Effect of Surface Nanocrystallization on Fatigue Behavior of Pure Titanium

The high-cycle fatigue behavior was investigated in pure titanium after surface nanocrystallization (SNC Ti). Compared with the coarse-grained titanium (CG Ti) samples, the SNC Ti samples exhibit an improved fatigue life. The SNC has a remarkable influence on the fatigue cracks initiation and growth of pure titanium. The results show that, because the free-surface cracking is suppressed by the surface nanogradient structure in the SNC Ti, the fatigue cracks initiation sites change from the free surface to the subsurface. Meanwhile, the fatigue crack growth rate decreases due to the microstructural feature and residual compressive stress. The deformation twins in the subsurface of SNC Ti have a marked effect on the fatigue crack initiation and the crack growth. The former effect is due to the twin boundaries being preferential sites for crack initiation, while the latter is associated with the barriers that the twin boundaries pose to the propagation of dislocations. Furthermore, microstructural analysis indicates that the dislocation distribution in SNC Ti gradually becomes homogenous as fatigue processes. This homogeneous microstructure is also beneficial to the improvement of fatigue life.