Development of nano-structured CoCrNi medium entropy alloy via equal-channel angular pressing

Abstract

Severe Plastic Deformation (SPD) is a post-processing technique that provides ultrafine grains by inducing very high strains in the bulk material. Equal Channel Angular Pressing (ECAP) is a prominent SPD procedure for developing submicron-sized grains with superior mechanical characteristics. Pure shear without considerable change in cross-sectional area is the phenomenon that causes ultrafine grain formation. In the present study two dies having 90⁰ and 120⁰ die angles have been utilized. Medium-entropy alloys (MEAs) constitute alloys with numerous primary alloying elements in approximately equivalent proportions with exceptional characteristics. CoCrNi is one such MEA with a promising mechanical response and an excellent balance of yield strength, fracture toughness, and ductility. Although the fracture toughness is superior to that of most existing HEAs, the lower yield strength limits its use in engineering applications. The goal of this research is to strengthen vacuum arc melted CoCrNi after grain refinement with ECAP. CoCrNi's initial yield strength and microhardness were found to be 250 MPa and 292.8 HV, respectively. The grain size of as-cast CoCrNi is also 546.33 μm. Improvement in yield strength and micro-hardness is obtained after two passes of ECAP at room temperature (RT) and 750 ⁰C. The maximum yield strength and microhardness are achieved at RT resulting because the largest grain refinement has been seen at RT. However, it should be noted that the die with a die angle of 90⁰ produced superior yield strength and microhardness when compared to the die with a die angle of 120⁰ when both dies were utilized at the same temperature.