Influence of Defect Thermodynamics on Self-Diffusion in Complex Concentrated Alloys with Chemical Ordering

Abstract

Self-diffusion kinetics in crystalline alloys depend on both equilibrium concentrations and diffusivities of point defects; mostly vacancy defects because of their lower formation energy than interstitial defects. In complex concentrated alloys (CCAs), which are often perceived as random solid solutions, chemical ordering, i.e., preferential bonding between certain groups of elements, generally exists, particularly at low temperatures. Such chemical ordering is expected to affect both point defect thermodynamics and self-diffusion kinetics in CCAs. Using hybrid molecular dynamics and Monte Carlo simulations and taking NiCoCr, HfNbTaZr and CoCuFeNiPd as model alloys, we show that chemical ordering affects the probability and spatial distributions of both vacancy formation energy and migration barrier. A negative correlation between vacancy formation energy and migration barrier is also identified. The probability distribution of formation energy determines the equilibrium vacancy concentration. The spatial distribution determines the diffusion path of vacancies, which preferentially consists of atomic sites with low formation energies. The development of short-range ordering can either increase or decrease self-diffusion when equilibrium vacancy concentration is considered. These findings highlight the importance of defect thermodynamics for understanding self-diffusion in CCAs with chemical ordering.