Mitigation of strength-ductility trade-off of CoCrNi medium entropy alloys with heterogenous structure by controlled Gd addition and annealing

In this work, we synthesized a series of (CoCrNi)(100-x)Gd-x (x = 0/0.3/0.5/1.0) medium entropy alloys (MEAs) using arc melting followed by homogenization, hot-rolling, and annealing at 700, 800, and 900 degrees C for 1 h. The effects of Gd content and annealing temperature on the microstructures and mechanical properties of Gd-doped CoCrNi MEAs were investigated. The heterogenous microstructure, consisting of the deformed grains, recrystallized grains and precipitates of GdNi5 phase, was obtained in the present study and it exhibited an excellent combination of strength and ductility. The optimum mechanical properties were obtained for CoCrNi MEA with 0.3-0.5 at.% Gd addition and annealed at 700 degrees C for 1 h to yield the heterogenous microstructure in the present study, and it had the tensile strength of similar to 1.3 GPa and fracture strain of 30-40%. The synergetic hardening mechanisms included solid solution, precipitation, grain refinement, nanotwinning and pre-existing dislocation hardening. The relations among compositions, microstructures and mechanical properties of (CoCrNi)(100-x)Gd-x MEAs were addressed.