Diffusive and displacive phase transformations in nanocomposites under high pressure torsion

Abstract

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 α/α' and β phases. In turn, the β 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 α/α' and β-phases into mixture of a and high-pressure ω-phase. The α → ω and β → ω phase transformations are martensitic, and certain orientation relationships exist between a and ω as well as β and → phases. However, the composition of →-phase is the same in all samples after HPT and does not depend on the composition of β-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, α-Ti and ω-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 a'-Ti and β-Ti before HPT.