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Phase diagrams and the lever rule

When a Cu-Ni alloy is solidified with rapid cooling, as in actual casting, which correctly describes the composition at the center of a dendrite?

  1. ARicher in Ni than the average alloy composition
  2. BRicher in Cu than the average alloy composition, with the outer parts richer in Ni
  3. CAlways equal to the average composition, because diffusion in the solid is fast enough
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Answer: A. Richer in Ni than the average alloy composition

The solid that forms early in solidification has the composition at the end of the higher-temperature tie line, so it is rich in Ni. With rapid cooling, diffusion in the solid cannot keep up, and the outer layers that solidify later are progressively richer in Cu, leaving segregation in which the composition changes from the center of the dendrite outward (a cored structure, or coring). The center is therefore richer in Ni than the average composition. Diffusion in the solid is far slower than in the liquid, so the equilibrium composition is not necessarily reached. The last regions to solidify have a lower melting point and may melt locally on reheating, so a homogenizing anneal is used to reduce the segregation.

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