Diffusion kinetics originating a bifurcation of the final states in the phase separation of the Ni₃Al_1-xV_x alloy system

The evolution of states related to D0₂₂ precipitation in the supersaturated L1₂ matrix to form the L1₂+D0₂₂ equilibrium state of the Ni₃Al_1-xV_x (0.40≤x≤0.60) alloy system was examined by transmission electron microscopy. Our results revealed that the microstructure of the initial L1 ₂ single state varied with the change in the alloy composition and that such a variation caused a bifurcation of the final states, i.e., the L1₂+D0₂₂ equilibrium state (x ≤ 0.40 and x≥0.55) and the metastable L1₂ single state (0.40<x<0.55). It was also found that three types of kinetic processes originated from the variation of the initial microstructure and led to the final bifurcation. A key factor causing the difference in the kinetic processes was thought to be the sensitivity of the atomic migration paths to a decrease in the crucial vacant sites in the L1₂ matrix from supersaturated to equilibrium ones during the isothermal process. The final bifurcation due to such sensitivity strongly suggests that the diffusion kinetics should be treated as an 'open system' in terms of the annihilation of vacancies under the present thermodynamic conditions.