Elucidation of Diffusion Blocking Effect During Anomalous Structural Change of L12+D022?L12 in Ni3Al1_xVx (0.4 < X < 0.55) Alloys by a First-Principles Calculation

The mechanical properties of Ni3Al1_xVx alloys (0.4 < x < 0.55) at elevated temperatures are deteriorated by the disappearance of D022-Ni3V regions even though two-phase equilibrium of L12-Ni3(Al, V) and D022-Ni3V is predicted from the phase diagram. This anomalous structural change of L12 + D022 -> L12 stems from the stagnation of V diffusion in the L12 matrix, but the mechanism of such a "diffusion blocking effect" has not been elucidated. In this study, a first-principles calculation was employed to quantify diffusion coefficients of Al and V in stoichiometric Ni3Al and V-doped Ni3Al from the intrinsic formation energies, migration energies and acti-vation energies required for their diffusion. The calculated diffusion coefficient of V was three orders of magnitude smaller than that of Al, and thus the shorter diffusion distance of V than the requisite diffusion length for the retention of D022 was found to be responsible for the diffusion blocking effect observed in the Ni3Al1_xVx alloys.