Abstract
The formation mechanism of α′′-martensite (α′′Mt) induced by tempering at 450-550℃ 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℃-3 s using a salt bath. However, α′′Mt was completely transformed into a single β phase by aging at 200℃ 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℃ and that a thermally activated α′′iso was generated at 450℃ 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℃) to 250℃ was performed to ensure a sufficient temperature difference, but α′′Mt was not generated at all.
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