In situ synchrotron X-ray diffraction investigations of the physical mechanism of ultra-low strain hardening in Ti-30Zr-10Nb alloy

A warm-rolled, metastable β-type Ti-30Zr-10Nb alloy exhibited a peculiar two-stage yielding behavior under uniaxial tensile loading, showing a first plastic stage with obvious strain hardening at 0.4%–10.4% strain and a second plastic stage with ultra-low strain hardening at 10.4%–23.5% strain. In situ high-energy X-ray diffraction (HE-XRD) was used to reveal the stress-induced martensitic transformation scenarios and physical mechanism of the different strain hardening rates. It was found that the deformation-induced phase transformation dominated the onset of the first plastic stage corresponding to the selection of favorable martensitic variants, and their elastic interaction contributed to the obvious strain hardening. HE-XRD experiments further verified that the ultra-low strain hardening rate in the second plastic stage was related to an interesting superelasticity of the martensite, which was characterized by the reversible, stress-induced reorientation of the martensite variants. This reorientation of the martensite variants was primarily due to the rigid lattice rotation of ∼23° about the [110]_α″ axis toward the tensile direction. Our investigations provide in-depth understanding of the mechanism of the excellent plasticity with ultra-low strain hardening in β-type titanium alloys.