Microstructure evolution and deformation behavior of Ti-7Mo-3Al-3Cr-3Nb alloy with different β grain sizes during tensile deformation

The microstructure evolution and deformation behavior of metastable Ti-7Mo-3Cr-3Al-3Nb (Ti-7333) alloy with β grain size ranging from 81 to 313 μm were investigated during room-temperature tensile test, combining electron backscattering diffraction and transmission electron microscopy. It shows that the main deformation products of Ti-7333 alloys are stress-induced α" martensitic transformation and martensite twins. As the β grain size increases, the amount of α" martensite decreases because the stress required to trigger martensitic transformation gradually rises. At the same time, the double yield phenomenon observed in the tensile stress strain curve disappears and the corresponding three-stage work hardening rate characteristics gradually weakens. The deformation mechanisms of Ti-7333 alloy evolved with the increasing of tensile strain. The stress-induced lath martensite and martensite lath reorientation or internal α" martensite twins at small strains. Then the widening and merging of martensitic laths emerge under medium strain levels (0.06). And finally, the α" martensite domain and various types of martensite twins formed under larger strains (fracture). As for the Ti-7333 alloy with an average β grain size of 313um, the martensitic laths trigger {130} < 31̅0 > α" deformation twins, {111} α" type I twins and {011} α" Compound twins multiple internal twins at smaller strains (0.03), while {112} α" type I deformation twins are characterized at larger strains (0.06). Stress-induced multiple types of martensitic twins provide more β/α′′ and α′′/α′′ interfaces that hinder dislocation movement effectively, which improve the work hardening rate and tensile mechanical properties of Ti-7333 alloy.