DZ445 镍基高温合金高温长时间氧化形成的多层膜结构

Nickel-based superalloys have been widely used in aero engines and gas turbines because of their excellent high-temperature strength and exceptional oxidation resistance. The oxidation resistance is obtained using the thermal barrier coatings and alloying elements. In the previous investigations, the oxidation time of the nickel-based superalloys has been focused on hundreds of hours. However, in practice, the superalloys last far longer. This study investigated the superalloy DZ445's oxidation behavior at 900^oC for 300-2600 h. An oxidation film with a four-layer structure is formed after oxidation at 900^oC for 500 h and above. The outermost layer is primarily composed of NiCr₂O₄, Cr₂O₃, and TiO₂. The subouter layer consists of CrTaO₄ and TiO₂, and the subinner layer consists of Al₂O₃, NiCr₂O₄, and NiO. The innermost layer is primarily Al₂O₃. The appearances of the subouter layer and subinner layer greatly reduce the oxidation rate of the alloy, which is represented by the dramatic increase in the oxidation kinetics equation exponent and a sharp reduction of the oxidation rate constant. The formation of subouter layer changes the oxidation mechanism from outward diffusion of alloy elements to O-inward diffusion. When the subinner layer is formed, the oxidation behavior is controlled using the outward diffusion of Ni and Cr and the O-inward diffusion. The multilayer structure gave the alloy an excellent oxidation resistance capacity.