Superplastic deformation behavior of a β-rich α+β titanium alloy

Abstract

Superplastic tensile test was performed on a β-rich α+β titanium alloy (SP700 alloy) sheet in the temperature range of 740–800 °C and strain rate range of 1 × 10−4-1 × 10−2 s−1. The effects of the deformation parameters on the flow curves, strain rate sensitivity (m-value), thermal activation energy (Q-value) and microstructural characteristics were investigated to elucidate the deformation mechanism. The results showed that the examined SP700 alloy exhibited excellent superplasticity with fracture elongation up to approximately 1800% deformed at 760 °C and 1 × 10−3 s−1, with the maximum m-value of 0.55. The shapes of the flow curves were dependent on the strain rate and showed a type of curves without an obvious steady-state flow at relatively high strain rates. For this β-rich α+β titanium alloy, when deformed at 740–780 °C and medium strain rates (approximately 5 × 10−3-5 × 10−4 s−1), the controlling deformation mechanism was the grain boundary sliding accommodated by dislocation slip. When deformed at 800 °C and a strain rate of 1 × 10−4 s−1, m-value decreased to 0.1 and the main deformation mechanism was dislocation slip with a high β volume fraction over 50% and average grain size over 5 μm. Meanwhile, the dominant texture changed from <0001> to <101¯0> or <21¯1¯0> because of the grain rotation caused by the grain boundary sliding.