Study Of The Corrosion And Tribocorrosion Behavior Of Ti-Zr-Nb-Ta Multi-principal Element Alloys

Multi-principal element alloy is a new alloy design concept,which uses multiple elements with equal atomic or near equal atomic ratio as the main component for alloy design.The synergistic effect between multi-principal components increase the mixing entropy of alloy and improve its properties,such as high strength,toughness,wear resistance and corrosion resistance.Refractory multi-principal element alloys composed of high melting point elements show excellent mechanical properties,and have a very broad application prospect.Currently,the research on refractory multi-principal element alloys mainly focuse on the microstructure,mechanical properties and high-temperature oxidation behavior.Refractory multi-principal element alloys are expected to become the material system for the design of corrosion resistant alloys,and their surface passive film will significantly affect the corrosion resistance.Therefore,it is particularly urgent to carry out the research on the surface passive film of refractory multi-principal element alloys and its contribution to corrosion resistance.At present,the research on the tribocorrosion behavior of refractory multi-principal element alloys are also rarely reported.Firstly,the characteristic of passive film on the Ti₂₅Zr₂₅Nb₂₅Ta₂₅ alloy was studied.The microstructure,chemical composition,surface nano morphology and electronic energy band structure of the passive film on the alloy were analyzed.According to the point defect model,the point defect diffusion coefficient Do of passive film under steady-state conditions was evaluated.The results show that the cations of each component were evenly distributed in the passive film on the nano scale,and the concentration ratio of each element in the film were close to that of the matrix.Moreover,the passive film on the alloy could maintain its homogeneous and dense amorphous structure even at high polarization potentials.In addition,it was found that the high Zr content in the TiZrNbTa multi-principal element alloys could promote the formation of ZrO2 nanocrystalline oxide in the passive film,which could destroy the structural disorder of the film,and lead to high internal stress and added interface structure in the passive film,resulting in reduced structural stability and compactness of the film.The surface oxide of Ti₂₅Zr₂₅Nb₂₅Ta₂₅ alloy presented an ultra wide and evenly distributed band gap of 6.2 eV.The calculation results showed that the Do in the passive film of Ti₂₅Zr₂₅Nb₂₅Ta₂₅ alloy was two orders of magnitude lower than that of pure Ti.Therefore,the passive film exhibited uniform composition,dense amorphous structure,low point defect diffusion coefficient,ultra wide and uniform band gap,which showed that the oxides had the characteristics of high entropy amorphous ceramics,improving the corrosion resistance of Ti₂₅Zr₂₅Nb₂₅Ta₂₅ alloy.Secondly,the local corrosion characteristics of Ti₂₅Zr₂₅Nb₂₅Ta₂₅ alloy were studied,and the relationshaips among the alloy microstructure,passive film composition distribution and metastable pitting were established.The results showed that metastable pitting of the alloy preferentially nucleated and developed on dendritic regions in the microstructure,which was related to the barren oxides of Ti,Zr and Nb above on dendritic regions.Combined with the point defect model,it was clarified that the Nb oxide enriched in the interdendritic regions region could enhance the cation doping effect in its passive film,inhibiting the nucleation of metastable pitting.The enriched Ti and Zr in interdendritic regions could improve the repassivation ability of metastable pitting and impede the development of pitting.This also reflected the synergistic effect of component oxides on improving the corrosion resistance of passive film.By comparing with the corrosion resistance of the alloy and pure metals,it was found that Nb and Ta elements were decisive to improve the corrosion resistance of the alloy.Subsequently,the effects of alloy element composition and content on the electrochemical corrosion and tribocorrosion behavior of TiZrNbTa alloys were studied.By studying the defects,composition and structure of passive film of equiatomic TiZr（Hf,Ta,Nb）alloys,it was found that the alloying of Nb and Ta were conducive to enhance the corrosion resistance of passive film by reducing the point defects in the passive film on Ti-50Zr（at%）alloy.Hf alloying promoted the formation of HfO2 crystal phase oxide in the passive film on TiZrHf alloy and reduced the compactness of the passive film.The corrosion resistance,point defects and chemical composition of the passive film of non-equiatomic TiZrNbTa alloys were studied by adjusting the content of Nb and Ta elements in a wide range.The results showed that with the increase of Nb and Ta alloying,the concentration of Nb and Ta cations in the passive film of TiZrNbTa alloys increased,and the point defect concentration in the passive film decreased,resulting in increased corrosion resistance.The increased concentration of Nb and Ta cations in the film could enhance the degree of cation doping,leading to reduced point defects in the passive film.Based on this,the mechanism of cation doping with gradient valence distribution to strengthen the protection of the passive film was proposed.By studying the tribocorrosion behavior of equiatomic TiZr（Hf,Ta,Nb）alloys,it showed that the order of tribocorrosion resistance and repassivation ability of the alloys was:TiZrHf>TiZrTa>TiZrNb.The worn surface of TiZrHf was easy to form tribofilm,which could inhibit abrasive wear and delamination wear.A large number of cracks were observed to produce on the worn surface of TiZrTa alloy,showing its dominant delamination wear.TiZrNb was easy to produce serious furrow,delamination and plastic deformation,resulting in its high tribocorrosion rate.Corrosion could intensify the delamination wear of TiZr（Ta,Nb）alloys,resulting in a higher proportion of corrosion-induced wear component in the total tribocorrosion volum.By studying the tribocorrosion behavior Ti45Zr45Nb5Ta5,Ti35Zr35Nb25Ta5 and Ti31.65Zr31.65Nb31.65Ta5 alloys,it was found that the tribocorrosion resistance and repassivation ability of alloys decreased with the increase of Nb alloying.Ti45Zr45Nb5Ta5 and Ti35Zr35Nb25Ta5 alloys mainly suffered from abrasive wear,and the formation of discontinuous tribofilm during tribocorrosion could reduce the degree of abrasive wear.However,the worn surface of Ti31.65Zr31.65Nb31.65Ta5 alloy produced serious furrow,delamination and plastic deformation.At the same time,the corrosion also intensified the degree of delamination wear,accelerating its tribocorrosion rate.