AlCoCrFeNi_2.1 eutectic high-entropy alloy (EHEA) ingots were successfully obtained by high-frequency melting and centrifugal metal-mold casting under an Ar flow. The microstructure of the ingots was investigated by trans-scale observations using optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and scanning transmission electron microscopy (STEM). The constituent phases of the ingots were identified as fcc and bcc phases by X-ray diffraction (XRD) analysis, and were not dependent on the position of the ingot. The microstructure was observed to have a primary fcc dendrite and fcc+bcc eutectic structure at the inter-dendrite region, regardless of the position of the ingots. The size of the solidification structure was affected by the cooling rate. Faster cooling rates resulted in finer solidification structures. TEM observations clarified the development of L1₂ ordering structures in the primary fcc dendrite phase, while the ordering peak could not be detected by XRD analysis.

This Paper was Originally Published in Japanese in J. JFS 89 (2017) 119–129. In order to more precisely explain the background, the purpose of the study, the experimental procedures, and the results, some parts of the contents were revised. The Refs. 21, 22), and 23) were added to clarify the source of pure elements. The Refs. 12) and 35) were added to discuss the solidification microstructure in more detail. Figure 1 was slightly modified to make the casting process and the position of the ingots clearly understandable. Figure 7 was slightly modified to discuss the solidification microstructure in more detail.