Studies On Hot Corrosion And High Temperature Oxidation Behaviors Of Al/Cr Composite Coating On The Surface Of Ti₂AlNb Alloy

Compared with other high temperature alloys,Ti₂ AlNb alloy has its own unique advantages,like good fracture toughness at room temperature and crack propagation resistanc,good creep resistance,high tensile strength and fatigue strength,and low thermal expansion coefficient.It has become a new generation of aero-industries materials.However,hot corrosion resistance and insufficient oxidation resistance are the main drawback which restricts the actual application of this alloy in many circumstances.A Al/Cr composite coating by combining the double glow plasma surface alloying technology and RF magnetron sputtering technology is prepared on the surface of Ti₂ AlNb alloy,to improve its high temperature oxidation resistance and hot corrosion resistance.The optimum process parameters of glow discharge plasma chromiuming on Ti₂ AlNb alloy are listed as follows: work bias is 920 V,substrate bias is 400 V,working pressure is 40 Pa,distance between target and work-piece is 15²0 mm,time is 3.5h,temperature is 800⁹00 ℃.The optimum process parameters of glow discharge plasma aluminizing on Ti₂ AlNb alloy are listed as follows: work-piece bias is 35³8V,working pressure is 0.5Pa,radio frequency power is 180 W,distance between target and work-piece is 20²5 mm,time is 3h.SEM was used to the detected the morphology of the oxidation film,EDS was used to detect the element distribution and XRD was used to identify the phase.It turns out that the surface of Al/Cr composite layer is compact and smooth,without any pour and crack.The mainly surface phase structure is Al.The layer is about 63μm thick and metallurgy combined withthe substrate,the bonding force is 57.8 N.The nano hardness in coating surface is 525 MPa,and the modulus of elasticity is 345 GPa.The microhardness of the alloy layer is 963.5 HV0.05.The results of high temperature oxidation studies at 750℃、850℃、900℃/100 h show that the weight gains of Al/Cr composite coating samples are much less than the substrates.The oxidation resistance of substrate decreases rapidly when temperature is higher than 850℃.A dense Al2O3 film is formed on surface of Al/Cr composite layer between 750℃-850℃.At 950℃oxidation,the surface has peeled lightly,due to its high content of Al,it still can form a protective Al2O3 scale and protect the substrate well.Cr diffuses to the spalling area and is oxidized to form Cr2O3,on the other hand,it promotes selective oxidation of Al and self-repair function of Al/Cr composite coating.The high temperature of oxidation resistance of Ti₂ AlNb alloy is improving by Al/Cr composite coating.The results of hot corrosion studies at 750℃、850℃/100 h and 900℃/40 h show that the weight gains of the Al/Cr composite coating samples are much less than the substrates.The hot corrosion resistance of substrate decreases rapidly when temperature is higher than 850℃.A dense Al2O3 film is formed on the surface of Al/Cr composite coating in Na2SO4 salt.At 900℃ hot corrosion,When the Al2O3 film is in alkaline fusion,Cr2O3 can be generated in dissolving area.Therefor,the Al/Cr composite coating can repair itself and inprove the corrosion resistance of Ti₂ AlNb alloy.