The Investigation Of Microstructure And Fatigue Properties Of Titanium Alloy Treated By Ultrasonic Surface Rolling Process And Plasma Electrolytic Oxidation

Fretting fatigue is an important failure cause for aero-engine and orthopedic implants.Metals with gradient nanostructure have significant advantages owing to the high strength,good wear resistance and fatigue resistance.Plasma electrolytic oxidation（PEO）could significantly improve wear resistance of titanium alloy.However,PEO treatment is not conducive to fatigue performance of titanium alloy.Ultrasonic surface rolling process（USRP）,is used to prepare a gradient nanostructure to improve the fatigue performance of titanium alloy.Therefore,in this study,the influence mechanism of USRP and USRP-PEO treatment on the microstructure,plain fatigue and fretting fatigue of titanium alloy are studied.The aim is to make new progress in improving fretting fatigue properties of titanium alloys.The main results are as follows:（1）USRP induces a SPD layer more than 400μm thick with a gradient nanostructure in the surface layer of the Ti-6Al-4V alloy.This nanostructure consists of:a roughly equiaxed nanograin layer,an elongated nano-lamellar layer,an elongated ultrafine lamellar layer,a refined grain layer,and a low-strain coarse-grained layer ranging from the treated surface to the interior.The surface nanocrystallization mechanism ofαphase in titanium alloy treated by USRP is dominated by complex dislocation activities in hcp-Ti grains without deformation twinning occurring.Besides,the phase transformation from hcp-Ti to fcc-Ti occurs as a supplementary process.The surface nanocrystallization mechanism ofβphase in USRP-treated titanium alloy is dominated by dislocation activities.（2）Compared with the untreated sample,the tensile strength increases slightly but the yield strength decreases obviously in USRP12-treated titanium alloy.Simultaneously,the USRP-treated sample has a good combination of strength and ductility.This phenomenon is associated with the gradient nanostructure surface layer,hcp to fcc phase transformation and a texture with（0002）plane at the surface layer.The main reason for the transition from hcp-Ti to fcc-Ti at the surface is attributed to exceedingly high strain and strain rate.And in the interior of the material where the strain and strain rate are low,the drive force for the transition from hcp-Ti to fcc-Ti is related to the stress concentration at theα/βphase boundary.（3）Both of USRP1 and USRP12 significantly improve the plain fatigue property of titanium alloy at low stess amplitude and the improvement effect of USRP1 is more significant.It is attributed to the synergistic effect of gradient nanostructure and CRS,where CRS plays a major role in improving the plain fatigue of titanium alloy.USRP1-treated sample has a better fatigue resistance due to the moderated grain size,the high strain-hardening capacity and the properly matched CRS.During the fatigue test,the CRS in USRP-treated sample reduces monotonously.After fatigue test,the grains in USRP1-treated sample surface layer is refined while the the grains in USRP12-treated sample surface layer is coarsened.（4）Both of USRP1 and USRP12 significantly improve the fretting fatigue property of titanium alloy at the low stress amplitude.It is attributed to that the synergistic effect of CRS and refined microstructure or work hardening layer inhibits the fretting fatigue crack initiation and early propagation.And the refined microstructure or work hardening layer plays a major role in improving the fretting fatigue of titanium alloy.The CRS induced by USRP increases first and then decrease with the number of fretting fatigue cycle.This is mainly related to the local plastic deformation and surface damage caused by the mutual sliding between the fretting pads and the sample.The fretting fatigue promotes the grains grow up on the USRP1-treated sample.（5）Surface nanocrystallization induced by USRP improves the chemical activity of titanium alloy,and accelerates the mass transfer of the titanium alloy surface during the PEO process,and thereby facilitates nucleation and growth of the PEO coating.The barrier layer in PEO coating is formed via O^2-and Ti⁴⁺inter-migration and diffusion.This layer undergoes a dynamic growth process that consisted of a clyclic process:“formation via ionic migration and diffusion→dielectric breakdown”.（6）PEO treatment severely impairs the plain fatigue performance of the titanium alloy at low stress amplitude.This is mainly attributed to the fact that the micropores and microcracks in the coating near to the substrate and the brittleness of the coating promotes fatigue crack initiation.The USRP pretreatment yields significant improvement in plain fatigue performance of PEO-coated titanium alloy.This is attributed to the synergistic effect of the gradient nanostructure and the CRS,which retards fatigue crack initiation and propagation.The CRS plays the dominant role in improving the fatigue life.（7）PEO treatment yields effective improvement in fretting fatigue performance of titanium alloy at low stress amplitude.This improvement results mainly from the fact that the high hardness of the ceramic oxide coating and the occurrence of CRS in the coating mitigates the fretting wear and retards the fretting fatigue crack initiation.USRP pretreatment yields significant improvement in fretting fatigue performance of titanium alloy.And the improvement effect of fretting fatigue performance at low stress amplitude is more significant.This results from the fact that the high hardness of the ceramic oxide coating retards crack initiation and the USRP-induced CRS and gradient nanostructure retards early crack propagation.Among these three factors,the CRS plays the dominant role in the aforementioned improvement.