Enhancement of tensile strength and ductility of titanium with a bimodal structure

A bimodal structured commercial pure (CP) titanium consisted of coarse grained alpha Ti plates with an average thickness of 1.60 mu m and ultrafine structured beta transformed domains was fabricated by high energy mechanical milling of a TiH2 powder, in-situ dehydrogenation of TiH2 and powder compact extrusion of the dehydrogenated Ti. It exhibited a high yield strength (YS) and ultimate tensile strength (UTS) of 731 and 903 MPa respectively and an appreciable tensile ductility (elongation to fracture: 8.8%). After vacuum annealing at 700 degrees C for 6 h, the hydrogen content was reduced from 0.36 wt% to 001 wt% and the microstructure of the titanium changed into another type of bimodal structure consisting of coarse equiaxed grains with an average size of 5.19 mu m and ultrafine grains with an average size of 0.325 mu m. This microstructural change caused the YS and UTS of the CP titanium to clearly increase to 794 and 932 MPa, and its tensile ductility to increase drastically (elongation to fracture:26.2%). Analysis of the correlation between the microstructures, tensile properties and fracture behavior of the CP titanium suggests that the drastic tensile ductility improvement with the change of microstructure after annealing can be attributed to the high capacity of equiaxed Ti grains to accommodate plastic deformation due to their large number of crystallographic orientations than the coarse grained a-Ti plates which have a limited number of crystallographic orientations. In the meantime, the grain boundary strengthening of the ultrafine grained domains of the annealed CP titanium still ensures a high strength.