First-principles study of self- and solute diffusion mechanisms inγ′-Ni3Al

Abstract

Multiple, competing diffusion mechanisms have been postulated in L1${}_{2}$ ordered compounds such as Ni${}_{3}$Al due to the presence of two distinct sublattices for self- and solute diffusion. They involve intrasublattice, intersublattice, two-pair vacancies, the six-jump cycle, and the antisite assisted jumps. In this work we systematically study these diffusion paths in L1${}_{2}$ ordered Ni${}_{3}$Al, postulated from three decades of experiments, by coupling density functional theory and the nudged elastic band methods. We determine activation barriers for self-diffusion (Ni and Al) atoms and solute diffusion (Cr, Co, and Ti) atoms, and also explore how solutes influence self-diffusion activation barriers. These calculations reveal that Ni vacancies mediate both self- and solute diffusion in ordered Ni${}_{3}$Al. Other findings include: (i) Ni atoms diffuse in their own sublattice via nearest neighbor jumps, (ii) Al atoms also preferentially diffuse in the Ni sublattice, (iii) solutes in the proximity of vacancies strongly affect the formation and also the migration energies in problems studied here, and (iv) diffusion mechanism(s) of solutes depend strongly on whether they occupy a Ni or Al site. This site occupancy is predicted by our activation barrier calculations and agrees with experiments. Our calculations reveal that antisite assisted solute diffusion has the lowest activation barrier. However, experiments report a higher activation barrier that correspond to the barriers for the six jump cycle mechanism determined by our calculations. Thus, our calculations together with published experimental work point to multiple, competing mechanisms driving the diffusion of ternary elements in Ni${}_{3}$Al.