Particle reinforced metal matrix composites are now being produced commerically, and in this paper the current status of these materials is reviewed. The different types of reinforcement being used, together with the alternative processing methods, are discussed. Depending on the initial processing method, different factors have to be taken into consideration to produce a high quality billet. With powder metallurgy processing, the composition of the matrix and the type of reinforcement are independent of one another. However, in molten metal processing they are intimately linked in terms of the different reactivities which occur between reinforcement and matrix in the molten state. The factors controlling the distribution of reinforcement are also dependent on the initial processing method. Secondary fabrication methods, such as extrusion and rolling, are essential in processing composites produced by powder metallurgy, since they are required to consolidate the composite fully. Other methods, suc...

Particle reinforced aluminium and magnesium matrix composites

The physical and mechanical properties that can be obtained with metal matrix composites (MMCs) have made them attractive candidate materials for aerospace, automotive and numerous other applications. More recently, particulate reinforced MMCs have attracted considerable attention as a result of their relatively low costs and characteristic isotropic properties. Reinforcement materials include carbides, nitrides and oxides. In an effort to optimize the structure and properties of particulate reinforced MMCs various processing techniques have evolved over the last 20 years. The processing methods utilized to manufacture particulate reinforced MMCs can be grouped depending on the temperature of the metallic matrix during processing. Accordingly, the processes can be classified into three categories: (a) liquid phase processes, (b) solid state processes, and (c) two phase (solid-liquid) processes. Regarding physical properties, strengthening in metal matrix composites has been related to dislocations of a very high density in the matrix originating from differential thermal contraction, geometrical constraints and plastic deformation during processing.

Particulate reinforced metal matrix composites — a review

Metal Matrix Composites

Crack tip shielding phenomena, whereby the “effective crack-driving force” actually experienced at the crack tip is locally reduced, are examined with reference to fatigue crack propagation behavior in metals, composites and ceramics. Sources of shielding are briefly described in terms of mechanisms relying on the production of elastically constrained zones which envelop the crack (zone shielding), on the generation of wedging, bridging or sliding forces between the crack surfaces (contact shielding) and on crack path deflection and meandering. Examples are taken from the fatigue behavior of high strength lithium-containing aluminum alloys, aluminum alloy-aramid fiber-epoxy laminate composites, and zirconia ceramics. It is shown that, whereas crack tip shielding can provide a potent means of enhancing “resistance” to crack growth, such extrinsic toughening mechanisms can result in the apparently anomalous behavior of “small cracks” and to the susceptibility of brittle materials to fatigue failure.

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Mechanisms of fatigue crack propagation in metals, ceramics and composites: Role of crack tip shielding☆

Micro-mechanisms of crack-tip shielding associated with the growth of fatigue cracks in metalmatrix composites are examined with specific emphasis on the role of crack bridging by uncracked ligaments. Simple analytical models are developed for such bridging induced by both overlapping cracks and by coplanar ligaments in the wake of the crack tip; the models are based on respective notions of a critical tensile strain or critical crack-opening displacement in the ligament. The predicted degree of shielding derived from these mechanisms is not large, but is found to be consistent with experimental observations in high-strength P/M aluminum alloys reinforced with 15 to 20 vol pct of SiC particulate.

Crack bridging by uncracked ligaments during fatigue-crack growth in SiC-reinforced aluminum-alloy composites

A study has been made of the mechanistic role of silicon carbide (SiC) particles during fatigue crack propagation in powder metallurgy AlZnMgCu metal matrix composites reinforced with 20 vol.% SiC particulates (SiC p ), with varying sizes of reinforcement phase. Crack growth and accompanying crack tip shielding (principally by crack deflection, closure and bridging) are examined in peak-aged alloys over a wide spectrum of growth rates from 10 −12 to 10 −4 m cycle −1 and are compared with corresponding behavior in the unreinforced matrix alloy. Crack growth resistance in the composites is found to be both superior and inferior to that of the unreinforced alloy, depending on how the SiC and SiC-matrix interface fracture. At low stress intensity ranges ΔK, the predominant fracture of carbides close to the crack tip results in low levels of crack closure and rapid growth kinetics for fine SiC p distributions, whereas for coarse SiC p distributions the rougher fracture surface promotes crack closure from asperity wedging and improved crack growth resistance. With increasing ΔK, the fracture of large carbides farther ahead of the crack tip leads to the development of non-uniform crack fronts, thereby promoting crack bridging via uncracked ligaments and a small improvement in crack growth resistance. At high ΔK levels approaching K Ic , growth rates are faster in the composites owing to their low toughness. On the basis of these results, the overall fatigue crack growth performance of the SiC p Al composites compared with that of the traditional monolithic aluminum alloys is briefly discussed.

Weikang Yu

Papers

Role of silicon carbide particles in fatigue crack growth in SiC-particulate-reinforced aluminum alloy composites