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

The formation mechanism of alpha ''-martensite (alpha ''(Mt)) induced by tempering at 450-550 degrees C for a short time was investigated using Ti-10Mo-7Al alloy. The solution treated and quenched (STQ) sample was composed of beta phase and a small amount of alpha ''(Mq), and a large amount of alpha ''(Mt) was generated by rapid tempering at 550 degrees C-3 s using a salt bath. However, alpha ''(Mt) was completely transformed into a single beta phase by aging at 200 degrees C for 3 min. Reversibility was observed between the alpha ''(Mt) transformation and the beta reverse transformation. In-situ high-temperature X-ray diffraction measurements revealed that alpha ''(Mq) -> beta reverse transformation occurred at 200 degrees C and that a thermally activated alpha '' iso was generated at 450 degrees C due to the slow heating rate. In-situ optical microscopic observation of STQ sample with rapid lamp heating revealed that alpha ''(Mt) was formed during heating process. However, alpha ''(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 degrees C) to 250 degrees C was performed to ensure a sufficient temperature difference, but alpha ''(Mt) was not generated at all. From the cross-sectional observation of the STQ plate, it was found that alpha ''(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 alpha ''(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 alpha ''(M).