Fundamental Core Effects in Transition Metal High-Entropy Alloys: “High-Entropy” and “Sluggish Diffusion” Effects

High entropy alloys (HEAs) are equimolar multi-principal-element alloys (MPEAs) that are different from traditional solvent-based multicomponent alloys based on the concept of alloy design. Based on initial work by Yeh and co-workers, HEAs were postulated to exhibit four “core” effects: high entropy, sluggish diffusion, lattice distortion, and cocktail effect. Out of these four proposed core effects, “high entropy” and “sluggish diffusion” effects were most debated in the literature as these core effects directly affect the thermodynamic and kinetic understanding of HEAs. The initial work on HEAs by several researchers utilized these effects to indirectly support the experimentally observed “unique” properties, without independent investigation of these core effects. The presumed implications of these core effects resulted in justification or generalization of properties to all HEAs, e.g., all HEAs should exhibit high temperature stability based on high entropy effect, high temperature strength owing to limited grain growth, good diffusion barrier application due to sluggish diffusion kinetics, etc. However, many recent studies have challenged these core effects, and suggested that not all HEAs were observed to exhibit these core effects.