Diffusive And Displacive Phase Transformations In Nanocomposites Under High Pressure Torsion

The high-pressure torsion (HPT) of Ti-Fe alloys with different iron content has been studied at 7 GPa, 5 anvil rotations and rotation speed of 1 rpm. The alloys have been annealed before HPT in such a way that they contained different amounts of alpha/alpha' and beta phases. In turn, the beta phase contained different concentration of iron. The 5 anvil rotations correspond to the HPT steady-state and to the dynamic equilibrium between formation and annihilation of microstructure defects. HPT leads to the transformation of initial alpha/alpha' and beta-phases into mixture of alpha and high-pressure omega-phase. The alpha -> omega and beta -> omega phase transformations are martensitic, and certain orientation relationships exist between alpha and omega as well as beta and omega phases. However, the composition of omega-phase is the same in all samples after HPT and does not depend on the composition of beta-phase (which is different in different initial samples). Therefore, the martensitic (diffusionless) transformations are combined with a certain HPT-driven mass-transfer. We observed also that the structure and properties of phases (namely, alpha-Ti and omega-Ti) in the Ti - 2.2 wt. % Fe and Ti - 4 wt. % Fe alloys after HPT are equifinal and do not depend on the structure and properties of initial alpha'-Ti and beta-Ti before HPT.