Surface self-nanocrystallization of α+β titanium alloy by surface mechanical grinding treatment

In this work, the heavy deformation was performed on Ti6Al4V alloy by using surface mechanical grinding treatment (SMGT) in order to obtain surface nanocrystalline layer. The phase structure and microstructures in the deformation zones were characterized by XRD, SEM and TEM. The nanocrystallization mechanisms of α and α phase were clarified. The results show that a gradient structure including nano grain, quasi nanograin and micro-grains was achieved within the depth of 500 μm below the surface. The depth of nanocrystallines layer was at least 20 μm. From the non deformation zone in the center to the heavy deformation zone close to treated surface, dislocation densities gradually increased. The original coarse grains gradually evolved into dislocation cell structures and subgrains through dislocation movement. In the depth of about 20 μm below the treated surface, the subgrains began to split into nano-scale grains with high angle grain boundaries by lattice rotation and tilting mechanisms. During SMGT, β phase deformed and refined prior to α phase, and inhibited the generation of twinning in α grain. The coordinated deformation between a and β phase promoted the formation of nanocrystallines.