On the transition from pushing to engulfment during directional solidification of the particle-reinforced aluminum-based
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THE high strength/density ratio makes particle-reinforced metal-matrix composites (PRMMC) with precipitation-hardenable matrices, e.g., 2XXX, attractive for use in aircraft and automobile structural parts. Nevertheless, their large-scale application demands competitive costs compared to their unreinforced counterparts. The most effective way to achieve high-quality-low cost PRMMC products is via casting processes, which implies intrinsic understanding of the mechanisms of microstructural evolution during solidification, especially with respect to the distribution of the reinforcing particles. Throughout the last 30 years, the introduction of particles into the liquid matrix,[1,2] the stability of the liquid dispersion,[3,4] and the incorporation of the particles into the solidifying matrix[5–9] have been in the focus of research. Mechanisms responsible for particle agglomeration have been investigated and different mathematical models have been proposed for their quantitative description. With respect to the interaction between inert particles and the solidification interface of the matrix, investigations concentrate mainly on the development of models for planar morphology of the interface and on their experimental validation. The different models use the same physical concept of steady-state pushing of a particle, based on equilibrium between a repulsive force/disjoining pressure and the correspondingly induced viscous drag force. Consequently, the transition from pushing to engulfment of a particle occurs in a given system as soon as processing conditions, e.g., velocity of solidification and thermal gradient, result in an U. HECHT, Project Scientist, and S. REX, Senior Scientist, are with ACCESS e. V., D-52072 Aachen, Germany. Manuscript submitted August 29, 1995. METALLURGICAL AND MATERIALS TRANSACTIONS A
exceeding of the drag force relative to the repulsive force. When assuming solidification under constant thermal gradient, as in a Bridgman process, the transition criteria leads to the definition of a ‘‘critical velocity.’’ Different mathematical formulations of the forces and, moreover, different considerations upon the effects of particle induced thermal and solutal distortions result in a variety of different expressions for the critical velocity.[10] Apparently, the difficulty in verifying or applying the theoretical models by experiments resides in the fact that when performed with impure matrices, predictions made by ‘‘pure matrix’’ models must fail as the critical velocity suffers extreme reduction even through impurities in the parts per million amount if these show strong segregation tendencies.[5,6,7] By now, the lack of experimental data on this aspect is evident, rendering it impossible to decide whether the models developed for the interaction between a single particle and a planar interface can be transferred to the applicably relevant dendritic interface, at least on a local scale. Therefore, there is no evidence to what extent interdendritic alignment (network) of particles, as obse
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