Mechanisms of near-linear elastic deformation behavior in a binary metastable β-type Ti-Nb alloy with large recoverable strain

A binary metastable β-type Ti-Nb alloy possessing large near-linear recoverable strain was designed to clarify the physical mechanisms responsible for high near-linear elastic deformability in near‑oxygen-free Ti-based alloys. The in situ synchrotron X-ray diffraction (SXRD) investigation reveals that in addition to the inherent elastic deformation, a reversible β↔α'' stress-induced martensitic transformation (SIMT) occurs during the near-linear elastic deformation. During the SIMT, the α'' martensitic variant 5 (V₅) forms preferentially due to its maximum phase transformation strains among all 6 equivalent α'' variants. The orientation dependence of phase transformation strains of the preferential V₅ leads to the appearance of different α'' diffraction peaks in the loading and Poisson's directions during the SIMT, which is beneficial for accommodating the external macroscopic deformation to the great extent. Our experimental results show that the high near-linear elastic deformability of present Ti-Nb alloy is attributed to a combined effect of the large inherent elastic deformation arising from low Young's modulus as well as a reversible SIMT that covers a wide stress range and exhibits small phase transformation strains limited by the lattice constants of β and α'' phases. These results could provide a comprehensive understanding of the mechanisms responsible for huge near-linear elastic deformability of β-type titanium alloys.