Study On The NiAl/Al₂O₃ Interface Structure By Electron Microscopy

NiAl alloy has been widely used as protective coating of superalloy or bond coats in thermal barrier coatings due to its excellent properties including high melting point,low density and good oxidation resistance.Ni Al alloy will inevitably be oxidized in a high temperature service environment.The high temperature oxidation behavior of Ni Al alloy is very complex.In the process of high temperature oxidation,the different phases of alumina may form,and voids may form in the oxide scale or at the oxide/alloy interface,so that the oxidation resistance of the alloy will deteriorate,or even make it failure.Although a large number of researches have been done about the oxidation behavior of Ni Al alloy at high temperature,most of them focus on the phase and morphology of oxide.The correlation between the phase transition of oxide and the morphology development of the oxide/alloy interface is still unclear.In this thesis,some electron microscopy characterization methods such as SEM and TEM were used to analyze the microstructure development at the oxide/alloy interface of Ni Al alloy after1 h,4 h,12h,24 h and 48 h oxidation at 1050℃and Ar-20%O₂.Some mechanism models about the interfacial voids development are proposed.It is helpful for understanding the high oxidation mechanism and improving the oxidation resistance.The main research contents and results are as follows:（1）Some characterization methods including SEM,EDS and TEM were used to analyze the microstructure development of the oxide/alloy interface after high temperature oxidation.The results show that a layer of blade-like alumina scale（γ-Al₂O₃）grows on the surface of the alloy and some large voids appear at the oxide/alloy interface at the initial stage of oxidation.With the extension of oxidation time,theγ-Al₂O₃ near the alloy substrate gradually transforms toα-Al₂O₃,forming aγ-Al₂O₃/α-Al₂O₃ double-layer structure.There are some large voids at theγ-Al₂O₃/α-Al₂O₃interface and the oxide/alloy interface.Moreover,the multilayer oxide withθ-Al₂O₃and Ni Al₂O₄ grow alternately in the voids at the oxide/alloy interface,whileθ-Al₂O₃appears in the voids at the double-layer oxide interface.（2）A mechanism about the development of the voids atγ-Al₂O₃/α-Al₂O₃ interface was proposed according to their microstructure characteristics.Firstly,the phase transition fromγ-Al₂O₃ toα-Al₂O₃ is accompanied by a volume decrease,resulting in the formation of voids at the interface of two-layer oxide.θ-Al₂O₃ forms in these voids owing to the low oxygen pressure,while no Ni Al₂O₄ forms in these voids since Ni in the alloy cannot diffuse outward through the compactα-Al₂O₃ scale.（3）A mechanism about the development of the voids at the oxides/alloy interface was proposed according to their microstructure characteristics.Due to the accumulation of Al vacancies at the oxide/alloy interface,the interfacial voids embed into the alloy at the initial stage of oxidation.With the prolonging of the oxidation time,some voids also appear inwards the oxide scale at the oxide/alloy interface,which is caused by the accumulation of the vacancies at the alloy/oxide interface.Firstly,due to the low partial oxygen pressure in the voids,Al atoms near the surface of the alloy substrate diffuse to the inner surface of the voids to formθ-Al₂O₃.Then,the surface of the alloy substrate with the depleted Al turns into the Ni-riched surface.Due to the low oxygen pressure in the interface voids,Ni atoms on the surface diffuse throughθ-Al₂O₃ to the inner surface of the voids and react withθ-Al₂O₃ to form Ni Al₂O₄.This process occurs repeatedly,leading to a multilayer oxide withθ-Al₂O₃ and Ni Al₂O₄ on the inner surface of the interface voids.