The alpha -> omega and beta -> omega phase transformations in Ti-Fe alloys under high-pressure torsion

The formation of omega-phase under high-pressure torsion (HPT) has been studied in Ti-Fe alloys. Seven alloys with Fe concentration from 0 to 10 wt % have been annealed between 600 and 950 degrees C, quenched and HPT-treated at 7 GPa, 1 rpm, 5 and 0.1 anvil rotations (equivalent strain e(eq) = 156 and = 3.1, respectively). The strain after 0.1 rot. corresponds to the transient state of HPT, and that after 5 rot. corresponds to the HPT steady-state and to the dynamic equilibrium between formation and annihilation of microstructure defects. A defect-rich high-pressure omega-phase forms after HPT and persists in the samples also after the pressure release. The amount of retained omega-phase after HPT depends on the iron concentration. It increases from 40% in pure titanium, reaches maximum of 95% at 4 wt % Fe and then decreases again to 10% at 10 wt % Fe. It is because the addition of iron influences the lattice parameters in beta and omega-phases in a different manner. The minimal lattice mismatch between beta- and omega-phases is reached at 4 wt % Fe. A good conformity between the lattices of the beta- and omega-phases enhances the probability of the martensitic (diffusionless) beta -> omega transformation. Based on the XRD and TEM observations, the crystallography and mechanisms of alpha -> omega and beta -> omega phase transformations (which can be diffusionless as well as controlled by mass transfer) under the influence of pure shear by HPT are discussed. (C) 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.