The influence of microstructure and texture on the hardening by annealing effect in cold-rolled titanium

Abstract

The purpose of this study was to compare the influence of microstructural and textural changes on the hardening by annealing effect in cold-rolled titanium. Ultrafine-grained (UFG) Ti Grade 2 was produced by multi-pass cold rolling (or warm rolling at 400 °C at the final stage) to different thickness reductions (90%, 95% and 97%) aimed at varying both the microstructural features of the material (dislocation density, grain size distribution and grain boundary characteristics) and its texture (the intensity and volume fraction of texture components). The hardening effect of UFG Ti Grade 2 sheets was obtained by a short-time annealing at 250 °C for 15 min. The highest strengthening, of about 4–5%, was observed for the UFG Ti Grade 2 sheet rolled to 90%; the strengthening gradually decreased for higher thickness reductions (down to ∼ 2% for 97%). The texture intensity and volume fractions of texture components for the annealed UFG Ti Grade 2 sheets were very close to their as-rolled counterparts, so this did not demonstrate any clear reason for the hardening effect. Instead, the dislocation substructure recovered during annealing at 250 °C, i.e. the dislocation density declined significantly, the remaining dislocations became rearranged, and it was easier to differentiate more nanosized subgrains of about 50–150 nm. The hardening by annealing effect was a result of the annihilation of mobile dislocations and the ordering of the dislocation substructure, mostly within coarse grains (>500 nm). The level of strengthening by low-temperature annealing was mainly affected by the fraction of coarse grains with a tangled-dislocation substructure in the as-rolled state, i.e. a higher fraction of coarse grains favored more pronounced strengthening. These observations seem to be very promising for optimizing the cold rolling process and short-time annealing of the UFG Ti Grade 2 sheets, which could be a simple and cost-effective way of enhancing their mechanical properties.