The orthorhombic α" martensite was formed in Ti-8 mass%Mo alloy by quenching from 1223 K. The purpose of this study was to investigate phase transformation of the α" martensite structure by isothermal aging. In differential scanning calorimetry curve of the quenched specimen, an exothermic peak that indicated decomposition from the α" martensite to α and β phases was observed near 780 K, so that isothermal aging was performed at 723 K and 923 K for 9.0 ks. Optical microscopy, X-ray diffraction and transmission electron microscopy were performed to these specimens. Band-like products that were composed of the single variant of ω phase were observed in the quenched α" martensite structure. On the other hand, (111)α" twins were observed in the 723 K-aged α" martensite structure. The quenched α" martensite structure indicated low elastic incline and good ductility, whereas the 723 K-aged α" martensite structure indicated high yield stress and brittleness. It was pointed out that the ω products were formed to relax the volume expansion from the β phase to the α" martensite, and the (111)α" twins were formed during the isothermal aging at 723 K with the extinction of the ω products.
