Gradient nanostructure, enhanced surface integrity and fatigue resistance of Ti-6Al-7Nb alloy processed by surface mechanical attrition treatment

Current Ti-based orthopedic implants often suffer from fatigue damage, therefore shortening their service lifespan. To solve this issue, in this study, mechanically polished Ti-6Al-7Nb (P-Ti6Al7Nb) was subjected to surface mechanical attrition treatment (SMAT). Effects of various SMAT process parameters, including ball diameter and treatment duration, on the surface integrity of P-Ti6Al7Nb were investigated, specifically in terms of surface quality, surface nanocrystalline layer, and residual stress. Subsequently, the microstructure, in-depth residual stress and microhardness distributions, surface roughness, and fatigue behavior in simulated body fluids of optimally SMATed Ti-6Al-7Nb (S-Ti6Al7Nb) were examined and compared to those of P-Ti6Al7Nb. Results showed that based on the experimental conditions established in the present research, the optimal parameters were determined to be a 3 mm ball diameter and a 15 min treatment duration, which resulted in excellent surface integrity; S-Ti6Al7Nb showed a 300 μm-thick gradient nanostructured layer comprising the thickest nanocrystalline layer of about 20 μm, a 1000 μm-deep residual compressive stress field with the maximum surface residual compressive stress, and a micro-concave topography but free of any defects or cracks. The microstructural evolution mechanism was also elucidated, revealing that the combination of multidirectional primary and secondary twins’ intersections and twin-dislocation interactions contributed to grain refinement. Compared to P-Ti6Al7Nb, S-Ti6Al7Nb exhibited a 40% improvement in fatigue strength, owing to synergistic effects of the gradient nanostructured layer, surface work hardening, high amplitude of residual compressive stress, and improved surface integrity. These factors effectively prevented the initiation of fatigue crack at the surface and shifted it to the sublayer, and inhibited the subsequent crack propagation.