Ultrafine Grain Formation, Grain Boundary Segregation and Phase Decomposition in CoCuNi Medium Entropy Alloy

FCC medium entropy alloys (MEAs), <i>e.g.</i>, CoCrNi, have been attracting much attention due to their excellent strength-ductility balance. The present talk will report the fabrication of an equiatomic CuCoNi MEA and its improvement of strength and ductility after high-pressure torsion (HPT).<br/>An equiatomic CoCuNi MEA was produced by high-frequency melting and was hot-rolled and swaged into a cylindrical shape with a 10 mm in diameter. The samples were homogenized at 1150 °C for 1 hour and quenched into room temperature water. Severe plastic deformation by high-pressure torsion (HPT) was applied on the disk samples with a thickness of 0.85 mm at room temperature with a pressure of 5 GPa and a rotation speed of 1 rpm to 10 rotations. After the HPT, the sample was annealed at 400 °C, 600 °C for 100 h, and at 800 °C for 1, 10, and 100 h. XRD, SEM-EBSD, EDS, TEM, APT, Vickers hardness, and tensile tests were conducted to investigate the microstructures and mechanical properties.<br/>The grain size of the sample before HPT was ~100 mm and was refined to ~90 nm after HPT. Tensile strength and elongation before HPT were 400 MPa and less than 10 %, respectively. HPT deformation improved both strength and elongation to over 1400 MPa and 16 %. The fracture surface observations revealed the intergranular fractured feature for the sample before HPT and ductile fracture for the one after HPT. APT analysis confirmed the segregation of a substantial amount of Cu along the grain boundaries in the sample before HPT. Severe deformation by HPT removed the grain boundary segregation, which may be the reason for the improved ductility.<br/>We also investigated the effect of post-deformation annealing on microstructures and mechanical properties. XRD revealed that the sample after HPT, and then annealed at 400 °C showed the single FCC peaks while the sample annealed at 600 and 800 °C showed the splitting of the FCC peaks, indicating the phase separation. EBSD-EDS of CuCoNi annealed at 800 °C for 100 h showed typical recrystallized microstructure containing twin boundaries in equiaxed grain boundaries. EDS analysis visually and quantitatively identified the phase separation into Cu-rich (~80 at%Cu-20 at%Ni) and Cu-depleted (~55at%Co-40at%Ni) phases with the volume fractions of 0.34 and 0.66, respectively. The boundaries between the Cu-rich phase and Cu-depleted phase often coincide with the grain boundaries or twin boundaries. But the phase boundaries also exist within a single grain[ST1] , suggesting the simultaneous progress of phase separation and recrystallization. The tensile tests indicated that prolonged annealing at high temperatures leads to improved ductility at the expense of strength, compared to those after HPT.