefficients between ceramics and metals. Forinstance, in the case of Al/SiC the coefficient of thennal

expansion for AI is 24xlO~K·I whilst it is only 3.8xl0·6K·1 forSiC. Thus on cooling from the high temperatures used duringmaterial processing, a thennal mismatch strain is generated acrossthe interface between the two components of the composite. Theassociated strain is then sufficient to generate local plastic yield inthe AI matrix [e.g. 42]. If the MMC is then subjected to thennalcycling, microstructuraUsubstructural damage can accumulateleading to dimensional instability or under low loads to acceleratedcreep which amounts to superplastic behaviour [e.g. 43, 44].

2. Fonning MMCs predominantly in the solid state2.1. In-si/u formation of /he dispersoid

With the widest possible defmition of an MMC it is possibleto think in tenns of standard phase transfonnations as a means bywhich MMCs are fonned. Within this very broad definition wouldthen come some of the more "designed" microstructures in steels[e.g. 26]. A more selective defmition would probably still includeinternally oxidised alloys [e.g. 45]. These are examples ofdispersion strengthened alloys which have been extensively studiedand modelled [e.g. 46]. An alloy, typically of Cu or Ag, is madeup as a dilute solid solution with a solute, e.g. AI, Si or Be whichhas a higher affmity for oxygen than does the solvent. This isthen heat treated in a partial pressure of oxygen which is belowthe threshold for surface oxidation. The result is diffusion ofoxygen into the alloy with the fonnation of oxide dispersions (e.g.AIP3, Si02 or BeO). Improved mechanical properties (e.g.strength) result without substantial loss of electrical conductivity.

2.2 Powder fanning

The simplest example of a powder-fonned MMC must besintered aluminium powder SAP [e.g. 47]. Only modestimprovements in strength and creep strength are achieved bypowder fonning Al powder with its air-fonned (or hightemperature oxidized) oxide film. Enhanced properties can beachieved by blending in additional AI20 3. Again these relativelysimple MMCs have been extensively studied and their mechanicalproperties accounted for in tenns of models and relationships suchas that expressed by the Hall-Petch equation [e.g. 48]. One of theattractions of this work is that it fonns a useful base for many ofthe studies of the more recently-developed MMCs which tend tohave much higher volume fractions of dispersoid. In particular,where the matrix is AI-based and powder fonned whatever themain dispersoid, fme AIP3 will be found as well and maycontribute to the fmal microstructure and properties [e.g. 49].

A very wide range of MMCs may be fonned using powdermetallurgy techniques (see Table 1); incorporating widevariations in volume fraction of reinforcement in particulate, shortfibre and long fibre fonn (see Table 1). By way of an example,figure 1 is a scanning electron micrograph (SEM) of the fracturesurface of a powder fonned and extruded MMC with 17.7% byvolume of approximately 3 [.Lm SiC particulate in 2124 aluminiumpowder matrix. Hot isostatic pressing was used to achieve fulldensification of this material. Table 3 gives a small selection ofthe mechanical property data collected from this material in thelongitudinal (L) and transverse (T) directions and as a function ofpercentage stretch after extrusion [50]. One rather specialised setof MMCs produced by powder fonning are the hard metals(usually WC dispersed in Co) [e.g. 51].

344 B. Ralfet al./Joumal ofMaterials Processing Technology 63 (1997) 339-353

Table 2. Examples of interactions in selected fibre matrix systems (from [29l).

System

C-AI

Cm-Ni

C(I1)-Ni

B-AI

B-Ti

SiC-AI

SiC-Ti

SiC-Ni

W-Cu

W-Ni

W-Fe

Steel-AI

TaC-Co·

Interaction

Fibre recrystallisation activated by Ni

Formation of borides

Formation of TiB!

No significant reaction below m.pt

TiSi!, TisSi3 and TiC form

Formation of nickel silicides

No significant reaction

AI,O dissolution (very little) gives pitsIn air, NiAIP4 formation

No interaction up to m.pt.

Recrystallisation of fibreDegredation of creep properties

Formation of Fe-,W6; dissolution of fibre

F~AIs formation

Dissolution (dissolution/reprecipitation)

Approx. temp.of significantinteraction (0C)

550

1150-1300

800

500

750

(m.pt 660)

700

800

(m.pt 660)

11001100

(m.pt 1083)

1000900

1000

500

1200

*Directionally solidified eutectic composite - representative of several carbide/metal eutectic composites.

B. Ralfet al./Journal ofMaterials Processing Technology 63 (1997) 339-353 345

Figure 1. Scanning electron micrograph of the fracture surfaceof a powder-formed SiC particulate reinforced 2124 AI alloyMMC. The fracture process in this case has cracked the SiCparticle (from [SO)).

Figure 2b. As Fig.2a but taken from a section transverse to therolling direction (from [53]).

Figure 2a. Light micrograph of a longitudinal cross-section of anAI MMC reinforced with 0.08 volume fraction of alumina (from[SO)).

Figure 3a. Light micrograph of a longitudinal cross section of anAI MMC reinforced with 0.14 volume fraction of alumina (from[53]).

346 B. Ralfet al./Journal ofMaterials Processing Technology 63 (1997)339-353

Figure 3b. As Fig. 3a but taken from a section transverse to therolling direction (from [53]).

Figure Sa. Scanning electron micrograph of TiB2 dispersion inan AI 8%Mg 1%Zr matrix produced by flux-assisted dispersion(from [61]).

Figure 4. Scanning electron micrograph of an alumina/AI MMCshowing poor banding at the end of the alumina particle (from[53]).

Figure 5b. As Fig. Sa but here the matrix is pure AI (from [61])

B. Ralfet al./Journal ofMaterials Processing Technology 63 (1997)339-353

Table 3. Tensile values for as-extruded and stretched material from [53]

347

Bar Orienta­tion

Ultimatetensilestrength (MPa)

0.2% proofstrength(MPa)

Elonga­tion(%)

As-extruded L 637 430 7.7As-extruded T 593 427 5.2Control L 600 417 5Control T 564 406 40.5% L 610 454 50.5% T 536 462 4.51% L 604 458 4.51% T 520 418 4.5

Table 4. Volume fraction of MMCs and geometric shape of the reinforcing alumina (from [53])

Volumefraction

Alumina geometry·

o

Averagelength (/-tm)

Aspectratio

90

Averagelength (/-tm)

Aspectratio

0.08 10.5 1.98 20.5 4.3

0.11 11.8 2.21 20.4 4.1

0.14 11.4 2.09 20.2 3.64

* 0° = longitudinal to rolling direction; 90° = transverse to rolling direction

Table 5. Experimental results of physical and mechanical properties of MMCs from [53].

Volume Sample Tensile Modulus Hamdess Resistivityfraction conditions strength (GPa) Hv (1O-9Om)

(MPa)

(0.08) Cold 00" 97 72.3 40.7 33.0rolled 90° 108 78 35.9Annealed 0° 85.2 73 31.8 36.8

90° 94 87 35.6

(0.11) Cold- o· 99.7 99.4 49.2 39.6rolled 90° 116 108.5 39.6Annealed 0° 97.3 83.1 32.6 35.9

90° 101.3 98.2 36

(0.14) Cold- 0° 90.7 78 51.1 40.1rolled 90° 121 120 39.9Annealed 0° 88.8 82.8 35.8 36.8

90° 98 103 39.7

* 00 = longitudinal to rolling direction; 90° = transverse to rolling direction

348 B. Ralfet al./Journal ofMaterials Processing Technology 63 (1997)339-353

As outlined in Table 1, mechanical alloying is an importantmeans of fabricating special MMCs which is alreadycommercially exploited [e.g. 30, 31]. Whilst a number ofpossible systems have been studied, most effort has been directedat alloys used in the high temperature parts of gas turbines, thatis nickel-base superalloys. In general, a rather low volumefraction of a rare earth oxide phase is used as the dispersoid.This is blended into a matrix which is made by mechanicalalloying where the elements are mixed and dispersed by diffusionbonding and fracture in a machine called an attritor. Extremelyuniform microstructures free of any segregation can be made bythis means.

2.3. Diffusion bonding

In principle, diffusion bonding may be used to form interfacesbetween the phases in an MMC and to form joints between MMCcomponents. Because of some of the inherent properties of Ti(that is to absorb large quantities of interstitial impurities and"consume" the air-formed oxide film) the development ofadvanced MMCs with Ti matrices and long fibre reinforcementshas been explored extensively [e.g. 36].

The Ti matrix/long fibre reinforced class of MMC might bethought of as rather exotic; certainly until critical applications forthis class of MMC are found they will remain expensive. Amuch less sophisticated and cheaper system has recently beenstudied [52, 53]. Figures 2 to 4 and Tables 4 to 6 come fromthis study where the aim was to produce a relatively low cost,reinforced Al which could be used for electrical powertransmission. Thus the aim was to retain the good electricalconductivity of pure Al but reinforce its mechanical propertieswith a dispersoid.

The processing chosen here was to anodize Al fIlm to increasethe thickness of the AlP3 layer. "Sandwich" structures werethen made from anodized and unanodized Al foils to generate aselected range of AlP3 volume fractions (0.08 - 0.14, see Table4). These sandwiches were then hot rolled (and in more, as-yetunreported, experiments were hot extruded) to roll bond themtogether. Subsequently, cold rolling was used to break up theoxide layers (see Table 4 and figures 2 and 3). A whole varietyof mechanical texts together with measurements of electricalresistivity were made (see Table 5) and the results compared withsimple rule of mixture models (Table 6). Whilst some goodimprovements in properties have been achieved more can beexpected once a full bond between the Al and ~03 is achieved.It is apparent from micrographs such as figure 4 that more workin this area is needed.

3. Forming MMCs predominantly in the liquid state3.1. Molten metal mix processing

In principle, this represents a very simple processing routewhereby reinforcements such as SiC or Al20 3particles are mixedinto a light alloy melt, subsequently cast and then fabricated in amanner analogous to conventional, unreinforced alloys [e.g. 54].In practice, there are three main considerations which distinguishthe reinforced melt from its unreinforced counterpart. Firstly,the presence of the particles leads to a large increase in therheological properties (viscosity) of the melt, which has to be

remembered when thinking of transferring the liquid. The secondfactor involves allowing for the sedimentation of the particleswhich have a higher density than the Al matrix. This is less of aproblem for more uniformly sized particles when the volumefraction of them is high. The fmal factor concerns the reactivityof the particles in the melt. In the case of SiC, its reaction withmolten Al to form Al4~ and Si can be prevented by using amatrix with a high (-10% Si) content [55]. Whilst Al20 3particlesare stable in pure Al, any Mg in the melt will encourage theformation of MgAlP4 spinel.

3.2. Processing by infiltration

In the infiltration process, the starting point is a preformmade from the reinforcing phase. The melt is then introducedinto this preform to fill all the open porosity. In systems wherethe melt wets the reinforcement the flow may be brought aboutsolely by the forces of capillarity; otherwise mechanical forcemust be applied to overcome the forces due to drag and capillarity[e.g. 56]. This process is seen to be a highly versatile, near netshape fabrication teChnique which offers very good control of themicrostructure. The disadvantages mainly involve the high toolingcosts and the fact that the reinforcement must be mechanicallyself-supported prior to inmtration. This latter point limits the typeof reinforcement geometries which may be employed.

3.3. Processing by dispersion

Potentially this is a relatively inexpensive way of making abroad range of MMCs whereby the dispersoid is added to thesurface of the melt and then becomes entrained in the melt byagitation and/or mechanical work [e.g. 56,57]. Variations of thisprocess involve semi-solid processing (rheocasting/thixoforming)[e.g. 33,58].

These dispersion techniques are versatile but suffer to variousextents from difficulties in controlling the microstructure anddefect content therein.

3.4 Processing by spraying

This is another relatively versatile way of forming MMCs;offering quite large production rates and the possibility of fonningto near net shape billets for subsequent, secondary processing[e.g. 56,59, 60]. Here the melt is divided into droplets (a varietyof "atomization" techniques are used) and sprayed on to or withthe reinforcement. The process can be used for surface coatingor for making monolithic composites.

Table 7 (from ref. [59]) illustrates some of the propertychanges which accompany two different ways of making MMCs.In the case of the SiC reinforced 2014 Al alloy MMCs, theforming process used was spray fonning (using the Ospreyprocess). By contrast, the AlP3 reinforced Al-3.5% Cu sampleswere produced by infIltration and squeeze' casting.

3.5. In-situ processes

Again depending on the breadth of the defmition of an MMC,cast irons could be said to come within this class. However, a

B. Ralfet al./Journal ofMaterials Processing Technology 63 (1997) 339-353

Table 6. Comparison of experimental and theoretical valuesof the tensile strength of MMCs from [53J

349

Volumefraction

Average experimental Average calculatedresults (MPa) results (MPa)

Annealed As-rolled Annealed As-rolled

0.08 94 108 83 1220.11 101 118 87 1340.14 103 121 92.8 146

Table 7. Tensile properties of composite and unreinforced alloys (from ref. [59»

Alloy Type Heat- Young's 0.2% Proof Tensile Elonga-and treated Modulus Stress Strength tionReinforcement condition (GPa) (MPa) (MPa) %

2014 (Ingot) ST - 153 402 21.72014 (Osprey)

+SiC ST - 210 406 U.5

2014 (Ingot) T6 73.8 432 482 10.22014 (Osprey) T6 - 437 489 7.42014 (Osprey)

+SiC T6 93.8 437 484 6.92014 (Osprey)

+SiC T8 - 484 521 8.7

Al-3.5%Cu T4 68.6 150 223 19.5AI-3.5%Cu-

AlP3 T4 90.9 134 319 2.3

Al-3.5%Cu T6 70.6 174 261 14.0Al-3.5%Cu-

AlP3 T6 95.4 238 374 2.2

ST = solution treated and cold water quenched

Table 8. Selected mechanical properties of A356 alloy series metal-matrix composites (from [62])

Material U.T.S. (MPa) 0.2% Proof Stress Elongation(MPa) (%)

A356 heat treated' 287 228 13

A356-4w/o TiB2 • 300 237 10

A356-5.5w/o TiB2 • 303 240 7

A356-15v/o SiC # 331 324 0.3

* Solution treated at 53SOc and aged for 24 h at 16O>C;# Duralcan™ material solution treated at 541°C and aged for 12 hat 16O>C.

350 B. Raifet al./Journal ofMaterials Processing Technology 63 (1997) 339-353

Table 9. Principle fibre types and their properties at room temperature (from [29]).

Fibre and Method of Diameter Specific Mean Axial Coeff. offonn prepara- (I'm) gravity fracture Young's thennal

tion stress modulus expansion(MPa) (GPa) K-1x106

Tungsten drawn 10-5<>0' 19.2 25003 400 5

Steel drawn 10-2503 7.8 25003 210 15(M or Y/C)

Boron (M/C) CVD 150 2.6 3500 400 8

SiC (M/C) CVD 150 3.4 3800 450 4.5

SiC (Y/C) PP 12 ± 3 2.6 2500 200 4.5

SiC whisker PP 0.1 - 2 3.2 10000 700 4.5(RlS)

a-Al20 3 (Y/C) PS 20 ± 5 3.9 1500 380 7

a-AlP3 (RlS) PS 3 ± 1 3.5 2000 300 7

Carbon (Y/C):

high modulus pp 10 2 3000 600 0

med. strength pp 8 1.9 4200 300 0

AlP3/27 %Si02 PS 3 3.0 850 150 -(RlS)

AlPi47%Si02 melt 3 2.7 1750 105 -(RlS)

S-glass (Y/C) melt 3 - 203 2.5 4000 90 3

1. C - continuous; S - short; M - monoftlament; Y - multifllament yam; R - random.

2. CVD - chemical vapour deposition; PP - pyrolysis of polymer precursor fibre; PS - pyrolysis orsintering of a salt andlor oxide suspension or gel in fibre fonn.

3. Fibre diameters can be chosen in this range. For metal fibres, the strength and pricedepend on diameter (extent of wire drawing).

B. Ralfet al./Journal ofMaterials Processing Technology 63 (1997) 339~353 351

number of MMCs coming within the more conventional defmitionof an MMC have been made by forming the reinforcing phase(such as TiB:z) by an in-situ reaction [e.g. 56, 61-63].

Table 8 compares the properties of an unreinforced Al alloy(A 356) with the same alloy reinforced reactively to give adispersion of TiB2 and reinforced with AlP3 by molten metal mixprocessing [62]. Figure 5 demonstrates the possibility ofproducing both uniform and ultrafme dispersions of TiB2 in Alalloys using a flux-assisted dispersion process [61].

3.6 Solidification processing

Directional solidification of superalloys to give highly alignedstructures or single crystals is now a part of the standardtechnology used to produce the compressor blades for the hightemperature zone in gas turbines for use in advanced aeroplanes[e.g. 64]. It is then possible to modify the base chemistry of thesuperalloy such that ceramic fibres (e.g. TiC) are producedduring the eutectic reaction [e.g. 28].

4. Secondary processing

The previous two main sections (2) and (3) have outlined themain primary processing steps used to fabricate MMCs. Asmentioned previously, in many cases a near net shape route isdesired so that the primary step represents the main step, as inthe case of using a pressure die casting route [e.g. 37].However, often there is a desire to use thermomechanicalprocessing steps on MMC billet (e.g. forging, rolling, extrusionand annealing) so as to be able to produce sections, etc., whichreplace those made in more conventional alloys and also so thatthe property advantages from thermomechanical treatments mayalso be exploited [e.g. 38, 65].

In terms of adjusting/optimising properties, the variouscomponents of the microstructure such as particles/fibres,precipiates/zones, grain size and dislocation substructure may allcontribute to the overall strengthening in an "additive" manner[e.g. 66]. Many of the working operations will help to align anyshort fibres within the microstructure but also to introduce somedegree of deformation texture into the matrix [e.g. 38].However, there will also be an interaction between the particlesand the matrix giving local deformation zones which will tend,in some cases, to randomise the texture [e.g. 38]. If these localdeformation zones do not cause particle-stimulated nucleations ofrecrystallisation then a stronger recrystallisation texture will result[e.g. 67].

It seems likely that the commercial development of MMCswill require considerable sophistication in the use ofthermochemical treatments, in order that properties may beoptimised.

5. Future perspectives

In terms of processing routes, this short overview hasdemonstrated that there is already a large number available forMMCs. However, as yet MMCs are only beginning to beexploited commercially. One reason for beginning this overviewfrom a treatment of polymer matrix composites (PMCs) was the

contrast in that PMCs are very widely used commercially and thisstatement also includes the expensive, advanced fibre reinforcedcomposites (AFRCs). In the caseofPMCs (including AFRCs) thedata bases and models linking processing to microstructure toproperties are well established and as yet this is not true forceramic matrix or metal matrix composites (CMCs and MMCs).However, this is a very active area of research and ourunderstanding of the processing/microstructure/property chain israpidly gathering momentum over such issues as stiffness strengthand toughness [e.g. 68-72] and fatigue [e.g. 59, 73-75].

Table 9 [from reference 29] demonstrates the wide range offibres available for reinforcement. MMCs have been made usingall of these and many particulate reinforcements but onlyrelatively rarely have the expected property enhancements beenfully realised. However, the "knowledge base" is improvingrapidly so that we may expect significant advances in theapplication of MMCs in the immediate future.

Acknowledgements

The authors are grateful to Dr. G.F. Fernando and Mr. C.Dometakis for valuable discussions.

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