High-temperature deformation behavior and microstructural evolution of as-cast and hot rolled beta 21S alloy during hot deformation

Abstract

In this study, the hot deformation behavior of an as-cast and hot rolled β21S alloy (Ti–15Mo–3Al–3Nb–0.2Si) was investigated. Based on the hot compression tests, which were conducted at high temperatures (800–1200 °C) using different strain rates (10−2 –10 s−1), a processing map and energy efficiency (η) value were obtained for the alloy. Furthermore, the deformation activation energy of the alloy was calculated and the tensile properties of the alloy after the thermal treatment were analyzed. The processing map results revealed that plastic instability occurred at a temperature of 900 °C and a strain rate of 10 s−1. Over the entire temperature range the hot deformation mechanisms indicated an activation energy of 269 kJ/mol and the ln Z values, calculated by the Zener-Holloman parameter, varied between 17 and 32 s−1, in which the stable flow was associated with dynamic recovery and dynamic recrystallization. Likewise, this study revealed that dynamic recovery is the dominant deformation mechanism in the β21S alloy at temperatures lower than 1000 °C, while dynamic recrystallization is prevalent at higher temperatures. Finally, the tensile properties of the β21S alloy depend significantly on the post-processing heat treatment. The rapid grain-growth phenomenon observed at 1200 °C suggests that the practical alloy rolling conditions may differ from that used for hot compression tests performed in the laboratory. These results demonstrate that the process optimization is significantly dependent on sample size which affects soaking time.