Grain boundary diffusion in compositionally complex alloys: A comprehensive review

High-entropy alloys (HEAs) have gained substantial attention over the past two decades, necessitating a comprehensive understanding of their intrinsic and extrinsic properties, including mechanical behavior, creep resistance, phase stability, environmental degradation, etc. Among these, atomic transport mechanisms, particularly along grain boundaries (GBs) play a pivotal role in determining material performance. This review critically evaluates the “sluggish” diffusion concept, focusing on its validity and applicability to GB diffusion in HEAs. It examines the influences of GB complexions, segregation phenomena, and precipitation processes on GB diffusion behavior in HEAs, comparing them with their counterparts in conventional binary and ternary alloys, both dilute and concentrated. The inherent challenges in accurately characterizing GB diffusion in multi-principal element alloys, given their broad compositional variability and complex microstructures are also highlighted. The contribution of inter-phase boundary diffusion in multi-component alloy systems is also identified and analyzed. Furthermore, the broader implications of GB diffusion on the mechanical and physical properties of polycrystalline HEAs are discussed in terms of their strength, ductility, and degradation resistance. By consolidating the current state of research on GB diffusion in HEAs and identifying the key research gaps, this review aims to catalyze focused and intensive research efforts into diffusion-related phenomena in HEAs and other compositionally complex alloys. Emphasis is placed on comprehensive understanding the interplay between GB structure, chemistry, and atomic transport phenomena to enable effective GB engineering stategies for these alloys. Insights from such studies will be instrumental in optimizing HEAs for advanced technological applications and in guiding the development of next-generation high-performance materials.