Effect of Zr on the as-cast microstructure and mechanical properties of lightweight Ti2VNbMoZrx refractory high-entropy alloys

Refractory high-entropy alloys (RHEAs) have attracted extensive attention due to their outstanding high-temperature performance. However, high density and poor ductility are the two bottleneck problems in industrial application. This study aims to exploit novel RHEAs with high specific strength and good ductility by adding light Group IVB elements. The effect of Zr on the as-cast microstructure, density, and mechanical properties of Ti2VNbMoZrx (x = 1, 1.5, 2, and 3) RHEAs was investigated in detail. All Ti2VNbMoZrx RHEAs exhibited a disordered body-centered-cubic solid-solution phase. The as-cast microstructures changed from dendritic morphology (x = 1 and 1.5) to equiaxed dendritic morphology grain (x = 2 and 3) with the increasing Zr content. The Ti2VNbMoZr2 RHEA displayed optimal mechanical properties with a compressive yield strength of 1421 MPa, a plastic strain of 31.6%, and a specific yield strength of 211 kPa · m3 · kg−1, superior to the most-reported RHEAs. The microbands along {110} and {112} slip planes, which was a typical poly slip configuration, were observed in the Ti2VNbMoZr2 alloy after compression test using ex-situ transmission electron microscopy. Furthermore, the Ti2VNbMoZr2 RHEA exhibited good high-temperature mechanical properties, showing a yield strength of 977 MPa at 600 °C and 386 MPa at 800 °C without fracture at a plastic strain of 50%. The solution strengthening model revealed that the high yield strength of Ti2VNbMoZrx RHEAs was mainly derived from the atomic radius misfit and shear modulus misfit of the Zr atoms.