Surface morphology and gradient microstructural evolutions in pure titanium via surface severe plastic deformation

A pure Ti plate with a smooth surface and a gradient nanostructured layer was achieved using surface mechanical attrition treatment method with 3 mm zirconia balls at room temperature. The twinning and dislocation slips layer, transition layer, ultrafine grains layer and nanograins layer appeared in sequence from the matrix to the topmost surface, representing different stages of microstructural evolutions. The dislocation slips and 101¯2 twins coordinated and competed with each other to adapt to plastic deformation. An additional misorientation and mirror symmetry line separation resulted from the interaction between <c + a> slips and 101¯2 twin boundaries, causing the twin boundaries to gradually transform into high angle grain boundaries. A stronger dislocation–twin boundary interaction in the transition layer triggered the continuous formation and accumulation of high angle grain boundaries, which subdivided the twins into blocks or submicronic polygonal grains. It is proved that a combination of continuous dynamic recrystallization and discontinuous dynamic recrystallization leads to the generation of nanograins.