Microstructural evolution and mechanical properties of CoCrFeMnNi cantor high-entropy alloy severely deformed by the accumulative roll bonding (ARB) process

High-entropy alloys (HEAs) with face-centered cubic crystal structure commonly exhibit excellent ductility, but they suffer from poor mechanical strength. This issue is important in applications where the strength-to-ductility trade-off is concerned. In this regard, a novel approach has been introduced in the current research to enhance the strength of CoCrFeMnNi HEA without a considerable reduction in the elongation. For this purpose, the HEA was fabricated by vacuum-arc melting followed by homogenizing heat treatment, cold-rolling, and annealing. Afterwards, a three-stage accumulative roll bonding (ARB) process was carried out at 450 °C. Microstructural examinations revealed that an ultrafine-grained structure with the average grain size of 205 nm and a large fraction of low-angle grain boundaries was achieved after 3 passes of ARB through the continuous and discontinuous dynamic recrystallization mechanisms. Moreover, Cr-rich sigma phase was formed during the ARB process despite the sluggish diffusion phenomenon associated with HEAs. It was found that the strain applied during severe plastic deformation promotes the formation of sigma phase. The development of nano-sized grain structure containing well-distributed hard sigma phase was found to be capable of providing excellent improvements in microhardness, yield strength, and ultimate tensile strength by 213 %, 347 %, and 169 %, respectively, while a reduction in the elongation was around 56 % compared to the annealed condition.