Macrosegregation in quiescent melting and liquid-phase sintering

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Macrosegregation during melting of alloys can arise if the densities of the alloy components differ. In particular, if the higher-melting component is not present in amounts sufficient to develop an interconnected structure, this component can sink (assuming it is more dense than the liquid) to the crucible bottom during melting. This produces a liquid head consisting of (to a first approximation) the pure lower-melting component. The resulting macrosegregation takes considerable time to eliminate since it exists over a macroscopic distance (that over which the particles sink). Similar considerations apply to liquid-phase sintering, also treated in this article. The tendencies for macrosegregation and the subsequent structure evolution in melted/sintered materials can be summarized in a melting map. Such a map has axes of alloy overall composition and alloy liquidus composition. The development and elimination of macrosegregation proceeds in stages. Following initial particle settling, the liquid in the liquid-plus-solid zone near the crucible bottom, which was generated by the settling, attains its equilibrium composition. Finally, elimination of the long-range liquid concentration gradient takes place. If the material has a composition corresponding to a liquid-phase-sintered alloy, the solid in the liquid-plus-solid zone then attains its equilibrium composition. The times for the various stages can be estimated simply. Melting maps can be refined by placing contours of equilibration time on them.

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Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science