Chapter 4 - Processing Nonequilibrium Composite (NMMC) by FSP

Abstract

Metal matrix composites (MMCs) combine the properties of two different materials to produce a material with superior mechanical properties. Table 4.1 shows the mechanical properties of some of the Al based MMCs. One of the major shortcomings of MMCs, as can be seen in Table 4.1 as well, is the low ductility which arises out of various reasons related primarily to the hard and brittle reinforcement [1–5]. If the primary reason for this be the brittle ceramic particles, can there be alternative reinforcements that can prevent this embrittlement? Harder metallic particles may be the answer. However, the equilibrium phase diagrams say that the metallic particles will either dissolve to form solid solution or react with aluminum to form intermetallic compounds if they have low solubility. Metals can be classified into two groups based on their solid solubility in aluminum as shown in Table 4.2 [6]. Metals from the low solubility group have to be chosen as reinforcement particles as they do not dissolve in aluminum. Ni, Ti, and W are attractive choices as reinforcements because of their higher strength and stiffness compared to aluminum. However, owing to their low solubility such metals will react and form brittle intermetallics when processed in equilibrium conditions by conventional routes. Moreover, the reaction is exothermic in nature that leads to rapid reaction kinetics [7]. Therefore the conventional composite processing routes such as powder metallurgy (PM) and stir casting cannot be used to incorporate metallic particles in aluminum [8–10].