In situ synchrotron X-ray diffraction study of deformation behaviour of a metastable β-type Ti-33Nb-4Sn alloy

In this study, the deformation behaviour of metastable β-type Ti-33Nb-4Sn alloys in different thermo-mechanical treatment states is investigated by tensile tests and in situ synchrotron X-ray diffraction (SXRD). In the case of solution-treated alloy, stress-induced martensitic (SIM) transformation takes place over a wide strain range of 0.5-14%. During this SIM transformation, the parameter of bα′′ ([020]α′′) increases with macroscopic strain within strain range of 1.5-4.7%, giving rise to that the α′′ variants, with bα′′-axis ([020]α′′) parallel to tensile direction, are formed preferentially to accommodate the macroscopic strain during loading. Similar SIM transformation behaviour also occurs in cold-rolled alloy with the exception that the extent of SIM transformation and pre-existing α′′ variants reorientation is much slighter than that in solution-treated specimen. This slighter SIM transformation gives rise to nonlinear deformation, instead of “stress plateau”, in the cold-rolled alloy. Upon annealing, the β phase survives against SIM transformation due to the suppression effects of dislocations and grain boundaries, resulting in a huge elastic deformability. Based on the results of tensile tests and SXRD, the activation sequence of different deformation mechanisms and the regions of different deformation mechanisms of Ti-33Nb-4Sn alloys, in solution-treated, cold-rolled and annealed states, are clarified unambiguously.