Study On Structure And Properties Of Vacuum Pulse Induced Nitriding Layer Of TC4 Titanium Alloy

TC4 titanium alloy has wide application prospects in aerospace,automotive,biomedical and other fields due to its characteristics of low density,high specific strength,excellent corrosion resistance and good biocompatibility.However,as far as the TC4 titanium alloy substrate is concerned,its low surface hardness,poor abrasion resistance,and easy corrosion in reducing acids（such as HF）greatly limit its application.In this paper,the vacuum induction rapid nitriding technology is used to treat the surface of the TC4 titanium alloy and the gradient strengthening layer is finally obtained.The relationship between the microstructure and performance of the gradient strengthening layer was discussed through the characterization of the microstructure and the performance testing and analysis of wear,corrosion,corrosion and wear.Research indicates:（1）After 1 h of vacuum induction rapid nitriding treatment,the TC4 titanium alloy can obtain a dense 20μm Ti N white bright layer on the surface.The surface microhardness reaches 1211.30 HV_0.25,which is more than 3 times the hardness of the untreated sample.The depth of the effective hardened layer is about 50μm,and the residual compressive stress on the surface increases by 157.48 MPa.（2）The surface layer after the nitriding treatment generates high dislocation density due to the high-frequency magnetic field,forming a dislocation cell structure,resulting in local dislocation inside the crystal grains,high-density stacking faults and obvious habit surfaces inside the Ti N crystal（112）;Theα-stabilizing elements（Al）andβ-stabilizing elements（V,Fe）in the surface layer segregated in theα-phase andβ-phase,respectively,resulting in uneven distribution of alloying elements inside the material,causing dislocation accumulation;no core grains were found The obvious defect is the non-coherent grain boundary at the junction ofαandβ.（3）After nitriding,the wear volume of the nitriding layer is reduced,the width of the wear scar and the roughness（S_a）of the wear scar are reduced,and the friction coefficient and wear rate of the nitriding layer increase as the load increases.During the wear process,the substrate mainly undergoes oxidative wear,and the wear surface after induction nitriding is mainly abrasive wear.（4）After nitriding,the corrosion resistance of F^-in the corrosion solution is enhanced,the self-corrosion potential(E_corr)increases,the corrosion current(I_corr)decreases,the charge transfer resistance（R_t）and the corrosion product film resistance（R_f）increases,and corrosion resistance increases.（5）After performing corrosion and wear tests under different F-concentrations and different loads,it was found that at 5 N and 0.2 mol/L concentration,the weight loss of nitriding samples was 29 times lower than that of untreated samples;with the increase of the corrosive liquid and load,the surface suffered corrosion and wear The synergy increases,the amount of wear increases,and promotes the failure of the surface;the nitriding layer has a certain resistance to the erosion of F^-,and the corrosion and wear performance increases;at the same time,the integrity of the surface is damaged due to wear and tear.During the wear process,the open circuit potential and self-corrosion potential decrease,the corrosion current increases,and the impedance arc radius decreases.During the wear process,the surface corrosion resistance decreases.After corrosion and wear,a large number of corrosion pits with F and O elements appear on the wear scar surface.Through the analysis of the microstructure and performance of the induction nitriding layer,it can be known that the vacuum induction rapid nitriding method can efficiently obtain the surface strengthening layer with excellent comprehensive performance,which has important guiding significance for extending the application range of the TC4 titanium alloy.