Microstructure and mechanical properties of Ti-Mo and Ti-6Mo-X-Y alloys

In this study, we investigated the microstructure, hardness, Young's modulus, and tensile behavior of binary Ti-4∼9 mass% Mo alloys, quenched from a temperature of 1223 K. Among the solution-treated binary Ti-Mo alloys, the lowest Young's modulus was observed in the case oftheTi-6Mo alloy, which was the border composition of α′ /α′. When the rolling deformation of 2%reduction was carried out, the Young's modulus of Ti-Mo alloys having less than 6 mass% Mo increased, whereas that of higher than 6 mass.% Mo reduced. The Young's modulus ofthe 6Mo alloy hardly changed with the rolling deformation. Ti-6Mo-X-(Y) alloys, where X and Y are 1 mass% of Al, Sn, Cr, and Fe, were prepared in order to investigate the influence of additional elements on the microstructure and mechanical properties ofthe 6Mo alloy. The phase constitution of quenched Ti-6Mo-X-Y alloys mostly corresponded with the value of the Mo equivalency (Moeq); however, Ti-6Mo-l Al-lFe (MAF) and Ti-6Mo-lFe (MF) exhibited slightly more β-rich structures for their respective Moeq values. The lattice parameters "a" and "b"of the α"-structure in the Ti-6Mo-X-Y alloys changed with Moeq in a manner similar to that in the Ti-Mo alloy; however, "c"exhibited a different behavior. The mechanical properties of Ti-6Mo-X-Y alloys except for the MF alloy were similar to those of Ti-Mo alloys corresponding Moeq. However, the Young's modulus of the Ti-6Mo-X-Y alloys was greater than that of the 6Mo alloy. The tensile properties of the MF alloy having β+ω structure were extremely unstable with respect to the fracture elongation. This peculiar behavior is attributed to the occurrence of deformation-induced β+ωα′ transformation. The formation of the α′ structure caused significant softening and local intense deformation in the α′ phase resulted in a brittle fracture. On the other hand a successive α′ formation induced by work hardening would result in good ductility. It was suggested that the unstable elongation inthe MF alloy resulted from competition between the reverse effects.