Formation Mechanism of Tempering-Induced Martensite in Ti10Mo7Al Alloy

The formation mechanism of α-martensite (αMt) induced by tempering at 450550° C for a short time was investigated using Ti10Mo7Al alloy. The solution treated and quenched (STQ) sample was composed of β phase and a small amount of αMq, and a large amount of αMt was generated by rapid tempering at 550°C for 3 s using a salt bath. However, αMt was completely transformed into a single β phase by aging at 200°C for 3 min. Reversibility was observed between the αMt transformation and the β reverse transformation. In-situ high-temperature X-ray diffraction measurements revealed that αMq→β reverse transformation occurred at 200°C and that a thermally activated αiso 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 αMt was formed during heating process. However, αMt 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 (1196°C) to 250°C was performed to ensure a sufficient temperature difference, but αMt was not generated at all. From the cross-sectional observation of the STQ plate, it was found that αMq 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 αMt 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 αM.