Formation of α″ martensite and {332} 〈113〉 twin during tensile deformation in Ti-40 mass%Nb alloy

The α″ martensite phase in the quenched Ti-40 mass%Nb alloy transforms reversely to the β phase upon short-time aging at 623 K for 0.9 ks, resulting in the β + ω microstructure in the aged specimen. The relationship between the tensile property and the microstructure of this specimen was investigated by the observation of the structural changes in the quenched and aged specimen during tensile deformation. Products of the β phase, which were similar to {332} twins, surrounded by the matrix of α″ martensite were formed in the quenched specimen upon tensile deformation. In the aged specimen, on the other hand, α″ martensites and {332} twins were formed upon tensile deformation. The amount of α″ martensite in the two specimens was larger in the nonuniformly deformed area near the fracture. In particular, concentrated slip occurred in the nonuniformly deformed area of the aged specimen accompanied by the extinction of ω phases. The results indicate that the α″ transformation and then the reverse α″→β transformation were involved in the formation of {332} twins. It is suggested that the formation of {332} twins was due to lattice instability, and that besides the role of deformation resistance, ω phases played a part in nonuniform deformation.