The discontinuous precipitation of a liquid phase in MoNi induced by diffusional coherency strain

Abstract

When a liquid phase sintered MoNi alloy is heat-treated at a temperature lower than that used for sintering where the solid and liquid phases coexist, the liquid films and grain boundaries between the grains migrate, leaving behind a new solid solution somewhat depleted of Ni. Since some liquid is produced during this migration, it resembles a discontinuous precipitation of the liquid phase. It is demonstrated experimentally that the driving force for this discontinuous precipitation arises from the coherency strain produced by Ni atom diffusion out of the grains. When two solute atom species simultaneously diffuse into or out of crystals in a ternary system, the resulting coherency strain depends on the size and concentration of the diffusing atoms and can be varied independently of the free energy of mixing. In particular, the coherency strain can be reduced to zero when the strain effects of the two atomic species exactly cancel each other. In this study, series of 90Mo10Ni alloy (by wt%) have been prepared by liquid phase sintering at temperatures between 1400 and 1520°C in order to produce solid MoNi grains of varying Ni concentration that are in equilibrium with the surrounding liquid matrix. The migration of liquid films and grain boundaries in the sintered specimens is induced by heat-treating them at 1400°C after adding various amounts of Fe to the liquid matrix. The migration does not occur when the estimated coherency strain is close to 0, although the free energy of mixing is finite. This result is a definitive demonstration that the driving force for the steady state migration is the coherency strain energy. The evidence that the grain boundaries exist in the sintered specimens and remain as grain boundaries during the migration is discussed.