Deformation behavior of harmonic structure designed CoCrFeMnNi HE alloys
Nurul Nadiah Mahmud, Mie Kawabata, Hiroshi Fujiwara, Bhupendra Sharma, Kei Ameyama
Abstract
This paper presents the deformation behavior of CoCrFeMnNi high-entropy alloy (HEA) produced from plastically undeformed and deformed powders. The investigation commenced with the preparation of ball-milled HEA powder at 90ks and 180ks, with a ball-to-powder ratio of 2:1, to induce severe plastic deformation on the outer layer of the HEA powder. Subsequently, the plastically undeformed and deformed HEA powders were successfully consolidated in a spark plasma sintering (SPS) furnace. The SEM observations revealed that the sintered plastically undeformed HEA powder exhibited conventional homogeneous microstructures (Homo), while sintered deformed HEA powders exhibited a regulated bimodal grain distribution known as harmonic structure (HS). The mechanical properties of the materials were characterized by performing tensile tests at room temperature (RT) and a low temperature of 193 K. The HS HEA exhibited an increase in proof strength of 16% to 40%, while the ultimate tensile strength (UTS) increased by 8% to 20% compared to the Homo HEA. Moreover, the strain hardening rate (SHR) curve indicates that both Homo and HS HEA exhibited oscillatory behavior at low temperature. However, Homo HEA exhibited more pronounced oscillation than HS HEA. The current results indicate that oscillatory behavior in the SHR curve is indicative of twin deformation. At low temperatures, the high rate of twinning deformation in Homo HEA was attributed to its higher ductility, while a good combination of high strength and high ductility produced by HS material was found to be influenced by HS design in addition to twinning deformation.