| DZ444is a directionally solidified Ni-based superalloy, which was newly developed for turbine and guide blades. It requires high temperature oxidation and hot corrosion resistance in rigorous working conditions. In the present paper, rare earth elements Y and Ce were added in the original DZ444alloy to improve its properties. The effects of Y and Ce addition on the DZ444alloy were investigated by comparing its microstructure, tensile properties, stress rupture properties, oxidation resistance and hot corrosion resistance. What is more, their mechanism was studied as well. The results show that:Rare earth elements Y and Ce can refine dendritic structure of DZ444alloy and increase the amount of (y+y’) eutectic in as-cast structure. The former situation is related to decreasing of nucleation energy due to Y and Ce addition and increasing of number of nucleus provided by oxide and sulfide of Y and Ce. The latter is associated with larger atomic radius of Y and Ce, which can exacerbate elements segregation in liquid phase near liquid-solid interface. Rare elements Y and Ce have few effects on tensile properties but weaken stress rupture properties slightly. It is because that there is no fine carbide chain precipitating in grain boundaries of the alloy with Y and Ce addition. However, the fracture mechanisms, which are both mixed fracture but ductile one mainly, are not changed with Y and Ce addition.Oxidation kinetics of DZ444alloy obeys parabolic law at700℃,850℃and950℃.. The oxidation behavior is controlled by the diffusion of Cr3+through the scale of Cr2O3, regardless of Y and Ce addition. Nevertheless, rare elements have an intense tendency to segregate at surface of the alloy. It accelerates the formation of protective film of Cr2O3, which can prevent further and internal oxidation. Besides, Y and Ce can decrease activation energy and thickness of oxidation film at950℃but cannot change the composition. A special area named decomposition zone of γ’ phase, can be obviously seen under oxidation film after oxidized for100hours at950℃. Thickness of the zone can be decreased with Y and Ce addition.Hot corrosion process of DZ444alloy consists of three stages:selective oxidation of Cr and other elements, sulfuration of oxidation layer and reoxidation after sulphur atom diffusing to the matrix. Rare earth elements Y and Ce are able to improve high temperature oxidation and hot corrosion resistance of DZ444alloy mainly through accelerating selective oxidation. |
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