A high-entropy alloy (HEA) is a new alloy concept, which was first introduced by the research of Yeh et al. and Cantor et al. in 2004. Conventional alloy systems, such as steels and aluminum alloys, are composed of one principal element and contain a small quantity of additional elements in order to enhance the properties of principal element or to gain secondary properties. On the other hand, HEA comprises several principal elements. Despite the preconception that microstructures of these multicomponent systems are complex due to the formation of intermetallic compounds or segregated phases, HEAs usually have a single phase, such as FCC or BCC, or a simple microstructure of two dominant phases. The simple structure of HEA was believed to result from one of the core effects observed in HEA (e.g., the high-entropy effect). In addition, the sluggish diffusion effect due to different sizes of the elements, which distorts the lattice, and limits the formation of new phases by diffusion.
Despite of the importance of understanding the characteristics of NMIs, there are few systematic studies focused on the effects of NMI in HEA. In order to utilize the promising properties of HEA, quantitative characterization of NMI is essential, and further development of methods to reduce NMI particle formation is also highly desired. Therefore, in this study, the content of impurities and the extent of NMI precipitation in the as-cast CoCrFeMnNi HEA were characterized. The impurity contents, such as carbon, sulfur, nitrogen, and oxygen, in the CoCrFeMnNi alloy were measured with combustion analyzers, and the NMIs that result from these impurities were predicted with a thermodynamic computation program, FactSage™7.0. The two-dimensional and three-dimensional morphologies, size distributions, and chemical compositions of the NMIs were investigated by employing scanning electron microscopy combined with energy dispersive spectroscopy (SEM-EDS) measurements. Finally, we evaluated the effect of these inclusions on the tensile properties of the HEA. Those characterizations were also conducted in order to examine the applicability of as-cast HEA as likely as many kinds of Al and Mg alloys in their use of as-cast alloys.
(a) Fractography at a magnification of 35´, (b) 500´ and (c) Mn-Cr-Al oxide particle in dimple on a fracture plane, (d) EDS spectra of the inclusion.