Relationship between the microstructure and mechanical properties of an equiatomic AlCoCrFeNi high-entropy alloy fabricated by selective electron beam melting

Because of their superior properties, high-entropy alloys (HEAs) are considered promising novel structural materials that can substitute conventional alloys. From the viewpoint of future applications, it is important to explore methods for producing complex shaped products with HEAs. In this study, selective electron beam melting (SEBM) was employed for fabricating equiatomic AlCoCrFeNi HEA specimens, and their microstructures and mechanical properties were evaluated by comparing them with those of a conventionally cast specimen. Both cast and SEBM specimens dominantly consisted of a nano-lamellar mixture of disordered body-centered-cubic (BCC) and B2 (ordered BCC) phases. The face-centered cubic (FCC) phase was also precipitated at the grain boundaries of the B2/BCC mixture phases on the SEBM specimens. The fraction of the FCC phase at the bottom part of the SEBM specimen was higher than that at the top part. The preheating procedure—a process unique to SEBM—is responsible for the precipitation of the FCC phase, because of the long-term exposure at sufficiently high temperatures. As a result, the hardness of the SEBM specimens gradually decreased as we approached the bottom part of the specimens due to the increased fraction of the FCC phase, which had lower hardness than the B2/BCC phases. Further, the SEBM specimen exhibited much higher plastic deformability than the cast specimen, without significant loss of strength.