Phase evolution and mechanical properties of Ti1.5NbVAlx(x=0.25, 0.5, 0.75, 1) lightweight refractory medium-entropy alloys

A series of Ti1.5NbVAlx(x = 0.25, 0.5, 0.75 and 1) lightweight refractory medium-entropy alloys was synthesized through vacuum arc-melting, and the effects of Al addition on the phase types, microstructure and properties were investigated. Al0.25 alloy showed a single-phase BCC structure, and the increase in Al content induced a transition from disordered-BCC to ordered-B2, which caused short-range ordered strengthening. The relationship between Al content and ordered B2 was revealed by TEM. The mechanical properties of this alloy system were measured by compression and tensile tests and results indicated that Al0.25 and Al0.5 show excellent tensile strength (800, 857 MPa) even without any microstructural homogenization or optimization, meanwhile, the tensile ductility is 20 and 19 %. TEM observation demonstrates that multiple slip systems are activated in Al0.5 alloy and highly concentrated dislocation bands cross each other to form substructures, which has an important effect on the strength and elongation. Furthermore, the four alloys showed excellent compression plasticity, and the deformation was more than 50 % without fracture. When the Al content increased to 0.75, the maximum compressive yield strength σ0.2 = 1.29 GPa. This work is of great significance for the design of high specific yield strength and good ductility RH/MEAs.