Initial texture dependence of nanocrystalline omega phase formation during high pressure torsion of commercially pure titanium

Abstract

The present investigation studied the influence of initial texture on large strain deformation behavior of hexagonal close packed (HCP) titanium in high pressure torsion (HPT). Discs of two distinct initial textures comprising of near basal and near prismatic-pyramidal orientations were subjected to HPT at room temperature. X-ray diffraction (XRD) revealed the formation of omega phase in HPT processed samples with the fraction of omega phase increasing with increase in strain. Transmission electron microscopy and precession electron diffraction showed the formation of nanocrystalline grains of alpha and omega phases in 5 turns samples for both the orientations. Although omega transformation initiated first in the prismatic-pyramidal textured sample, the basal textured sample showed higher omega volume fraction at higher strain. The mechanical response based on micro-hardness measurements is different for the aforementioned two different initial textures. This is because hardness depends not only on the omega phase fraction, but also on the grain size and distribution of the nanocrystalline omega phase. Further, the relative contributions of strengthening imparted by the omega phase and hardening caused by the accumulated strain component determines the overall hardness profile. Also, synchrotron XRD measurements were performed to estimate crystallite size, microstrain and dislocation densities. The latter analysis coupled with the microstructural analysis indicates that initial texture and crystallographic orientation relationships of omega transformation influence the distribution of omega phase and strain partitioning between the alpha and omega phases, resulting in a strong texture dependence of hardness response.