Microstructure And Properties Of Gradient Nanostructures Prepared On TC11 Titanium Alloy

Ti alloys are generally broadly used in aerospace,marine,petrochemical and other industrial fields because of their low density,high specific strength,corrosion resistance and good fatigue resistance.However,Ti alloys also have defects such as low surface microhardness,poor wear resistance and fatigue sensitivity,and Ti alloy components often serve in harsh and complex environments,which can easily damage the surface and lead to a sharp decrease in their service life.Therefore,it is essential to find efficient,economical and environmentally friendly surface strengthening methods to improve the surface microstructure of Ti alloy components to enhance their service performance.Surface nanocrystallization is a proven surface strengthening method to prepare gradient nanostructures on the surface of components to extend their service life.In this study,the gradient nanostructures were prepared on the surface of TC11 alloy with lamellar microstructure by two surface nanocrystallization technologies,supersonic fine particle bombardment(SFPB)and laser shock peening(LSP).The surface integrity,microstructure evolution and mechanical properties of TC11 alloy after surface nanocrystallization were also studied systematically by using scanning electron microscope(SEM),three-dimensional morphometer,X-ray diffractometer(XRD),transmission electron microscope(TEM),X-ray stress analyzer,microhardness tester and universal mechanical tensile tester.By changing the experimental parameters such as SFPB impact time,gas pressure and LSP pulse energy to regulate the multi-scale microstructure and properties of TC11 alloy surface gradient nanostructure,we optimize the processing parameters to match the best microstructure and optimal properties,which provides important technical support for the application of Ti alloy in industrial fields.The TC11 alloy is treated with both SFPB and LSP to obtain gradient nanostructures.The surface microstructure is broken and the grain size is refined to the nanometer scale.The nanocrystalline grain size does not change much with the extension of SFPB time,and gradually decreases with the increase of SFPB gas pressure and LSP pulse energy.The surface of TC11 alloy forms honeycomb craters after SFPB,and microcracks are formed on the surface when the impact time was too long and the gas pressure was too high.The surface roughness shows a trend of first decreasing,then increasing and slightly decreasing with the extension of the impact time,and the trend of gradually increasing with the increase of the gas pressure.After LSP,microcracks are also formed on the surface,and the surface roughness gradually increases with the increase of pulse energy.By SFPB and LSP treatments,compressive residual stresses are formed in the surface of TC11 alloy,the surface microhardness is significantly increased,and the depth of the hardened layer increases with increasing SFPB time,gas pressure and LSP pulse energy.Both surface nanocrystallization processes resulted in a significant increase in strength,while the elongation does not change significantly.The strength value increases significantly with the increase of SFPB time,gas pressure and LSP pulse energy.However,the microcracks forms on the surface when the impact time is too long and the gas pressure is too high lead to the decrease of the strength.The fracture mode changes from typical ductile fracture to ductile-brittle mixed fracture after both SFPB and LSP treatments.