Solid solution and grain refinement achieve high strength and good plasticity of NbTaTi_xV refractory high entropy alloy

NbTaTi_xV refractory high entropy alloys were fabricated using MA and SPS. The effects of different Ti content on the microstructures and mechanical properties of the NbTaTi_xV RHEAs were systematically investigated. The results showed that the alloys were consisted of BCC phase and TiO phase. With the Ti content rised, the grain size of the NbTaTi_xV alloys correspondingly increased. The compressive fracture strength of the NbTaTi_xV alloy decreased gradually with increasing Ti content, while the compressive strain initially increased and then decreased. Specifically, the NbTaTi₁.₅V alloy exhibited a compressive fracture strength of 2977 MPa and a compressive strain of 17.58 %. Comparing with the NbTaTiV alloy (3100 MPa), the fracture strength decreased by 4 %, while the fracture strain increased by 19.7 %. Theoretical calculations and TEM analysis indicated that significant atomic mismatches caused lattice distortion in NbTaTiV alloys. Concurrently, a certain number of dislocations were generated, which enhanced the compressive yield strength. The reduction in atomic mismatches in the NbTaTi_1.5V alloy led to a decrease in lattice distortion within the alloy. Through the analysis of the established strengthening model, it was found that the high strength and large strain of the alloy were primarily attributed to the synergistic effects of grain boundary strengthening, solid solution strengthening, and interstitial solid solution strengthening. These factors collectively contributed to achieving both high strength and high plasticity in the alloy.