Microstructure evolution in the phase separation of the Ni ₃Al_0.40v_0.60 alloy

It has been found that phase separation, i.e., D0₂₂ precipitation in the supersaturated L1₂ matrix to form the L1 ₂ + D0₂₂ equilibrium state, stagnates in the Ni ₃Al_0.52V_0.48 alloy under a certain thermodynamic condition. This stagnation originates from the suppression of the long-range vanadium diffusion in the LI: 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 Ni₃Al_0.40 V_0.60 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 20nm 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 L1 ₂ cuboidal domains. A shorter migration length than that in the case of the Ni₃Al_05.2V_0.48 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 Ni ₃Al_0.40V_0.60 alloy was attributed to the reduction in the size of the cuboidal domains in the initial L1₂ matrix.