In-situ composites and nanocomposites cations in engineering are SiC whiskers/polycrystalline

11.3.2.4 In-situ composites and nanocomposites cations in engineering are SiC whiskers/polycrystalline

Alloy microstructures, such as eutectics and eutectoids, alumina (cutting-tool tips), graphite fibres/borosilicate

containing a dispersion of fibres or lamellae within a

374 Modern Physical Metallurgy and Materials Engineering matrix phase are well-known products of phase trans-

formation. They form an intimate combination of two or more quite different phases and, understandably, there has been strong interest in developing in-situ (self-assembled) composites which offer the advan- tages of the orthodox composites produced by adding

a matrix phase to preformed fibres. This new class of materials, with its micron-scale structures, offers the prospect of unique physical (electrical, thermal, magnetic), mechanical and chemical properties. Pro- vided that the alloy system is carefully chosen and manipulated, it is possible to produce unidirection- ally solidified structures with a uniform distribution of aligned fibres over extended distances. The prob- lem of fibre/matrix interaction so often associated with conventional fabrication routes is eliminated. Eutectic systems are immediately attractive because the solid phases produced simultaneously by this type of reac- tion are essentially in thermodynamic equilibrium and are held together by strong interfacial bonds. Fibres are preferred to lamellae because the latter are more likely to be weakened by the presence of transverse cracks.

In-situ composites are commonly produced by the classical laboratory technique in which molten alloy is withdrawn vertically downwards from a furnace held at a constant temperature. The rate of withdrawal is necessarily very low for some alloys ⊲<10 mm h ⊳ , increasing the risk that the materials of containment

will contaminate the melt. Because of the low produc- tion rates, costs tend to be high. As the eutectic reaction proceeds, solid phases nucleate and grow perpendicu- lar to the horizontal melt/eutectic interface in a cooper- ative manner. Interdiffusion of alloying elements takes place in the melt adjacent to this advancing interface. If both matrix and reinforcement phases have a low entropy of fusion, the eutectic structure grows in a uniform manner and the interface is essentially planar. Fortunately, most alloy systems of industrial interest tend to freeze with this morphology when solidifi- cation is closely controlled, even in cases where the reinforcing phase (e.g. carbide) has a known tendency to form crystallographically oriented facets because of its high entropy of fusion. When both phases have a high entropy of fusion, each phase has a strong ten- dency to form facets and the final structure is likely to be very irregular with much branching. This last combination is intrinsically unsuitable for producing in-situ composites.

Severe limitations on phase composition and phase ratio may apply to these melt-grown composites. For instance, binary alloy systems can often only develop

a relatively small volume fraction of a strong fibrous phase. In the Ni–Al system, which has been exten-

sively studied, the volume fraction of strong Al 3 Ni

filaments within the aluminium-based matrix is 0.1. (Beyond a certain volume fraction in a given system, laminae have a lower interfacial energy and tend to form in preference to fibres.) Complex ternary alloy systems are more promising: a monovariant eutectic

reaction at a ‘valley’ line in the liquidus surface of the Co–Cr–C system can produce 30% v/v carbide fibres in a solid solution matrix of cobalt and chromium. For high-temperature applications, such as turbine blades in aero engines, in-situ composites can compete with directionally-solidified (DS) nickel-based superal- loys and single-crystal superalloys (e.g. stress-rupture properties in directions parallel to the reinforcement). Aligned two-phase structures can be grown from the alloy melt; phase compositions can be varied quite widely without preventing such structures from form- ing. There is also valuable scope for alloying additions (Cr, Al, W, Re) which enhance properties such as resistance to oxidation, corrosion, creep and thermal fatigue. Examples are the NITAC and COTAC series of eutectic superalloys, which are based upon nickel and cobalt, respectively. In typical NITAC alloy, the ductile matrix consists of -Ni and a fine precipitate of

Ni 3 Al particles (gamma-prime phase, 0 ), and is rein- forced with a carbide phase (e.g. TaC, Cr 3 C 2 , NbC). In service, the integrity of in-situ composites can be jeopardized by oxidative attack, temperature gradients and/or thermal cycling. The last condition is potentially disruptive because a mismatch in the thermal expan- sion characteristics of the two phases will cause stiff fibres to fracture and the ductile matrix phase to fail by fatigue.

In the late 1980s, researchers began to extend the composite principle beyond the micron scale and into the nanometre range (1 –50 nm). The ‘jumps’ in vari- ous properties achieved with these new nanocomposite microstructures were considerable. An early prototype was a nominally single-phase ceramic structure based upon a conventional polycrystalline matrix of micron- sized alumina grains. A uniform dispersion of 5% sili- con carbide inclusions, about 10 nm in size, throughout this matrix gives a fourfold increase in strength. The processing route for this ceramic is based upon estab- lished sintering technology: the main stages are the

Figure 11.21 SiC inclusion/alumina nanocomposite (from Brook and MacKenzie, Jan 1993, pp. 27–30; by permission of the Institute of Materials) .

Plastics and composites 375

Feest, E. A. (1988). Exploitation of the metal-matrix compos- ders, hot-pressing ⊲1750 ° C⊳ and annealing ⊲1300 ° C⊳.

grinding and mixing of fine ˇ-SiC and ˛-Al 2 O 3 pow-

ites concept. Metals and Materials, May, 273–278, Insti- The structural character of the ceramic is shown

tute of Materials.

schematically in Figure 11.21. It has been proposed Harris, B. (1986). Engineering Composite Materials. Institute that the large number of dislocations present form a

of Metals, London.

sub-structure within each grain, with SiC particles at Hertzberg, R. W. (1989). Deformation and Fracture Mecha- nics of Engineering Materials , 3rd edn. Wiley, Chichester. the nodal points. In general terms, this combination of

Hughes, J. D. H. (1986). Metals and Materials, June, pp. micron-sized grains and much finer nanometre-scale

365–368, Institute of Materials. sub-structures gives exceptional strength and, impor-

Hull, D. (1981). An Introduction to Composite Materials. tantly in the case of ceramics, good reproducibility.

Cambridge University Press, Cambridge. Study of nanoscale structures has been extended to

Imperial Chemical Industries, Plastics Division (1974). Ther-

a wide variety of metallic systems. Again, fabrication moplastics: Properties and Design , (ed. R. M. Ogor- is based on pure, ultrafine powders. These powders

kiewicz), Chap. 11 by P. C. Powell, Wiley, Chichester. are mainly produced by either condensation from a

Kelly, A. (1986) Strong Solids. Clarendon Press, Oxford. vapour, which minimizes the risk of contamination, or King, J. E. (1989). Metals and Materials, 720–6. Institute of

Materials.

high-energy ball-milling. Two or more metal powders Lemkey, F. D. (1984). Advanced in situ composites. In are compacted at high pressure (1 –5 GN m ) to

Chap. 14, Industrial Materials Science and Engineering give a unique nanometre-scale grain structure. An

(ed. L. E. Murr). Marcel Dekker. extremely large proportion of atoms are sited at or near

Mascia, L. (1989). Thermoplastics: Materials Engineering. grain boundary surfaces and properties dependent upon

2nd edn, Elsevier Applied Science, London. grain boundary characteristics come into prominence.

McLean, M. (1983). Directionally-Solidified Materials for For instance, these multiphase metallic nanostructures

High-Temperature Service . Metals Society, London. Metals and Materials are stable and highly resistant to grain growth. The . (1986). Set of articles on composites,

Institute of Materials: