Microstructure and anisotropy of the mechanical properties in commercially pure titanium after equal channel angular pressing with back pressure at room temperature

Abstract

In this work, we report on the anisotropy of the mechanical properties and the results of in-depth microstructural analysis of commercially pure (CP) grade 2 titanium after severe plastic deformation. CP-Ti was successfully processed at room temperature via four consecutive passes of equal channel angular pressing (ECAP) with very high back pressure (BP). An ECAP-BP die with circular channel cross-section, channel angle φ=90° and arc curvature angle ψ=0° was used. A sub-microcrystalline structure with a grain size of ~150nm exhibits promising mechanical properties, as determined by hardness measurements and tensile and compression tests in different directions. We observed a significant mechanical anisotropy related to the strong texture. Considering the ID, ED and TD to be the insert, extrusion and transverse directions of the ECAP die, respectively, the highest compression strength was attained for samples with the major axis in the ID and in a direction inclined 22.5° from the ID toward the TD (σmax~1150MPa). In contrast, the lowest strength was observed in the ED and at 45° from the ID toward the ED (σmax~940MPa). Although a fracture occurred during compression of the samples tested along the ID, compression along the ED exhibited perfect plasticity with balanced hardening and softening mechanisms. Transmission electron microscopy (TEM) examination after ECAP-BP revealed a small amount of high-pressure hexagonal ω-phase. The occurrence of this phase was induced by a combination of severe plastic deformation and high pressure.