Superplasticity of Ti-6Al-7Nb alloys with different initial microstructures processed by high-pressure torsion

Ti-6Al-7Nb alloy was quenched below and above the β-phase field and processed by high-pressure torsion (HPT) to produce different grain sizes in the ultra-fine range. The influence of temperature and strain rate over the superplastic properties, activation energy and strain rate sensitivity were evaluated by high temperature tensile tests. The mechanical properties were complemented with Vickers microhardness measurement of heat treated and HPT-processed samples. The crystallographic analysis was performed through XRD and EBSD studies in the grip and tip of the high temperature tensile specimens. Microhardness measurements show significant hardening with respect to the imposed strain after HPT. Meanwhile, high superplastic elongation of 1200% was accomplished at 850 °C with a strain rate of 2x10^-3 s^-1. EBSD studies of the failed specimens suggest that the tip part exhibited dynamic recrystallization during tensile test, which produced a larger grain size of ~5.1 µm and higher fraction of β phase of 3.7%, with respect to the grip part under static recrystallization, which attained ~3.4 µm grain size and 3.3% β phase fraction.