A critical test for the coherency strain energy as the driving force for the discontinuous precipitation in Mo-Ni alloy
When 90Mo-10Ni alloys prepared by liquid phase sintering are heat-treated at 1300°C, discontinuous precipitation of MoNi phase in the form of discrete particles or rods occurs in the Mo-Ni grains. In order to test the diffusional coherency strain hypothesis as the driving force for the discontinuous precipitation, Fe has been added at varying concentrations to the matrix of the liquid phase sintered MoNi specimens before the heat-treatment at 1300°C. The discontinuous precipitation is observed to be suppressed by the grain boundary diffusion of Fe and does not occur at all when the Fe concentration in the solidified matrix is increased to a certain range, where the coherency strain energy is estimated to be close to the minimum value of 0. The result thus shows that the coherency strain energy is the driving force for the discontinuous precipitation in this alloy. When the Fe concentration in the matrix is high, the chemically induced grain boundary migration (commonly referred to as DIGM) occurs due to the predominant effect of Fe diffusion, producing a tensile coherency stress.