Microstructure and mechanical properties of (CrCoNi)97Al1.5Ti1.5 medium entropy alloy twisted by free-end-torsion at room and cryogenic temperatures

Abstract

In this study, the swaged (CrCoNi)97Al1.5Ti1.5 medium entropy alloys (MEAs) were twisted by free-end-torsion at room and liquid nitrogen temperatures. Microstructure in different strained layers of the twisted sample was characterized by combined use of energy dispersive spectroscopy, electron channeling contrast imaging, and electron backscatter diffraction techniques. The microhardness and overall tensile properties of the twisted sample were analyzed. The results show that the MEAs exhibit higher torque and lower torsion angle during low-temperature torsion. The deformation mechanism during torsion is dominated by dislocation slip, and gradient dislocations and lamellar dislocation substructures gradually increasing from core to surface are formed in the twisted samples. The twin content gradually decreases in the surface layer, especially when twisted at room temperature or at liquid nitrogen temperature but with a small torsion angle. The reduction of twins is related to the addition of Al and Ti, the critical shear stress of twinning, and the unique strain state of torsional deformation. The twisted samples have gradient distributions of microhardness. Torsion can effectively improve the tensile strength of the MEAs at the expense of ductility.