定向凝固镍基高温合金DZ466表面CoAl涂层的氧化及组织演变

Oxidation and Microstructure Evolution of CoAl Coating on Directionally Solidified Ni-Based Superalloys DZ466

采用低压化学气相沉积法(LP-CVD)在定向凝固镍基高温合金DZ466表面制备Co Al涂层,通过900℃下约5000 h恒温热暴露实验,研究了DZ466合金及其表面Co Al涂层的高温氧化行为和内部组织演变。结果表明,由于Co Al涂层Al含量较高,促进了表面Al_2O_3的形成,改善了DZ466合金900℃抗氧化性能。揭示了热暴露过程中Co Al涂层基体相及析出相的演变规律,涂层的基体相由b-Ni Al/Co Al相逐渐转化为g'-Ni3Al相,涂层与基体合金间的元素互扩散促使基体相转变优先在靠近基体合金侧进行。热暴露后,Co Al涂层中间层中析出a-Cr相,a-Cr相倾向于在碳化物附近形核并依附于碳化物生长。涂层内层中有针状TCP相(m相)析出,m相的整齐排列形态与g/g'的立方化组织密切相关。涂层内部组织演变存在遗传效应。

Aluminide coatings are widely employed to protect internal cooling channels of high grades blades and buckets in gas turbines have always been in severe conditions including high temperature oxidation and hot corrosion. There is a major concern for the application of aluminide coatings that refer to the inter-diffusion between aluminide coating and superalloy substrate at high temperatures. Diffusion of Al from the coating to the underlying substrate usually leads to depletion of Al in the coating, resulting in inferior oxidation resistance of the coating. Accordingly, Ni declines to diffuse counter currently from the substrate into the coating, as well as other refractory elements, such as Cr, Mo and W etc.. The inter-diffusion between aluminide coating and superalloy substrate results in degradation or various evolution behaviors of aluminide coatings, in other words, substrate composition significantly affects the properties of aluminide coatings. Co Al coating was prepared on directionally solidified superalloy DZ466 by low pressure chemical vapour deposition（LP-CVD）. Oxidation behavior and microstructure evolution of Co Al coating was investigated during long term（about 5000 h） exposure at 900 ℃. Results suggested that,high concentration of aluminum did help to form Al_2O_3 on the surface of coating, improving oxidation resistance of DZ466 at 900 ℃. Evolution of matrix phase and precipitates in the Co Al coating during exposure was displayed, b-Ni Al/Co Al phase in the coating transformed gradually to g＇-Ni3 Al phase, higher transformation rate for the g＇ phase closed to the substrate due to the diffusion between the coating and the sub strate superalloy. During exposure, a-Cr phase precipitated in the middle layer, which inclined to form close to carbides and grow by consuming them. Needle like TCP phase（m phase） grew in the inner layer that arranged in order, which was due to the cubic microstructure of g/g＇. Heredity-effect was in company with the precipitates evolution.