Formation Mechanism of Tempering-Induced Martensite in Ti–10Mo–7Al Alloy

The formation mechanism of α"–martensite (α"_Mt) induced by tempering at 450–550 °C for a short time was investigated using Ti–10Mo–7Al alloy. The solution treated and quenched (STQ) sample was composed of β phase and a small amount of α"_Mq, and a large amount of α"_Mt was generated by rapid tempering at 550 °C for 3 s using a salt bath. However, α"_Mt was completely transformed into a single β phase by aging at 200 °C for 3 min. Reversibility was observed between the α"_Mt transformation and the β reverse transformation. In–situ high–temperature X–ray diffraction measurements revealed that α"_Mq → β reverse transformation occurred at 200 °C and that a thermally activated α"_iso was generated at 450 °C due to the slow heating rate. In–situ optical microscopic observation of STQ sample with rapid lamp heating revealed that α"_Mt was formed during heating process. However, α"_Mt did not generate under following conditions; that is, a slow heating rate, thin sample plate, and a small temperature difference until tempering by preheating. On the other hand, rapid tempering using thick plate from liquid nitrogen (–196 °C) to 250 °C was performed to ensure a sufficient temperature difference, but α"_Mt was not generated at all.
From the cross–sectional observation of the STQ plate, it was found that α"_Mq was hardly formed on the surface of the sample, but was formed abundantly inside the sample. On the other hand, in the rapidly tempered plate, a large amount of α"_Mt was distributed in the surface layer than inside sample. These results suggest that the thermal compressive stress induced by rapid heat treatment contributes to the formation of α"_M.