Study Of Initial Oxidation Of Ni-based Single Crystal Superallov DD5

Ni-based single crystal superalloy has excellent high temperature mechanical properties in high temperature environment,such as high fatigue resistance and creep resistance,and it has been widely used in turbine blades of aero gas turbine engine.Turbine blade could be oxidized and corrosion by high temperature air in service environment,induced change of the microstructure of the surface layer,led to decrease of creep strength of the blade in service process and shorten service life of the blade.The oxidation occur in the early stage during heating process is a inevitable experience of the ni-based single crystal superalloy used in turbine blades in service process.The understanding of the initial oxidation behavior help to understand the oxidation behavior and mechanism of the alloy occur in service process.In this paper,we study the initial oxidation behavior of ni-based single crystal superalloy DD5 in the process of heating process from room temperature up to 1150℃ under the water vapor environment,place emphasis on the initial formation and development of the internal oxide.In-situ oxidation of the specimen of DD5 alloy in an environmental scanning electron microscope.The surface and cross-section morphology of the oxided sample,and the structure and composition of the oxides have been characterized and analyzed by using scanning electron microscope,transmission electron microscope and energy spectrum technology.The formation and growth of amorphous alumina oxide and the nucleation and growth of inner oxide γ-Al₂O₃ on the interface of amorphous alumina oxide and γ1 have taken place on the surface of γ’ phase.On the surface of γ’ phase,the external diffusion and oxidation of elements such as Ni,Co,and Cr happens,formed Ni_xCo_1-xO and Ni_xCo_1-xCr₂O₄,and the nucleation and growth ofγ-Al₂O₃ occur on the subsurface of y phase.The y-y’ phase boundary perpendicular to the surface of alloy promotes the initial formation of the amorphous alumina oxide,and the γ-Al₂O₃ formed on this region at 900℃ with the γ-Al₂O₃ formed on the subsurface of y phase merge into one big γ-Al₂O₃ particle at 980℃.The oxides form on the surface of the y’phase have an effect on the composition of which form on the surface of the γ phase:1)The amorphous alumina oxide grow into the interface of Ni_xCO_1-xCr₂O₄ and γ-Al₂O₃ which form on the surface of the y phase;2)The Al³⁺ spread beyond the amorphous alumina oxide to the interface of NixCo1-xO and Ni_xCo_1-xCr₂O₄,dissolve into NixCO1-xCr2O4 and cause decomposition of Ni_xCo_1-xCr₂O₄.The rapid growth of the local amorphous alumiina oxide to the substrate side and the formation and growth of γ-Al₂O₃ successively became the two major internal oxidation forms of DD5 alloy.On the subsurface of DD5 alloy,y phase,γ’ phase and the γ-γ’ phase boundary perpendicular to the surface of alloy can reach the nuclear condition at 600℃C,810℃ and 900℃,respectively.The nucleation rate,the coarsening rate,and the migration rate of reaction front of γ-Al₂O₃ are decided by the difficulty level of the internal diffusion of O^2- and the external diffusion of Al.The reaction front of γ-Al₂O₃ formed on different position tend to situate in the same plane which parallel to the surface of the alloy at 1150℃.The nucleation and growth of γ-Al₂O₃ cause compressive stress on the subsurface of DD5 alloy,and it could cause partial deformation on the external amorphous membrane or be released by the way of external diffusion and oxidation of Ni,Co,and Cr or those elements diffuse into the interface of amorphous membrane and γ1 along the subsurface of alloy and form a γ2.