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2 µm - eTheses Repository - University of Birmingham

2 µm - eTheses Repository - University of Birmingham

The main factors

The main factors controlling the elastic modulus of MMCs are the reinforcement aspect ratio and volume fraction fo the reinforcement. The presence of interfacial reaction layers can lower the transverse moduli of many composites. Young’s modulus is one of the properties least sensitive to microstructural features. It has been shown that the simple rule of mixture approach is not valid for estimating the effective modulus of particle reinforced MMC. Hashin and Shtrikman (55) proposed upper and lower bounds for prediction of a composite Young’s modulus Ec with a reinforcement volume fraction Vf: E 1 + E 2 1 − E V 1 f 1−V + 3E 1 f ≤ E c ≤ E 21 2 + E 1 1−V 1 − E 2 f V f + 3E where E1 and E2 are the moduli of the matrix alloy and the reinforcement, respectively. 2 Equation 5 As an alternative to analytical modelling, numerical techniques such as FEM have become increasingly popular. One common approach is to use a unit cell model, where one or more reinforcement particles are embedded within the Al matrix, to simulate a composite material with a periodic array of reinforcement. A unit cylindrical particle has a greater stiffening effect than a spherical particle. As reported by Feest et al. (56) , the elastic modulus and thermal conductivity values remain relatively constant at particulate diameters greater than 10-15 µm. However, below this threshold, the modulus increases and thermal conductivity decreases as the diameter decreases. Two possible explanations are matrix microstructural refinement and increased surface area for interfacial reactions. As a result, poor correlations between predicted and measured property values may occur for small diameter reinforcements and require the use of specific bounds.

General models to predict fracture toughness have been shown to be inappropriate in many MMCs (57) . It is becoming recognized that, for composites, toughness is an engineering concept and not a material property (58) . Clustering of reinforcements is a typical manufacturing problem. Studies suggest that global thermomechanical effective properties are generally unaffected but properties such as fracture toughness, where crack initiation is a function of the local stress, are influenced by inhomogeneous distributions (59) . Much of the driving force behind the development of MMCs has been that monolithic lightweight alloys have inadequate fatigue resistance for many demanding applications. MMC fatigue failure, which is governed by the growth of very small microcracks, is also not well described by linear elastic fracture mechanics (LEFM). The single most important factor is the reinforcement volume fraction. For particulate-reinforced composites, the matrix properties dominate at low volume fraction, and the reinforcement properties dominate at high (41) . The shape or aspect ratio and alignment of the reinforcements are also important factors. For random reinforcement orientations, a minor difference in the predicted property value is seen between different reinforcement shapes, particularly at low volume fractions. The use of high stiffness ceramic reinforcement in particulate form, such as Al2O3, can result in a substantial increase in fatigue resistance. Several studies have shown that both, increasing volume fraction and decreasing particle size, result in enhanced fatigue resistance. In a composite, most of the load is carried by the high modulus, high strength reinforcement so, for a given stress, the composite undergoes a lower average strain than the unreinforced alloy. Thus the fatigue lives of particle-reinforced metal matrix composites are generally longer than those of unreinforced metals (42) . For a given reinforcement volume fraction, the reinforcement interparticle spacing decreases with decreasing particle size, resulting in more barriers for the reversible slip motion that takes place during fatigue, and a decrease in strain localization by cyclic slip refinement. Above a critical particle size, reinforcement fracture is predominant 22

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