Processing Nonequilibrium Composite (NMMC) by FSP

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]. Friction stir processing (FSP) is a solid state processing route that can offer the solution to incorporate metallic particles in their elemental form in aluminum. This chapter presents a detailed overeview of how metal particles are incorporated as reinforcement in an aluminum matix by FSP to process composites. Such composites are defined as nonequilibrium composites (NMMCs) since, the metallic particles cannot be retained in their elemental state in an aluminum matrix under equilibrium conditions. Two types of particle reinforcements, one having low solid solubility in aluminum (nickel and titanium) and other having high solid solubility (copper) are used as reinforcement in pure [GRAPHICS] aluminum matrix. The feasibility of processing nonequilibrium composite in a wrought aluminum alloy (AA5083) with tungsten (W) as reinforcement particles, is also shown. The effect of the metal particles on the microstructure evolution and the mechanical properties is discussed in detail. Finally, the thermal stability of the microstructure of the processed composites against abnormal grain growth (AGG) is evaluated and the role of the metal particles and their own thermal stability is discussed.