Effects of annealing on microstructure and mechanical properties of nano-grained titanium produced by combination of asymmetric and symmetric rolling

Nano-grained (NG) commercially pure (CP, grade 2) Ti with average grain size of 80 nm was produced by the combination of asymmetric and symmetric rolling at room temperature. The effects of annealing on mechanical properties and microstructure of this NG Ti were investigated systematically. Examination using transmission electron microscopy (TEM) revealed that the annealed Ti samples possessed ultrafine-grained (UFG) structure in which grain size was below 200 nm even though annealed at 400 °C for 30 min, which suggested considerable thermal stability of the microstructure. X-ray diffraction (XRD) results showed that the preferred orientations along the (0 0 0 2) and (1 0 1¯ 3) crystal plane were formed in the NG Ti and both the two preferred orientation were slightly decreased with the increase of annealing temperature. It was found that annealing of this NG Ti with appropriate temperature and time resulted in the abnormal effect that the microhardness, strength and ductility were simultaneously enhanced as compared to the as-processed state. Further increasing of the annealing temperature and time leads to the decrease of microhardness and strength. Strain rate jump tests revealed that the strain-rate sensitivity (SRS) parameter m of the NG Ti was slightly enhanced with the increase of annealing temperature. The mechanisms of variations in mechanical properties were analyzed based on microstructure evolution of the annealed NG Ti.