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

Abstract

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 ω-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 °C for 24 h and quenched. The microstructure of the initial Ti–1.5 wt.% Mo alloy consisted of the α-phase and α’-martensite, and the initial Ti–15 wt.% Mo alloy contained polycrystalline β 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 ~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.