Diffusion blocking, path variability, and bifurcation of the final state in the phase separation of Ni3 (Al,V)1-δ alloys

When a diffusion field has a regular atomic and/or vacancy configuration, diffusion blocking acts as a nonlinear perturbation in the diffusion kinetics and its appearance should substantially influence the temporal evolution of the macroscopic state in a diffusion-controlled phenomenon. In this study, we systematically review, on the basis of experimental results, the kinetic process of the L 12 →L 12 +D 022 phase separation of Ni3 (Al,V)1-δ alloys, which is dominated by long-range atomic diffusion in the diffusion field with the L 12 ordered structure. Our results demonstrate that diffusion blocking actually dominates the diffusion kinetics when the size of the L 12 single domains, in other words, the migration length of atoms in the L 12 diffusion field, is larger than a certain critical value. Therefore, the state with the critical domain size becomes a branching point in the evolution of the macroscopic state because of the change in the diffusion kinetics. We have also found that, under the diffusion-blocking mode, the vanadium (V) content of the domain boundaries accumulated during the evolution from the initial to the branching state directly varies the kinetic path, causing a bifurcation of the final (L 12 +D 022 or L 12 single) state. These results imply that the formation of the final state should be explainable based on the kinetic description, not on the free-energy description, from the viewpoints of the blocking effect and the path variability in the kinetic process. This conversion of the description represents the discrepancy between the ensemble and time averages of the system in the present state change.