The multi-effects of Ta element in refractory high entropy alloys with dual BCC structure: Strength-plasticity synergy and high temperature strengthening

The challenge of high strength and low plasticity in refractory high entropy alloys (RHEAs) limits the processability and applicability. This study aims to achieve superior room temperature strength-plasticity combination and high temperature strengthening of dual-phase solid solution structure RHEAs by controlling Ta content. Therefore, TiZrNbAl_0.1MoTa_x (x = 0, 0.3, 0.5, 0.7, 1.0) RHEAs were prepared. The addition of Ta cannot change the dual BCC structure, but significantly promoted the increase of BCC1 phase content. Elevated Ta addition contributes to continuous enhancement of compressive yield strength. The optimized Ta_0.7 alloy possesses a yield strength of 1463 MPa and a plasticity of 21.4% at room temperature. The molecular dynamics (MD) simulation results show that the harder BCC1 phase has higher resistance to plastic deformation, thereby effectively hindering the dislocation movement and enhancing the strength. The softer BCC2 phase preferentially deforms during the loading process, thereby suppressing dislocation motion and achieving strength improvement. However, the excessive addition of Ta can aggravate the mechanical incompatibility between BCC1 and BCC2 phases, causing stress concentration and ultimately reducing the fracture strain of Ta_1.0 alloy. The strengthening model indicates that the increase in room temperature yield strength is primarily attributed to solid solution strengthening reinforcement from the BCC1 phase. Moreover, the yield strength at 1000 °C increases significantly from 323 MPa of Ta_0.0 alloy to 583 MPa of Ta_1.0 alloy. The EBSD and TEM characterization demonstrated that Ta delays the dynamic recrystallization (DRX) process, allowing the alloy to retain a high dislocation density, which accounts for the improved high temperature strength.