Effects of Al alloying on microstructure and mechanical properties of VCoNi medium entropy alloy

The FCC-structured medium entropy alloys (MEAs) possess excellent ductility, but their strength is still insufficient for engineering application. In this work, the effects of aluminum alloying and thermomechanical treatment on the microstructures and mechanical properties of an equiatomic VCoNi MEA were systematically investigated. The results showed the structure changed from a single FCC structure to an FCC and BCC dual-phase structure in the as-cast alloys as the Al content increased, and would further evolve into a multiple-phase structure after thermomechanical treatment. The Al alloying and thermomechanical treatment led to the formation of a refined and heterogeneous microstructure with nanoscaled precipitates, and the (VCoNi)93Al7 alloy exhibited very comprehensive properties of 1.3 GPa yield strength, 1.7 GPa ultimate tensile strength and 22.7% elongation. The strength increment was mainly due to the enhanced strengthening contributions from the dislocation, solid solution and precipitation as compared with the undoped alloy, while the good ductility was resulted from the optimal morphology and volume fraction of the second phases. This work was useful for developing high-performance medium entropy alloys by optimizing composition and treatment.