A critical test for the coherency strain effect on liquid film and grain boundary migration in Mo-Ni-(Co-Sn) alloy

A critical experiment has been performed to test the coherency strain hypothesis for the chemically induced migration of liquid films and grain boundaries. The liquid films and grain boundaries in liquid phase sintered 95Mo-5Ni alloys migrate when Co is added to the liquid matrix. Behind the migrating boundaries form Mo-Ni-Co solid solutions. The migration also occurs when Sn is added to the liquid matrix. Because of the diluting effect of Sn, the Ni concentration in the solid formed behind the migrating boundaries is lower than that in the initial Mo-Ni alloy. By adding both Co and Sn to the liquid matrix at different ratios, the coherency strain in the diffusion zone ahead of the moving boundaries can be varied from negative to positive. When the coherency strain is 0, the migration velocity becomes 0, although the free energy of mixing is finite. The results show definitively that the driving force for the liquid film and grain boundary migration is the coherency strain energy as proposed by Hillert.