High ductility CrCoNi medium entropy alloy prepared by liquid nitrogen temperature rolling and short time annealing at moderate temperature

Abstract

As-cast CrCoNi Medium Entropy Alloy (as-casted MEA) was obtained by vacuum arc melting. The as-casted MEA was rolled 3 times at the temperature of liquid nitrogen (77 K), the sum of the total deformation was 50%. Then, the rolled CrCoNi Medium Entropy Alloy (as-rolled MEA) were annealed with different parameters, the annealing parameters were 923 K/30min, 973 K/30min and 1073 K/30min respectively. The experimental samples were characterized by uniaxial tensile test at room temperature, electron back scatter diffraction (EBSD) and transmission electron microscopy (TEM). The tensile test results showed the CrCoNi MEA was deformed by Liquid Nitrogen rolling, the tensile strength increased from 421 MPa to 1312 MPa, but the elongation was only 7.4%. The as-rolled MEA annealed at moderate temperature and short time, the ductility of the CrCoNi was improved significantly. When the as-rolled MEA annealed at 1073 K/30 min, the elongation of CrCoNi increased to 65.3%, while the tensile strength maintained 800 MPa. The EBSD and TEM results showed that the CrCoNi MEA was deformed by Liquid Nitrogen rolling, there were a lot of high density dislocation regions in the alloy. At the same time, a large number of bunches of stacking faults (SFs), intersected SFs and deformation twins (DT) were found in the rolled MEA. The EBSD and TEM results also showed that when the CrCoNi MEA was annealed, a large number of type B and type C annealing twins and 9 R structures were found in the alloy. The growth of annealing twins were achieved by the mechanism of grain boundary migration caused by stacking faults annexation. After annealing, the transformation of FCC→9 R structure was occurred, and the 9 R structure caused more boundary nucleation of annealing twins. The annealing twins and 9 R structure played a key role in improving the ductility of the annealed CrCoNi MEA. Adopt the technology of moderate temperature combining short time annealing process, can effectively control the properties of CrCoNi MEA.