Effect Of Deformation Texture And Substructure On Nitriding Composite Aging Microstructure Of Ti-6Al-4V Alloy

Ti-6Al-4V(TC4)alloy is the most widely used titanium alloy in the world,but its hardness is low and its wear resistance is poor.this paper designs a compound process of820°C solution treatment+cold deformation+450°C low temperature plasma nitriding and aging to improve the overall performance of Ti-6Al-4V alloy.Metallographic microscope,Scanning Electron Microscope(SEM),EDS energy spectrum analysis,Transmission Electron Microscopy(TEM),X-Ray Diffraction analysis(XRD)and Electron Backscatter Diffraction(EBSD)were used to characterize the transformation of deformed microstructure and nitriding microstructure.The performance changes were characterized by friction and wear test and microhardness test.The research of deformed microstructure shows that,during the solid solution process,the Ti-6Al-4V alloy has aβ→αtransition,with the increase of the deformation,metastableα’andβ’phases that may exist in the matrix further decompose to formβ-Ti during rolling,the content ofβ-Ti increases and is distributed at theα-Ti grain boundaries,grain rotation causes the Burgers orientation relationship is destroyed;After rapid water cooling,there are a lot of residual stresses in the specimen and a large number of dislocations are produced at the grain boundary,dislocation intertangles form dislocation walls and dislocation cells,as the degree of deformation increases,the dislocations continue to move toward the interior of the grains,multiply and increase in density.The texture components of alloys changed with the increase of deformation from approximately transverse texture to transverse texture,the basal plane orientation is concentrated in the TD direction and the cylinder orientation is concentrated in the RD direction.The research of nitrided microstructure shows that,Ti-6Al-4V alloy after deformation nitriding,there are not obvious interface between nitrided layer and diffusion layerin the specimen.N element deep into the alloy matrix penetration through the entire specimen which generated a high hardness TiN phase,the content of TiN in the surface is higher than the content in the core,and the content of TiN in the sample with 30%deformation is the most and the distribution is the most uniform.After nitriding,the content ofβ-Ti in the specimen increased,and the part ofα-Ti was changed toβ-Ti.Part of the generated TiN{001}crystal plane and{110}crystal plane are parallel to the{110}and{111}crystal planes ofβ-Ti and takeβ-Ti nuclei atα-Ti grain boundaries;The other part of the{110}plane of TiN is nucleated along the dislocation of theα-Ti{112~—0}plane in theα-Ti grains.In the process of nitriding at 450℃for 22h,the recrystallization of undeformed samples occurred.The grain size was reduced by about one time,and the grain boundaries at small angles were decreased.The specimens with 30%deformation and the specimens with 50%deformation resumed recrystallization during the nitriding.The degree of recrystallization of the specimens with 50%deformation was not as good as that of the specimens with the deformation of 30%.There are three main factors that influence the formation of TiN:dislocation density,recrystallization degree and grain orientation.The content of TiN is determined by the combination of the three factors.The higher the dislocation density,the higher the degree of recrystallization(the more the number of grain boundaries),the closer the crystal texture is to the transverse texture(the basal plane orientation is concentrated in the TD direction and the cylinder surface orientation is in the RD direction)Therefore,the TiN content of the 30%deformation specimens with the three combined effects is the highest.The research of performance shows that,cold rolling deformation improves the wear resistance of the alloy.After nitriding,the friction coefficient and volumetric wear rate are further reduced as a whole,and the volumetric wear rate of 30%deformation specimen is the smallest.The wear mechanism of the specimen before nitriding is mainly adhesive wear+fatigue wear+a small amount of abrasive wear,after cold rolling deformation,abrasive wear increases,abrasive wear phenomenon of 30%deformation sample is most obvious;After nitriding,the wear mechanism of the specimen is mainly abrasive wear,while the hardness of nitrided 22h specimen heart greatly improved,it shows that the cold rolling deformation pre-penetration+low temperature aging nitriding process can effectively improve the surface and core performance of the specimen,and the overall performance of the material has been greatly improved.