Phase transformation of α martensite structure by aging in Ti-8 mass%Mo alloy

The orthorhombic alpha 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 alpha martensite structure by isothermal aging. In differential scanning calorimetry curve of the quenched specimen, an exothermic peak that indicated decomposition from the alpha martensite to alpha and beta phases was observed near 780K. 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 omega phase were observed in the quenched alpha martensite structure. On the other hand, (1 1 1)alpha twins were observed in the 723 K-aged alpha martensite structure. The quenched alpha martensite structure indicated low elastic incline and good ductility, whereas the 723 K-aged alpha martensite structure indicated high yield stress and brittleness. It was pointed out that the omega products were formed to relax the volume expansion from the beta phase to the alpha martensite, and the (1 1 1)alpha twins were formed during the isothermal aging at 723 K with the extinction of the omega products.