Vacancy Diffusion in Multi-Principal Element Alloys: The Role of Chemical Disorder in the Ordered Lattice

Abstract

Many of the purported virtues of Multi-Principal Element Alloys (MPEAs), such as corrosion, high-temperature oxidation and irradiation resistance, are highly sensitive to vacancy diffusivity. Similarly, solute interdiffusion is governed by vacancy diffusion -it is often unclear whether MPEAs are truly stable, or effectively stabilized by slow interdiffusion. The considerable composition space afforded to these alloys makes optimizing for desired properties a daunting task; theoretical and computational tools are necessary to guide alloy development. For diffusion, such tools depend on both a knowledge of the vacancy migration barriers within a given alloy and an understanding of how these barriers influence vacancy diffusivity. We present a generalized theory of vacancy diffusion in rugged energy landscapes, paired with Kinetic Monte Carlo simulations of MPEA vacancy diffusion. The barrier energy statistics are informed by nudged elastic band calculations in the equiatomic CoNiCrFeMn alloy. Theory and simulations show that vacancy diffusion in solid-solution MPEAs is not necessarily sluggish, but can potentially be tuned, and that trap models are an insufficient explanation for sluggish diffusion in the CoNiCrFeMn MPEA. These results also show that any model that endeavors to faithfully represent diffusion-related phenomena must account for the full nature of the energy landscape, not just the migration barriers.

Mechanism of Local Lattice Distortion Effects on Vacancy Migration Barriers in FCC Alloys

“…• 2-Atom cluster (different symmetry) set #1: [8,6], [8,12], [9,7], [9,13], [10,7], [10,13], [11,6], [11,12];…”

“…• 2-Atom cluster (different symmetry) set #3: [6, 2], [6,5], [7,3], [7,4], [12,14], [12,17], [13,15], [13,16]; [14,18], [15,18], [16,18], [17,18].…”

“…Diffusion kinetics in metallic alloys and associated material mechanisms (e.g., aging), which control properties such as strength and ductility, are critically dependent upon vacancy-mediated migration of matrix atoms and substitutional solutes [1,2]. A migrating species in an alloy encounters complex and varying local chemical environments, especially in multicomponent alloys, which in turn change the energy barrier ∆E a of a vacancy migration event between two adjacent lattice sites [3][4][5][6][7]. Accurate descriptions of such local chemical effects on ∆E a are necessary to construct the kinetic master equations in mesoscale methods, such as kinetic Monte Carlo (kMC) [8][9][10][11], phase-field crystal (PFC) [12,13], and diffusive molecular dynamics (DMD) simulations [14], to study diffusion and precipitation.…”