Studies On High Temperature Oxidation And Hot Corrosion Behaviors Of Al-Cr Composite Coating On The Surface Of TC21 Alloy

As one kind of high-damage tolerance titanium alloy, TC21 alloy has potential to become a new generation of aero-industries materials due to its advantages like high strength, ductility, fraeture toughness and low crack growth rate. However, insufficient oxidation resistance and hot corrosion resistance, resulted from the low content of anti-oxidizing element, are the main drawback which restricts the actual application of this alloy in many circumstances. By combining the double glow plasma surface alloying technology and RF magnetron sputtering technology, a Al-Cr composite coating is prepared on the surface of TC21 alloy to improve its high temperature oxidation resistance and hot corrosion resistance.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 smooth and compact, without any pour and crack. The mainly surface phase structure is Al. The layer is about 48μm thick and metallurgy combined with the substrate, the bonding force is 58.4 N. The nano hardness in coating surface is 3.53 GPa, and the modulus of elasticity is 142 GPa. The microhardness of the alloy layer is 993.4 HV0.05.The results of high temperature oxidation studies at different temperatures（650 、750 、850 、950 /100h） and different time（850℃/10h、20h、50h） show that the weight gains of Al-Cr coating samples are much less than the substrates. The oxidation resistance of substrate decreases rapidly when temperature is higher than 650. A dense Al₂O₃ film is formed on surface of Al-Cr layer between 650-850. 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. The oxidation of Al-Cr coating is not only a process of film forming, breaking and peeling off, but also a process of mutual diffusion of elements. 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 coating. The high temperature of oxidation resistance of TC21 alloy is improving by Al-Cr composite coating.The results of hot corrosion studies at different temperatures（750 、800 、850 /100h） and different time（800 /2h、5h、10h、20h、50h） show that the weight gains of the Al-Cr coating samples are much less than the substrates. A dense Al₂O₃ film is formed on the surface of Al-Cr coating in 75% Na2SO4+25%NaCl mixed salt. When the Al₂O₃ film is in alkaline fusion, Cr2O3 can be generated in dissolving area.Therefor the Al-Cr coating can repair itself and inprove the corrosion resistance of TC21 alloy. In the mixed salt, Al-Cr coating corrosion front advances by vulcanization-oxidation mechanism, the oxide is in the outer and the sulfide is in the front. With the increase of temperature, the oxide layer thickness increases, and the sulfide layer thickness decreases. During the low temperature corrosion at 750, the Al₂O₃ in the surface is firstly alkaline melted, and then acid melted, which indicates that the diffusion mode of hot corrosion is successively connected.