Microstructural refinement and deformation mode of Ti under cryogenic channel die compression

Commercial purity Ti was grain-refined with a channel die compression method at −190 °C. Repeated cryogenic compression cycles with brief intermittent annealing treatments at 620 °C resulted in the reduction of the grain size from 50 μm to 100 nm. The microstructure in the final state consisted of narrow lamellar structure intermixed with equiaxed grains surrounded by high angle grain boundaries. The effective microstructural refinement was attributed to the suppression of the dynamic recovery and maximization of the stored energy. Due to the fine grain size and the residual dislocation density, the room temperature yield strength and the ultimate tensile strength increased by 80% over the starting values. Using the electron back-scattered diffraction analysis, the deformation mechanism of individual grains was studied in terms of the Taylor axis distribution. While the grains without mechanical twin showed the predominant concentration of the individual grain misorientation axes along [0 0 0 1] the ones containing many twins revealed random distribution of the axes. Based on this observation, a future study was suggested as to the control of the initial texture to enhance the cryogenic deformability of Ti and subsequently a finer microstructure.