Gradient nano microstructure and its formation mechanism in pure titanium produced by surface rolling treatment

A large depth of 800 µm gradient microstructure was produced in pure titanium (Ti) by means of surface rolling treatment (SRT). The microstructural characteristics with different depths from the top surface were analyzed by optical microscopy, transmission electron microscopy, and electron backscattered diffraction. The results showed that, on the outmost surface, nanograins with an average grain size of ∼100 nm were achieved. In the subsurface, i.e., the deformation twinning region, a large volume fraction of({10ar 12})deformation twins together with a low fraction of({11ar 22})and({11ar 24})twins were identified. Based on the microstructural analysis, the grain refinement mechanisms with increasing strain are summarized as: (i) prior division by deformation twinning, (ii) refinement effect of subgrain boundaries resulting from the accumulation of high density of dislocation, and (iii) transformation effect from low angle grain boundaries to high angle grain boundaries. The results of tension tests also show that the titanium sample after SRT shows higher strength than the as-received titanium sample.