(The) interface migration and instability inducedby diffusional coherency strain in Mo-Ni alloy

Abstract

Diffusion in solid can cause strains to develop in the diffusion zone when the lattice parameter is a function of composition. The coherency strain effect on the chemically induced interface migration and morphological instability of solid-liquid interface has been investigated using the liquid phase sintering technique. In chapter one the concept of the coherency strain and the theory of liquid film migration based on coherency strain hypothesis are introduced. Also, the concept of coherency breaking and coherency strain in ternary alloy are explained. In chapter two and three, the hypothesis of coherency strain energy as the driving force for chemically induced migration of liquid film (LFM) and grain boundary (CIGM) has been tested. The liquid films and grain boundaries in liquid phase sintered 95Mo-5Ni and 85Mo-15Ni respectively alloys migrate when Co is added to the liquid matrix. Behind the migrating obundaries 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. Under a constant driving force the average migration distance increases linearly with heat-treatment time. When Co and Sn are simultaneously added at different ratios. the coherency strain in the diffusion zone ahead of the moving boundaries is expected to vary from negative to positive, and the average migration rates are observed to vary nearly parabolically with the estimated coherency strain, becoming 0 in the range where the strain is 0. The result definitively shows that the coherency strain energy is the driving force for LFM and CIGM. The conditions under which the coherency breaks and hence no migration occurs are displayed by a coherency map. A kinetic model of CIGM based on the coherency strain hypothesis has been also tested experimenally in chapter four by comparing the observed magrati...