Microstructure Evolution in the Phase Separation of the Ni3Al0:40V0:60 Alloy

It has been found that phase separation, i.e., D022 precipitation in the supersaturated L12 matrix to form the L12 þ D022 equilibrium state, stagnates in the Ni3Al0:52V0:48 alloy under a certain thermodynamic condition. This stagnation originates from the suppression of the long-range vanadium diffusion in the L12 matrix, so-called diffusion blocking. Because diffusion blocking is inherent to the L12 structure, its occurrence depends largely on the morphological features of the L12 matrix. In this study, the microstructure evolution during the phase separation of the Ni3Al0:40V0:60 alloy was examined from the viewpoint of the relation between the variation in the initial microstructure and the appearance of the effect of diffusion blocking. Our results showed that cuboidal domains of about 30 nm on a side formed in the initial L12 matrix and that the D022 regions appeared at the domain boundaries. The microstructure evolution of the alloy was found to proceed via the rearrangement, combination, and growth of these D022 regions, accompanying vanadium migration of about 20 nm in the L12 matrix. The requisite migration length was determined by the size and density of the initial D022 regions, which depend on the size of the L12 cuboidal domains. A shorter migration length than that in the case of the Ni3Al0:52V0:48 alloy was presumed to be advantageous to concealing the effect of diffusion blocking. On that basis, it was concluded that the occurrence of the phase separation in the Ni3Al0:40V0:60 alloy was attributed to the reduction in the size of the cuboidal domains in the initial L12 matrix.