First Principle Study Of Oxidation Mechanism Of NiAl Alloy

NiAl alloy with high melting point,low density,and good oxidation resistance,has been widely used as a bond coating in Thermal Barrier Coatings and other systems.In high temperature service environment,with the continuous oxidation of NiAl alloy,the stress concentration at the oxide/alloy interface will cause cracking of the bond coat/top coat interface,and eventually lead to the failure of the thermal barrier coating.The oxidation process of alloys is very complex.The composition of alloy,ambient temperature,atmosphere（such as O₂,H₂O,CO₂）,oxygen pressure and other factors will affect the oxidation behavior of NiAl alloy,including the growth rate of Al₂O₃ film,the morphology of NiAl/Al₂O₃ interface,the adhesion of NiAl/Al₂O₃ interface and so on.At present,although many efforts have been made to elucidate the high-temperature oxidation behavior of NiAl alloy,a complete understanding of its atomic oxidation mechanism is still lacking.For example:in the process of the continuous growth of the alumina film formed by the oxidation of NiAl alloy in the water vapor environment,the adsorption,decomposition and diffusion mechanism of H₂O molecules on the surface of Al₂O₃ is not clear;Moreover,the atomic mechanism about how the water vapor affects the growth of alumina film is not clear.In addition,it is well known that the doping of reactive elements can significantly improve the oxidation resistance of the alloys（by reducing the growth rate of oxide film and increasing the adhesion of the interfacial）,but how the reactive elements migrate and segregate through the oxide/alloy interface is still unclear.In this thesis,we analyzed:（1）the adsorption,decomposition and diffusion process of H₂O molecule on Al₂O₃ surface,（2）the formation,migration and aggregation of Al and O atom vacancies in Al₂O₃ affected by hydrogen protons obtained by H₂O molecule decomposition,（3）the diffusion and aggregation process of the reactive element Hffrom NiAl alloy matrix to NiAl/Al₂O₃ interface,and to Al₂O₃ film by first-principles calculation method.Our results revealed the atomic mechanism of the influence of water vapor on the oxidation process of NiAl alloy（i.e.the growth of Al₂O₃ ）and the influence of active elements on the alloy/oxide interface.It is helpful for understanding the oxidation mechanism of alloys and improving the oxidation resistance of alloys at high temperature.The main research contents and conclusions are as follows:（1）We investigated the effects of perfect and defectγ-Al₂O₃ surface structure on the adsorption,decomposition of single H₂O molecule and the inward diffusion of H ions.Our results show that the adsorption and decomposition into OH and H ion of single H₂O molecule onγ-Al₂O₃ （001）,（110）,（111）surfaces are energy-favorable processes.The defects of γ-Al₂O₃ surface not only reduce the energy barrier of H₂O molecular decomposition,but also alter the decomposition products of H₂O molecule.The migration of H ions on the surface is affected by the crystal structure.H ions produced by the decomposition of H₂O molecules cannot generally diffuse into theγ-Al₂O₃ lattice,but V_Aldefect ofγ-Al₂O₃ （100）sub-surface makes H ions migrate from the surface to the bulk ofγ-Al₂O₃ .Our results indicate that the decomposition state of H₂O can be controlled via adjusting the crystal orientation and surface defects ofγ-Al₂O₃ .（2）We investigated the effect of water vapor on the formation,migration,and aggregation of Al,O atom defects in the three dominant aluminum oxide phases（α-Al₂O₃ ,γ-Al₂O₃ andθ-Al₂O₃ ）involved in the high-temperature oxidation of alumina-forming alloys.In the water vapor environment,the formation energy and migration energy barrier of V_Aland V_Odefects in Al₂O₃ reduce due to the presence of H ion.For two Al atom vacancies,H reduces the repulsion between the vacancies.For two O vacancies,H makes O vacancies easy to aggregate.The lower concentration of dissolved H ions mainly promotes inward diffusion of O atoms,and forming of protectiveα-Al₂O₃ which slows down the oxidation of NiAl.In contrast,the high concentration of dissolved H ions slows down inward diffusion of O by passivating empty O sites and accelerates the outward diffusion of Al,thus promoting the growth of metastable Al₂O₃ .These results provide atomistic insight into resolving the long-standing controversy in the experimental results of the effect of water vapor on the oxidation of alumina-forming alloys and suggest ways of manipulating the oxidation kinetics of alumina-forming alloys via controlling the oxidizing atmospheres.（3）We investigated the atomic processes governing the interfacial dynamics of hafnium with γ-Al₂O₃ and θ-Al₂O₃ during the oxidation of NiAl alloy.Our results show that Al vacancies in the Al₂O₃ overlayer play a critical role in influencing the interfacial segregation of Hfatoms and HfO₂ formation.For the NiAl（100）/γ-Al₂O₃ （001）interface,the presence of interfacial Al vacancies inγ-Al₂O₃ drives the interfacial segregation and aggregation of Hfatoms from the NiAl substrate,thereby resulting in interfacial nucleation of HfO₂.By contrast,for the NiAl（100）/θ-Al₂O₃ （100）interface,the presence of interfacial Al vacancies steers the migration of Hfatoms from the NiAl substrate across the oxide/alloy interface deeper into the oxide overlayer,thereby promoting the HfO₂ formation in the bulk of theθ-Al₂O₃ lattice.These results may find broader applicability for manipulating the interfacial transport process of the reactive element by controlling the phase and stoichiometry of the transient oxide phases.