Torsion fatigue behavior of pure titanium with a gradient nanostructured surface layer

The effect of surface nanocrystallization (SNC) on torsion fatigue property of pure titanium was investigated. Compared with coarse-grained Ti (CG Ti), the SNC Ti exhibits the longer fatigue life at the same cyclic stress amplitude and the higher fatigue strength limit. Both CG and SNC Ti display cyclic softening during torsional loading. The cyclic softening rate of SNC Ti is lower than that of CG Ti. Microstructural analysis reveals that, fatigue crack initiation is suppressed in the SNC Ti since the surface gradient nano/ultrafine grained layer restrains strain localization. As a result, the mechanism of crack initiation changes from grain boundary and twin boundary cracking to shear band cracking. Parallel dislocation lines, embryonic cells and twins are observed in the fatigued CG Ti. In comparison, no obvious variation of substructures can be distinguished in the SNC Ti before and after torsion fatigue. The relative stable microstructure in the SNC Ti facilitates the improvement of fatigue life. Furthermore, the large residual compressive stress on the surface restrains the fatigue crack propagation in the SNC metals. Therefore, the SNC Ti displays the better torsion fatigue properties than the CG Ti.