Design Of Light Refractory High Entropy Alloy And Wear And High-temperature Oxidation Resistance Behaviors Of Its Laser Cladding Coating

Poor wear resistance and high temperature oxidation resistance are the key problems that restrict the long-term service of titanium alloy under complex working conditions.The results show that it is still difficult to greatly improve the wear resistance and high temperature oxidation resistance of titanium alloy through composition design,alloy melting and traditional surface modification technology.Laser cladding(LC)coating is widely favored because of its controllable thickness,dense structure and metallurgical bonding with the substrate.However,there are many intermetallic compounds in LC coating,which makes it show certain crack sensitivity.High-entropy alloys(HEAs)developed rapidly in recent years have become the focus of researchers because they can form a single solid solution structure and have excellent wear and corrosion resistance,high temperature oxidation resistance and mechanical properties.Therefore,the design and LC preparation of HEAS coating on titanium alloy surface is expected to improve its wear resistance and high temperature oxidation resistance.Aiming at the disadvantages of poor wear resistance and high-temperature oxidation resistance of titanium alloy in service,combined with the composition and microstructure characteristics of titanium alloy,without losing its inherent properties.Firstly,a series of low-density refractory HEAs based on MoNbTiV are designed by using the first principle,combined with quasi harmonic approximation(QHA).The structural stability,mechanical and thermal properties of a series of HEAs were calculated by the model.Then,MoNbTiVX(X = Al/Cr)series low-density refractory HEAs coatings were prepared on the surface of Ti6Al4V(TC4)alloy by LC technology.The microstructure and structural characteristics of HEAs coatings were characterized by means of XRD,SEM and TEM,and the mechanical,wear resistance and high-temperature oxidation resistance of HEAs coatings were tested.Finally,the internal relationship between the phase structure of the coating and its wear resistance and high temperature oxidation resistance was discussed,and the wear resistance and high temperature oxidation resistance mechanism of LC light refractory HEAs coating on titanium alloy was analyzed.First-principles calculations show that the four equal atomic ratio HEAs MoNbTiV,AlMoNbTiV,CrMoNbTiV and AlCrMoNbTiV all have a stable bodycentered cubic structure,and their theoretical densities are 7.358,6.409,7.331,and 6.510g·cm^-3 respectively.And lattice constants of the equilibrium structure are 3.177,3.175,3.116,and 3.118?,respectively.The relative lattice constants of BCC obtained from XRD data of the coating are 3.221,3.171,3.193,and 3.159?,respectively,which are very close.There is also a trace of C15 Laves second phase in CrMoNbTiV and AlCrMoNbTiV coatings,and their lattice constants are6.9989? and 6.9684?,respectively.SEM images show that the coating structure is typical dendrite,Ti is enriched in the dendrite region,and other alloy elements have no obvious segregation.The high-resolution TEM characterization further confirmed that the almonbtiv coating had a single BCC Structure and the lattice parameter was 3.016 ?;The main phase of crmonbtiv coating is BCC,the second phase is Laves phase,and the lattice parameters are 3.026? and 7.060?.The addition of alloying element Al/Cr will cause the structural distortion of the coating and affect its mechanical properties.The results show that the four coatings are elastic anisotropic and ductile.The order of Young's modulus and hardness is AlCrMoNbTiV > MoNbTiV > CrMoNbTiV > AlMoNbTiV.The modulus values of the four HEAs coatings were 174.78,146.51,157.47,and179.90 GPa,respectively.The hardness values are 7.694,6.451,7.806,and 8.939GPa,respectively.The QHA model is used to calculate the thermal expansion coefficients of four types of HEAs and ?-Ti and ?-Ti titanium alloys.Their values are very close,and the difference in linear expansion coefficients is less than1.16×10^-5/K.The addition of the active element Al/Cr makes it easy to form Al and Cr oxide films on the surface of the HEAs coating.The hard oxide film can effectively reduce the adhesion and wear of the coating in the initial stage.At the same time,the second phase strengthening effect of C15 Laves makes CrMoNbTiV coatings and AlCrMoNbTiV coatings have high H/E ratio and high yield stress.The combined effect of these factors makes the AlCrMoNbTiV coating have the best wear resistance.Its friction coefficient is 0.2,which is about 1/3 of the TC4 substrate.And its wear rate is 7.25×10^-5mm³·N^-1·m^-1.After high temperature oxidation,a dense composite oxide layer composed of Al₂O₃,TiO₂,Al_0.2Nb_0.2Ti_0.6O₂,AlMoVO₇ and Nb_10.7V_2.38O_32.7 is formed in the area of almonbtiv coating close to TC4 substrate,which can well block the oxidation corrosion of O₂ to titanium alloy substrate and significantly improve the oxidation resistance of titanium alloy in high-temperature environment.