Investigations On Surface Grain Refinement And Nitriding Of Pure Titanium

Titanium and its alloy have been widely used in various fields due to their high specific strength,good corrosion resistance and biocompatibility.However,the weak points of titanium alloys are their low strength and poor wear resistance.Gas nitriding is commonly used in surface strengthening of titanium and its alloy,and nitriding layer has high hardness,good wear resistance and seizure resistance.However,conventional nitriding process in industry is very slow as a result of the reaction kinetic limitations.Nitriding temperature of titanium changes between 800?900?,and nitriding duration can often reach 50?80 hours,which have greatly increased the costs of industrial nitriding.High-temperature nitriding also lead to workpiece deformation and surface toughness.Therefore,it is of great significance to improve the nitriding rate and reduce the nitriding temperature in industrial production.The ultrafine-grained materials have abundant structural defects such as grain boundaries,dislocations and so on,and these defects can serve as a fast channel for atom diffusion.In addition,crystal defects increase energy storage in the ultrafine-grained material,which is very beneficial for chemical reactions and phase transitions.Therefore,formation of a ultrafine-grained layer on the surface of titanium alloy is helpful for enhancing diffusion of nitrogen atoms and reducing nitriding temperature.In this paper,nanocrystalline and submicron layers were prepared on the surface of pure titanium samples by surface mechanical attrition treatment(SMAT)and mechanical shot blasting(MSB),respectively.Then the titanium samples were subjected to gas nitriding.The microstructures and properties of the ultrafine grained and nitrided layers were tested and analyzed by means of metallographic microscope,X-ray diffraction,scanning electron microscopy and transmission electron microscopy.Below are key research findings:Study on surface micro structure after SMAT and performance of the nitriding layer(1)Nanocrystallization of the pure titanium samples were achieved by surface mechanical attrition treatment.The average grain size of the surface nanocrystalline layer was 18.48 nm.Thermal stability studies showed that the nanocrystalline layer had good structural stability below 500?.(2)Surface mechanical attrition treatment improves nitriding ability of pure titanium.After gas nitriding at 550? for 5 h,nitrided layer with thickness of 15?25 ?m was formed on the surface of the SMAT sample,whereas nitrided layer was almost not found on the surface of the coarse-grained sample.(3)Microhardness,wear resistance and surface toughness of the nitriding sample were significantly improved after surface mechanical attrition treatment.Study on surface micro structure after MSB and performance of the nitriding layer(1)Experiments confirmed the suitable MSB processing parameters:800 rpm,for 2 h.A gradient submicron crystal structure layer was prepared on the surface of titanium plate and the average grain size of the surface layer was about 200 nm.(2)The diffusion of nitrogen atoms in pure titanium plate could be accelerated by mechanical shot blasting treatment.In temperature range of 600?850?,the thickness of nitrided layer on the surface of the MSB nitriding titanium plate was always higher than that of coarse-grained nitriding titanium plate,and the difference of thickness reached to maximum at 750?.(3)Surface microhardness and wear resistance of the MSB nitriding titanium plate had been improved compared with that of the coarse-grained nitriding titanium plate.However,the surface nitriding brittleness grades were both ???,so the surface mechanical shot blasting treatment did not improve the toughness of nitrided layer.(4)By comparison of the SMAT nitriding sample,the MSB nitriding sample and the coarse-grained nitriding sample,it was found that the gradient nanostructure layer and the submicron structure layer effectively promote the nitriding process.