Ultra-high temperature diffusion in multi-principal element alloys: Experiment, simulation and theory

Multi-principal element alloys (MPEAs) have garnered significant attention due to their exceptional performance under extreme conditions such as high temperatures and irradiation, yet their diffusion behavior and mechanisms at elevated temperatures remain elusive. In this work, we investigate the diffusivity of a Ni_xCoCr alloy system under high temperature conditions as a model for MPEAs. Our findings reveal that, the alloys with high mixing entropy exhibit unexpectedly diffusivity at ultra-high temperatures, challenging the conventional wisdom that diffusion in high-entropy alloys is typically sluggish. Based on tight-binging model, it is revealed that severe lattice distortion and electron interaction in high-entropy systems markedly weaken the atomic bonding strength. This phenomenon significantly reduces the vacancy formation energy and substantially increases the vacancy concentration especially at high temperature, thereby counteracting the inhibitory effect of reduced vacancy jump frequency on diffusion due to lattice distortion. This discovery not only provides new insights into the diffusion mechanisms of high-entropy alloys under extreme conditions but also holds significant implications for the design and optimization of high-performance materials suitable for extreme environments.