When a liquid phase sintered 85Mo-15Ni alloy (by weight) is heat-treated at the sintering temperature in contact with Mo-W-Ni alloys, the nearly spherical grains of the Mo-Ni alloy become corrugated by dissolution of the initial solid at some grain surface regions and reprecipitation of Mo-W-Ni alloy at the adjacent surface regions. A W concentration gradient exists in the sintered specimen because of the slow diffusion of W and the reprecipitation of the W-containing alloy. The corrugation is apparently driven by the diffusional coherency strain at the dissolving surfaces and inhibited by the curvature developing at the solid-liquid interface. When the W content in its source alloy of Mo-W-Ni is relatively low at 10 %, the corrugation proceeds only momentarily, and the spherical grain shape is recovered as the intruding interface segments reverse their direction when they attain a critically high curvature. The reversal is attributed to the coherency breaking when the velocity of the intruding interface becomes critically low because of the increasing curvature. When the driving force for the corrugation is relatively high with 20% W in the source alloy, the corrugation can occur throughout the whole grains, producing single crystalline skeletons of Mo-W-Ni solid with the liquid matrix meandering through each. When Fe of 10-30% are added to the same 85Mo-15Ni specimen instead of W, the corrugation occurs. As Fe concentration increases further, the wavelength of corrugation becomes shorter and the velocity of intruding interface does faster, but the intruding interface does not continue to advance into the grain center. When the Fe content exceeds to 25%, chemically induced recrystallization (CIR), in which new grains nucleate at the surface of a sintered grain and grow into the interior of the sintered grain, is followed by corrugation, which forms a particular contrast to that with high W.
When a liquid phase sintered 90Mo-10Ni is heat-treated at 1400℃ after replacing its liquid matrix by Cu-10Fe melt, CIR occurs. The concentration profiles of Fe and Ni in a recrystallized grain show characteristic ones, which is characteristically determined by grain boundary diffusion. At 1540℃ the recrystallization stops to advance when the coherency strain is reduced to zero at the recrystallization front. The grain boundaries formed during recrystallization retreat backward to the original surface and the nucleated grains disappear completely, leaving behind regions with higher Fe and extremely lower Ni than the original grain. The growth of nucleated grains in CIR is also driven by the diffusional coherency strain energy.
To test a misfit hypothesis for nucleation in CIR, the misfit is controlled by varying the relative amounts of Fe and W with lattice parameters less and larger, respectively, than that of Mo. CIR occurs only at the central region with higher misfit and lower chemical free energy, while only chemically induced interface migration(CIIM) occurs at the outer region of the specimen with lower misfit and larger chemical free energy than that of center region, because of a W concentration gradient. Over the critical misfit occurrence of CIR is attributed to misfit dislocations which is formed after coherency breaking. The density of misfit dislocations required to nucleate CIR is calculated to be close to the critical dislocation density for recrystallization in the cold-worked solids. The result is that nucleation in CIR is induced by solute diffusion producing misfit dislocations after coherency breaking.
