Effect Of Re And Ppm Level S Addition On The Oxidation And Hot Corrosion Behavior Of Ni-base Single Crystal Superalloys

Ni-base single crystal superalloys are widely used in thermal end components of gas turbines due to their good high temperature strength,structure stability and excellent oxidation and hot corrosion resistance.In this dissertation,effect of different Re and ppm level S on the hot corrosion behavior of Ni-base single crystal superalloys at 900°C and influence of different Re and ppm level S on isothermal oxidation behavior under different temperature were systematically investigated.Scanning electron microscopy?SEM?equipped with an energy dispersive spectroscopy?EDS?,electro-probe microanalyzer?EPMA?and X-ray diffraction?XRD?were used to characterize the original microstructure of the alloys,the corrosion morphologies and to identify the corrosion products.The mechanisms were discussed in detailed.The increase of ppm level S contents has no obvious effect on the oxidation properties of alloys at 900^oC.The increase of oxidation weight was more obviously with the increasing of S content at 1000^oC and 1100^oC.The effect of S contents on the oxidation behavior of alloys was more obvious with the higher temperature.Although the increase of ppm level S did not change the oxidation products of the alloys,it changed the structure of the oxide layer.The increase of ppm level S accelerates the external diffusion of Ta,affects the formation and stability of alumina and continuous oxidation layer containing Ta spinel.The increase of ppm level S reduces the adhesion between the oxide film/substrate,increases the holes and cracks on surface,promotes the formation of nitride,and exacerbates the isothermal oxidation properties of the alloys.The increase of ppm level S content obviously reduces the thermal corrosion gestation period of the alloy,which leads to the increase of the final oxidation weight of the alloy in turn,which exacerbates the thermal corrosion performance of the alloy.The increase of ppm level S content obviously reduces the hot corrosion incubation period of the alloys,which leads to the increase of the final oxidation weight and exacerbates the hot corrosion properties of the alloys.The increase of ppm level S content reduces the stability of the outer layer in molten salt,promotes the formation of mesh Al₂O₃ and the occurrence of internal sulfuration,leads to the swell and spalling on surface,and reduces the hot corrosion resistance of the alloy.The increase of Re makes the oxidation properties of the alloy decrease at the first stage and then enhance.The oxidation properties of 1.0Re alloy is the worst,and 2.0Re alloy is the best.The reason of competitive growth of Ta and Al was principle which increase of Re promotes the diffusion rate of Ta,affects the formation and stability of alumina and the continuous oxidation film containing Ta spinel.When Re=1.0wt.%,the diffusion rate of Ta and Al is basically the same,which reduces the compactness of the oxide film and exacerbates the isothermal oxidation properties of the alloys.The increase of Re contents obviously lengthens the hot corrosion incubation period of the alloys,which leads to the decrease of the final oxidation weight,and enhances the hot corrosion properties of the alloys.The increase of Re content enhances the stability of the outer oxidation layer in molten salt,promotes the formation of continuous Cr₂O₃ and hinders the occurrence of internal sulfuration,and enhances the hot corrosion resistance of the alloys.