Control of nonmetallic inclusions in high-entropy alloys

High-entropy alloys (HEAs) have garnered considerable attention in recent years owing to their exceptional mechanical properties, including high yields and ultimate strength as well as remarkable resistance to oxidation and corrosion. These properties make them suitable for various engineering applications, particularly in demanding environments such as aerospace, nuclear reactors, and chemical processing industries. The typical composition of HEAs, which typically consist of five or more principal elements in near-equimolar ratios, results in a high configurational entropy (usually >1.5R) that stabilizes the solid-solution phase. Consequently, their performance is superior to that of traditional low-entropy alloys, i.e., low-alloy steels, stainless steels, and nickel-based superalloys. However, despite their promising potential, the widespread industrialization of HEAs is limited by their high manufacturing costs. Currently, HEA production primarily relies on the use of pure metal elements, which are expensive and limit the scalability of these materials. Existing fundamental studies have been mainly focused on the preparation of high-purity nickel-based alloys by vacuum induction melting (VIM). By contrast, preparation of high-purity HEAs has been rarely attempted because of the fundamental differences between the thermodynamic and kinetic behaviors of impurity removal from nickel-based alloys and HEAs; thus, detailed investigations are required to understand the optimal process parameters for producing high-purity HEAs. One of the critical issues in HEA preparation is the presence of impurity elements, even in high-purity metal raw materials. Impurity elements, such as carbon, oxygen, sulfur, nitrogen, and aluminum, are inevitably introduced into HEAs, forming nonmetallic inclusions, which can degrade the mechanical properties and corrosion resistance the HEAs. Notably, in addition to high-purity metal materials, impurities can be generated from diverse sources, such as refining slags, refractory materials used in the melting process, and specific preparation methods. The interactions between these impurities and the HEA melt are complex, and thus, investigating the mechanisms of impurity removal and the formation and transformation of inclusions in HEAs is a challenging task. To the best of the authors’ knowledge, studies on controlling impurity elements during the preparation of HEAs by VIM are scarce. With the aim to address these challenges, this paper presents a comprehensive review on existed literature and experimental data, which can provide insights on the mechanisms by which impurity elements and nonmetallic inclusions affect the performance of HEAs. The findings can offer theoretical guidance for preparing high-purity HEAs in the future, highlighting the importance of controlling impurity levels and optimizing the refining process. Ultimately, this study is expected to contribute to the development of more cost-effective and scalable methods for producing HEAs, paving the way for their broader application in high-performance engineering fields. The insights gained from this study advance our fundamental understanding of HEAs, and practical recommendations for overcoming the current limitations in their production are provided to facilitate their transition from laboratory-scale research to industrial-scale manufacturing.