Structures And Properties Of TC18 After Cr,Nb And Zr Implantation

As the preferred material for key components of aircraft,new high-strength titanium alloy TC18can effectively reduce weight and improve aircraft maneuverability.However,they often suffered from poor tribological and anti-corrosion properties,and its fatigue property is sensitive to surface defects and damage under alternating loads,which seriously impaired the service life of such aircraft components.In this paper,Ti substitution solid solution elements Cr,Nb and Zr with different atomic radius are used to improve the surface properties of TC18 titanium alloy using ion implantation.Not only the wear resistance of TC18 titanium alloy was to be improved,but also the mechanisms that how its corrosion resistance and fatigue properties were affected by Cr,Nb and Zr ion implantation are also explored.This provides experimental data and theoretical basis for the future application of ion implantation in the engineering of surface modification of high-strength titanium alloy.Firstly,the effects of MEVVA ion implantation on the surface morphology,elemental distribution and micro-structure of titanium alloy were evaluate using a series of analysis and measurement methods,such as SEM,XPS,TEM,XRD and three-dimensional topographic analyzer.Then the effects of Cr,Nb and Zr ion implantation on wear behavior,corrosion behavior and fatigue behavior of TC18 titanium alloy were studied at the dosages of 1.0×10¹⁶ions/cm²,5.0×10¹⁶ions/cm²and1.0×10¹⁷ions/cm².Finally,the relationship among ion implantation parameters,elemental distribution of titanium alloy surface layer,microstructure and properties of titanium alloy was explored based on the results of XPS,TEM,XRD,and et al.The influence of MEVVA ion implantation on the wear resistance,corrosion resistance and fatigue resistance of TC18 alloy and the related mechanism were also studied in this paper.It is found that the depth and concentration of Cr?Nb and Zr elements in the implanted layer directly affected the effect of solid solution strengthening.The average dislocation density and average crystal size of the surface layer directly determined the effect of dislocation strengthening and fine grain strengthening,respectively.It can be concluded from the results of XPS that,with the increase of implantation dosage,the content of Cr and Zr in the implated layer increases gradually,and accordingly the related solid solution strengthening effect shows an upward trend;the content of Nb in the implanted layer firstly increases and then decreases,so the comprehensive effect of solid solution strengthening and dispersion strengthening shows a upward and downward trend.Using the whole pattern fitting and Rietveld methods,the phase,average crystal size,micro-strain and average dislocation density were comparatively studied.It was found that with the increase of implantation dosage,the average crystal size of alpha phase in the Cr implanted layer firstly increased and then decreased,and the average dislocation density firstly decreased and then increased.The average crystal size of two phases in Nb implanted layer firstly decreased and then increased,and the average dislocation density increased.The average dislocation density of the alpha phase in the Zr implanted layer firstly decreased and then increased,and the average dislocation density firstly increases and then decreased with the increase of implantation dosage.The results of TEM study show that,due to the bombardment of high-energy ions,a mixed structure of amorphous and nanocrystalline with different thickness formed on the surface layer of Cr and Zr implanted samples,and their inner layer was polycrystalline.While a complete amorphous surface layer and the polycrystalline inner layer formed after Nb implantation.The wear properties of the implanted TC18 titanium alloy are directly related to the effect of solid solution strengthening of alloying elements,the strengthening effect of dispersed phase and the surface microstructures.After Cr and Zr implantation,the key strengthening mechanisms of the implanted layer are solution strengthening,dislocation strengthening and fine crystal strengthening.The key strengthening mechanisms of Nb impalnted layer are dispersion strengthening,solution strengthening,dislocation strengthening and fine crystal strengthening.Among all of them,Nb implantation exhibited the best comprehensive strengthening effect in the implanted layer,followed by Zr implantation and Cr implantation.It is also found that the comprehensive effect of strengthening factors in the Cr,Nb and Zr implanted layers differed with the increase of implantation dosage.As the implantation dose increased,the comprehensive strengthening effect of Cr implantation in surface layer increases gradually,while that of Nb and Zr implantation firstly increased and then decreased.It aslo can be concluded that,when the dosage of Cr and Zr implantation were 1.0×10¹⁷ions/cm² and 1.0×10¹⁶ions/cm²,the hardness was the highest and wear resistance of titanium alloys was best as a result of the best comprehensive effect of solution strengthening,dislocation strengthening and fine crystal strengthening.When the Nb implantation was 5.0×10¹⁷ions/cm²,due to the best effects of the solid solution strengthening,dispersion strengthening,dislocation strengthening and fine crystal strengthening,and the hardness and wear resistance of implanted samples were found to be highest.The thickness and structure of oxide film,defect density,grain boundary and passivation film in aggressive solution are the main factors affecting the corrosion performance of the implanted TC18titanium alloy.Based on the analysis of the damage effects of dislocations and grain boundaries,it is found that,the improved corrosion resistance of the Cr,Nb and Zr implanted TC18 alloy in 3.5%Na Cl solution are mainly attributed to the formation of a more stable and dense oxide film on the surface of titanium alloys,which outweighed the damage effect due to the defects and dislocations.The results of electrochemical test show that the corrosion resistance of Zr implanted samples was the best,followed by Nb implanted specimens and Cr implanted samples.With the increase of implantation dose,the corrosion resistance of the three alloying elements implanted specimens varied.The corrosion resistance of Cr and Nb implanted samples firstly decreased and then increased.The corrosion resistance of Zr implanted samples increased gradually.In addition,the thickness and composition of the oxide layer formed on the surface varied after Cr,Nb and Zr implantation with different dosages based on XPS analysis.With the increase of implantation dosage,the thickness of oxide layer on the surface of Cr and Zr implanted layer first decreased and then increased,while that of Nb implanted layer decreased after a certain dose.Based on the fitting of AC impedance,it was found that formed two protective passive films on the surface of all implanted TC18 titanium alloy during electrochemical reaction,but the performance and protective effect of the films differed.Among them,the charge transfer resistance of surface passivation layer in Cr and Nb inplanted zone increases significantly,which provided an important protection function for substrate.For Zr implantation,the charge transfer resistance of bottom passive layer increases significantly,which provided the key protection.When the Cr,Nb and Zr implantation dosages were 1.0×10¹⁷ions/cm²,1.0×10¹⁶ions/cm² and 1.0×10¹⁷ions/cm²,respectively,the charge transfer resistance of the surface passivation layer were the largest,their comprehensive protection effect was the best,and accordingly the corrosion resistance was best.The major factors affecting fatigue properties of the implanted samples are surface morphology,solution strengthening and dispersion strengthening effect,residual stress,dislocation density and grain size.Based on the change rules of strengthening factors,residual stresses and notch sensitivity coefficients of different metal elements implanted layers with implantation dosage,it is found that,the the median fatigue life of substrate under low loads was significantly improved by high dose Zr ion implantation.This is because the effect of surface strengthening factor and residual tensile stress were stronger than that of the roughness factor.Due to the weaker negtive effects of the strengthening factors than that of the roughness and residual tensile stress factors,the fatigue properties of the Cr implanted samples with small dosages decreased to varying degrees.The positive effects of solution strengthening,fine grain strengthening and dislocation strengthening outweighed the unfavorable factor of roughness and residual tensile stress.The fatigue properties slightly improved to a certain extent after Cr implantation with high doasgae.Due to the weaker effect of the roughness and stronger effect of surface strengthening factors,the the median fatigue life of the all Nb implanted TC18titanium alloy under low loads decreased in varying degrees.Arola-Ramulu model was used to evaluate the stress concentration factors and related fatigue stress concentration factors caused by the change of surface roughness after implantation.The results showed that the effective fatigue stress concentration factors of Cr and Nb implanted samples increased to a different degree after ion implantation,which implied that the effective fatigue stress concentration impaired the fatigue resistance.With the increase of implantation dose,the notch sensitivity coefficient of Cr-implanted samples firstly decreased and then increased,while that of Nb-implanted samples firstly increased and then decreased.Zr ion implantation has little effect on the effective fatigue stress concentration of TC18 titanium alloy.