Challenges in High-Entropy Alloy Synthesis from Industrial Wastes: A Review of Impurity Control Strategies and Compositional Homogeneity

High-Entropy Alloys (HEAs), as a novel class of multi-component materials, have shown high potential in advanced engineering applications. Simultaneously, industrial wastes, especially electronic wastes (E-waste) and metallurgical wastes (slags, dusts, and sludges), are becoming an "Urban Mine" (Urban Mining) as rich secondary resources for strategic metals (such as Cu, Ni, and rare earth elements). The direct synthesis of HEAs from these wastes is an attractive "Upcycling" route within the framework of the circular economy. However, the use of this heterogeneous feedstock faces two fundamental challenges, which are addressed in this review paper. The first challenge is Impurity Control. Industrial wastes contain harmful non-metallic elements such as oxygen (O) and sulfur (S), which lead to "oxygen-induced embrittlement" and "sulfur segregation at interfaces", and severely degrade the final properties. The second challenge is achieving Compositional Homogeneity. Contrary to the initial assumption, high entropy alone does not guarantee the formation of a homogeneous solid solution. Elements like copper (Cu), which is abundant in E-waste, have a strong tendency for Dendritic Segregation during casting solidification and accumulation in inter dendritic regions or precipitation as secondary phases, which leads to a severe drop in mechanical properties. This review paper critically examines the existing metallurgical strategies to overcome these two challenges. In the first part, impurity control methods, from feedstock refining (such as bioleaching and hydrometallurgy) to in-process control (such as hydrogen annealing [۴] and oxygen-resistant alloy design), are discussed. In the second part, advanced synthesis methods for achieving homogeneity are evaluated. The results show that traditional casting methods are challenging for this purpose due to inherent segregation [۱۳], especially with the presence of copper. In contrast, advanced methods such as Powder Metallurgy (PM) via Mechanical Milling (MM) and Additive Manufacturing (AM), due to solid-state mixing or very rapid solidification rates, offer much more effective strategies for achieving compositional homogeneity. This review also emphasizes the importance of modern computational tools (such as CALPHAD and Machine Learning) in guiding the design of these complex alloys from unconventional resources.

کلیدواژه ها:

High-Entropy Alloys (HEAs) ، Industrial Waste ، Upcycling ، Circular Economy ، Impurity Control ، Compositional Homogeneity ، Segregation