Phase transformation and mechanical properties of nanocrystalline Ti–2Fe-0.1B alloy processed by high pressure torsion

In this paper, nano-Ti-2Fe-0.1B alloys with different grain sizes were processed by high pressure torsion (HPT) deformation. The study examined the microstructural evolution of the Ti-2Fe-0.1B alloy during HPT and evaluated its mechanical properties through microhardness and tensile tests. The results of the microstructural observations revealed that the grain size of the Ti-2Fe-0.1B alloy progressively decreased from 3.14 μm in the as-annealed state to approximately 20nm after 10 turns. In the initial state, the alloy consists of -phase and -phases, with a volume ratio of approximately 5:1. During the HPT process, transformations of -phase and -phase to -phase were observed. The primary reason for this is that the shear forces applied during the HPT process facilitated the phase transformation of the -phase. Additionally, the presence of Fe element in the alloy altered the lattice compatibility between the grains of -phase and -phase, thereby promoting the phase transformation of the -phase, with the accumulated turns of HPT process, the fraction of -phase increased in the beginning gradually, then decreased, which reached to the maximum 80.8% in the 5 turns. This is higher than that of pure titanium and Ti-Fe alloys under the same conditions, because the proportion of the -phase increases with the increase in Fe content. Studies of mechanical properties demonstrated that HPT can substantially enhance the hardness of Ti-2Fe-0.1B alloys to as high as 483 HV, and the tensile strength of the Ti-2Fe-0.1B alloy was observed to increase from 725 MPa to 1568 MPa after 5 turns, which is much higher than Ti-6Al-4V subjected to the identical processing conditions. It can be ascribed to the exist of mass -phase as well as finer grain size in nanocrystalline Ti-2Fe-0.1B alloy.