Microstructure Evolution And Mechanical Properties Of Commercially Pure Titanium After Equal Channel Angular Pressing At Room Temperature

Equal Channel Angular Pressing (ECAP), as one of the main techniques to producebulk Ultrafine-grained (UFG) materials, can not only refine grain by severe approximatepure shear deformation but also improve the mechanical properties. The hot-rolledannealed commercially pure titanium was used as the research material. The smooth andcrack-free samples were successfully fabricated by ECAP for4passes via route C (thebillet is rotated180°clockwise along its longitudinal direction between adjacent passes)at room temperature using a die with channel angle Φ=90°and an outer curvature ψ=20°. The microstructure evolution and mechanisms of microstructure refinement andstrengthening during ECAP were investigated by optical microscope, transmissionelectron microscope and mechanical properties test.Lots of banded structures with dislocation tangle zones and dislocation cell blockswere formed by shear deformation and twins were activated and accommodated thedeformation during the initial stage of ECAP at room temperature. With increasingnumber of the ECAP passes, subgrains of low angle grain boundaries was graduallyformed by dislocations sink in dislocation cell walls and then evolved into high anglegrain boundaries by rotation. Finally, the dislocation density reduced. A great quantityof subgrains and high angle grain boundaries were gradually formed and equiaxedultrafine grained structures were achieved.The microstructure evolution of the corner of CP Ti subjected to ECAP wasinvestigated by optical microscope and transmission electron microscope. The results show that dislocation slip is the main deformation mechanism of CP Ti during the ECAP.Twins are activated and accommodated the deformation. The number of twins close tothe inside of the channel corner is more than that of close to the outside of the corner.The results show that the original grains are refined to about170nm, and themicrohardness increases significantly and the microhardness values are distributedhomogeneously throughout the cross-sectional plane after four passes of ECAP. There isexcellent strengthening after ECAP with the ultimate tensile strength increasing from438to728MPa．Meanwhile，there is also reasonable ductility with an elongation tofailure of21.4%．...