GRAIN-BOUNDARY MIGRATION WITH PRECIPITATION AND DISSOLUTION OF A LIQUID-PHASE IN MO-NI ALLOY

When 85Mo-15Ni (by weight) alloys prepared by liquid phase sintering at 1380-degrees-C are heat-treated at 1520-degrees-C, the grain boundaries and liquid films between the grains migrate, leaving behind them a new Mo-Ni solid solution with Ni content higher than that in the initial solid formed during the liquid phase sintering treatment. The grain boundaries migrating during this discontinuous dissolution of the liquid phase have a flat shape. When the temperature change is reversed by first sintering at 1520-degrees-C and subsequently heat-treating at 1380-degrees-C, the grain boundaries and liquid films again migrate, with the composition change of the solid reversed. Liquid precipitates form at grain boundaries and migrate with them, their size and total volume increasing during the migration. The grain boundaries have a curved shape and their initial migration rate is considerably higher than that during the discontinuous dissolution. This contrasting migration behaviour between the discontinuous dissolution and precipitation is attributed to the coherency strain energy operating locally on both grain boundary segments and liquid droplets, and to the high mobility of fine liquid droplets.