Al-induced trace B2 phase enhances strength-ductility synergy in Ti2Zr0.75NbVAlx high-entropy alloys through dislocation bundling

The Ti 2 Zr 0.75 NbVAl x (x = 0.1, 0.3, 0.5, 0.7) lightweight high-temperature resistant high-entropy alloys (HEAs) were prepared by vacuum arc melting. Increasing Al content shifts the alloy phase from a single disordered body-centered cubic (BCC) to a coexistence of disordered BCC and ordered B2 phases. The appearance of the B2 phase significantly affects the mechanical properties of the alloy. The trace amount of B2 phase increases the yield strength of Al0.5 alloy from 770 MPa to 900 MPa, with specific yield strength reaching 156.66 MPa cm 3 g −1 and plasticity rising from 4.5 % to 22.5 %. In contrast, higher B2 content in Al0.7 reduces yield strength to 760 MPa and plasticity to 4.9 %. This paper explains that a trace amount of B2 phase has a positive effect on plasticity enhancement in alloys, while the formation of large amounts of B2 phase causes significant inhibition of material plasticity when the B2 phase content is too high. Transmission electron microscopy analyses show that a trace amount of B2 phase enhances the orderly arrangement of dislocations by promoting the generation of an appropriate amount of dislocations, thus making it easier to form parallel dislocation bundles. These bundles reduce dislocation energy and form a solid dislocation wall at grain boundaries, effectively suppressing strain localization and enhancing plasticity. Molecular dynamics simulations show that alloys with a trace amount of B2 phase enhance tensile deformation and plasticity by promoting dislocation alignment and bundle formation. Alloys containing dislocation bundles exhibit optimal plasticity in tensile tests.