Critical Assessment of Interdiffusion-Driven Phase Transformations: from Binary Alloys to Complex Concentrated Alloys

Interdiffusion-driven phase transformations govern microstructural evolution at material interfaces and critically influence the performance of coatings, joints, and layered components in engineering systems. Although the fundamentals of interdiffusion and phase growth are well established for binary alloys, their direct extension to complex concentrated alloys (CCAs) introduces new scientific challenges arising from multicomponent interactions, local chemical complexity, and non-ideal diffusion behavior. Here, CCAs are used as the broader classification of multicomponent alloys, with high-entropy alloys (HEAs) representing the most widely studied subclass within CCAs. Recent reviews on diffusion phenomena in multicomponent alloys primarily focus on diffusivities determined by radiotracer analysis and the interdiffusion approach, with limited attention to the actual growth behavior of interfacial phases, which is an equally vital aspect in real-world applications. The objective of this review is to critically assess existing theories, experiments, and advanced approaches for interdiffusion-driven phase transformations, and to evaluate their applicability and limitations when extended from binary alloys to CCAs. The review first revisits the fundamentals of interdiffusion and intrinsic diffusion, emphasizing mechanistic descriptions of phase growth, interface motion, local equilibrium assumptions, and diffusion-controlled kinetics. Through key examples, interdiffusion-driven reaction layer formation in binary diffusion couples is then examined, with attention to simultaneous and sequential growth, interface morphology, growth kinetics, Kirkendall effects, and interface instabilities. Building on this foundation, recent advances in understanding diffusion and phase growth in CCAs, particularly HEAs, as the most widely studied subclass, are critically discussed, focusing on atomistic transport across multicomponent intermetallic phases, the role of local chemical ordering, potential energy fluctuations, and lattice distortion. Engineering implications are analyzed through case studies on bond coats for thermal barrier coatings, interdiffusion zone (IDZ) evolution, diffusion barriers in electronic materials, and dissimilar metal joining processes, including welding, brazing, and diffusion bonding. This review identifies key gaps in current understanding and outlines approaches for improved analysis and control of interdiffusion-driven phase evolution. It concludes with an outlook on future research directions for interface design in CCAs.