Omega Phase Formation and Mechanical Properties of Ti-1.5 wt.% Mo and Ti-15 wt.% Mo Alloys after High-Pressure Torsion

The paper analyzes the effect of severe plastic deformation by the high-pressure torsion (HPT) on phase transformations, in particular, on the formation of the !-phase, and on mechanical properties, such as hardness and Young's modulus, in Ti alloys with 1.5 and 15 wt.% Mo. Both alloys were pre-annealed at 1000 ffiC for 24 h and quenched. The microstructure of the initial Ti-1.5 wt.% Mo alloy consisted of the ff-phase and ff'-martensite, and the initial Ti-15 wt.% Mo alloy contained polycrystalline fi solid solution. The hardness tests of the samples were carried out under the load of 10 and 200 mN. The annealed alloys were subjected to HPT, and the micro- and nanohardness of both deformed samples increased up to similar to 1 GPa compared to their initial state. It turned out that the values of hardness (H) and Young's modulus (E) depend on the applied load on the indenter: the higher the applied load, the lower H and higher E. It was also found that the HPT leads to the 30% increase in E for an alloy with 1.5 wt.% Mo and to the 9% decrease in E for the alloy with 15 wt.% Mo. Such a difference in the behavior of the Young's modulus is associated with phase transformations caused by the HPT.