Microstructure Evolution of Surface Gradient Nanocrystalline by Shot Peening of TA17 Titanium Alloy

A surface gradient nanocrystalline structure (SGNS) was obtained by shot peening (SP) on the TA17 near α titanium alloy to improve its surface properties. The effect of shot peening time was investigated by characterizing the grain size of the surface nanocrystalline layer, the thickness of the severe plastic deformation (SPD) layer, the microstructure evolution of the SGNS and the hardness change. The experimental results show that the grains of TA17 titanium alloy can be refined to a nano-scale of about 22 to 26 nm when shot peened at 0.6 MPa pressure for 5 to 10 minutes. The thickness of the SPD layer increases from 55 to 88 μm with the SP duration from 5 to 10 minutes and tends to be saturated afterward. The SGNS is composed of a surface nanocrystalline layer and a transition layer. During the SP treatment, the coarse grains are first divided into small blocks by intersection of twins, then dislocation walls, dislocation tangles and dislocation bands lead to the formation of low-angle grain boundaries, which subdivide the subgrains into a finer scale until nanograins with a stable size are obtained. The topmost surface hardness can be improved significantly to twice the hardness of the matrix due to both grain refinement and work-hardening.