Activation volume dominated diffusivity of Ni₅₀Al₅₀ melt under extreme conditions

In this work, ab initio molecular dynamic simulations investigate diffusivities of Ni and Al in Ni₅₀Al₅₀ melt under high pressures. While the tracer diffusion coefficients at the equilibrium state predicted by Green-Kubo and the Einstein equations follow the Arrhenius relationship and agree well with the experimental and theoretical results, a linear relationship between the logarithm of tracer diffusion coefficients and the external pressure is addressed in term of an activation volume modified Arrhenius relations. It is understood that the autocorrelation velocity functions (VACF) characterizes the capacity of an atom memorizing the velocity, which has also been considered as one of important approaches revealing thermodynamics, kinetics and structure relaxation dynamics of melts. The onset of the negative region of VACF is in line with the plateau of the meaning square displacement representing the cage effect of the system. The initial damping oscillations of VACFs are due to the momentum relaxation caused by the collision between the tagged atoms and its neighbor ones while the subsequent rises result from the energy relaxation. This work provides an insight into the physical factors controlling these dynamic behaviors under extreme conditions of Ni-Al, which could be extended to the other advanced materials.