High Strength Ti of Substantial Interstitial Contents Produced by Back Pressure Equal Channel Angular Consolidation of Dehydrided Particles

Back pressure equal channel angular (BP-ECA) processing was utilised to consolidate a dehydrided (DH) Ti powder of high interstitial content (1.15 wt.% O, 0.09 wt.% N) at 630°C into fully dense bulk ultrafine-grained (UFG) Ti. The consolidated samples showed an increase in the contents of oxygen (1.34 wt.%) and nitrogen (0.3 wt.%). The measured densities of 4.53 g/cm3 for the consolidated samples after 1 and 3 passes were very close to the theoretical value of pure Ti. TEM revealed the formation of a bimodal microstructure in the one-pass sample, comprising equiaxed grains of several micrometers in size with ultrafine grains of the order of 100 nm uniformly distributed at the triple grain junctions. Most grains had high-angle grain boundaries with some boundaries exhibiting non-equilibrium characteristics. Upon further BP-ECA processing to three passes, the micrometer-sized grains were refined down to the ultrafine level and copious nanoscale deformation twins were introduced by severe plastic strain into those ultrafine grains of the order of 100 nm. As a result of high interstitial contents and refined grains, the sample after processing for 3 passes exhibited remarkably enhanced true yield and ultimate strengths of 1510 and 2050 MPa, respectively. Significantly, a noticeable compressive ductility was simultaneously attained despite such a high interstitial content, thanks probably to the non-equilibrium grain boundaries, bimodal grain structure and the occurrence of deformation twinning.