Study On Microstructure And Properties Of Ultrafinegrained Pure Titanium After ECAP And ECAP+CR At Room Temperature

Equal Channel Angular Pressing(ECAP) is an effective method to produce ultrafine-grained materials, which can refine the grains and improve mechanical properties by severe plastic deformation. Ultrafine-grained pure titanium was successfully processed by ECAP with small angle dies and traditional cold rolling at room temperature in this paper. Microstructure and properties of pure Ti after ECAP and cold rolling were investigated by optical microscope, transmission electron microscope, uniaxial tensile test and microhardness test.Pure titanium was successfully processed by ECAP for 4 passes via route Bc with a die of 90°. During the initial stage of ECAP, lots of banded structures were formed by severe shear deformation and dislocation density increased rapidly. Few twins were activated and accommodated the deformation simultaneously. With the increasing of ECAP pass, dislocations moved to dislocation cell walls. Dislocations were piled up and rearranged and disappeared at dislocation cell walls, and then subgrains with low angle grain boundaries were formed. The decrease of dislocation density and rotation of subgrains leaded to the evolution from low angle grain boundaries to high angle grain boundaries during the later stage of ECAP. Original coarse grains were refined to about 160 nm. The mechanical performance results show that the ultimate tensile strength increases from 284 to 599 MPa, and the microhardness increases from 1040 to 1792 MPa. Meanwhile, there is reasonableductility with an elongation to failure of 17%.After a single pass of ECAP and cold rolling, a lot of deformed microstructures and equiaxed ultrafine grains were formed. This makes ultimate tensile strength and elongation increase at the same time. The annealing experiments results show that the recovery process occurred when the annealing temperature was below 400℃, which can make grain boundaries gradually clear and dislocation density decrease. When the annealing temperature was higher than 400℃, ultrafine grains grew slowly, ultimate tensile strength and microhardness decreased obviously. When the annealing temperature at 500℃, ultrafine grains grew rapidly, and the average grain size was 2μm.