Corrosion Behaviour In Ultrafine-grained Pure Titanium Processed By High-pressure Torsion

Due to the increase of the demand towards microstructure product,microforming technology has developed sharply.Plastic microforming technology is a kind of low-cost and high-productivity forming method of microstructure deformation,therefore the technology possessed vast potential for future development.Conventional coarse-grained materials have obviously scale effect in microforming processing,which results that the properties and sizes of the products have been influenced.However,ultrafine grained and nano-crystalline metallic materials processed by severe plastic deformation(SPD)improved the uniformity of the common materials,and had high plastic forming property.Therefore,the application of ultrafine-grained materials in microforming technology will be a significant developed trend.Pure titaniumpossesses the excellent corrosion resistance and biocompatibility,is one of the most potential medical metal materials.The grain-refinement of pure Ti is an effective method to improve the mechanical properties.For this purpose,high-pressure torsion technology(HPT)to prepare ultrafine industrial pure titanium.The electrochemical properties of commercially Ti have been measured by open circuit potential,polarization curves and AC impedance graphic,combined with the measure method of surface morphology by transmission electron microscopy(TEM)and scanning electron microscope(SEM),the corrosion behavior of ultrafine industrial pure titanium processed by HPT in sulfuric acid solution and simulated body fluid environment were researched.The effect of different torsion turns on the microstructure and grain sizes of the ultrafine industrial pure Ti processed by HTP have been researched.The result indicated that when high-pressure torsion process reached 0.25 t,a large number of twins and dislocations emerged in the grains of the processed samples due to the vast shear stress and torsion deformation.When the turn reached 1t,there is no decrease in the size,and the microstructure consisted of dislocation substructure such as slips,dislocation tangles and dislocation cells.When the time reached 5t,the grains were refined to about 0.3?m,and the microstructure was composed of fine dislocation cells and submicron grains.When the time arrived at 10 t,the grains were refined obviously,and the average grain size was about 150 nm.In this case,the grains had been obviously refined,and possessed high uniform properties..The influence of annealing on the microstructure of ultrafine industrial pure titanium was studied.The results indicated that when the annealing temperature remained at 200 oC,the dislocation number of ultrafine pure titanium reduced,but the grain boundary was still in non-equilibrium condition.When the annealing temperature was 400 oC,the reversion emerged in the non-equilibrium structure,but the difference among grains become bigger,and the recrystallization nucleation stage disappeared.Besides,the proportion of dislocation tangles in the internal zones become larger.When the annealing temperature was at 600 oC,the recrystallization emerged and the dislocation density decreased gradually,even the internal stress in the grains reduced.Besides,the microstructure mainly consisted of fine,uniformed grains.The corrosion behavior of HPT-processed ultrafine industrial pure titanium is investigated in sulfuric acid solution.The results showed that the corrosion resistance after HPT processing is lower than for the original coarse-grain titanium,but the corrosion susceptibility of the HPT-processed specimen decreases with increasing torsional circles.When the processing time was at 0.25 t,the corrosion resistance of the ultrafine samples was the worst,while the time was at 10 t,the corrosion resistance was the best.The corrosion resistance as the increase of annealing temperature rose.When the sample remained 0.5h under the 600 oC,the corrosion resistance was the best,and the current density decreased obviously.Besides,the surface-corrosion production distribution is uniformed,and the compactness was the best.The passivation coating component of original titanium and ultrafine pure titanium in the sulfuric acid solution was Ti O 2.The research in the simulated condition of body fluid showed that the corrosion occurred at the grain boundary of original titanium and ultrafine titanium,but the corrosion resistance of original titanium was best.As the increase of torsion circles,the corrosion electric potentialmoved positively and corrosion curren t reduced dramatically.Besides the high frequency capacitance arc radius of AC impedance and low frequency phase angle increase,and the current resistance of fitting passive coating was increased,and the corrosion resistance improves sufficient.With the increase of annealing temperature,the corrosion resistance improves,and when the sample remained 0.5h as 600 oC,the corrosion resistance was best.