First-principles Study On Oxidation Of NiAl Alloy In CO₂ Environment

NiAl alloys are widely used in aerospace engines for their excellent properties in heat resistant components.Oxidation of NiAl alloys is a key factor affecting the reliable application of the alloy.Among them,the atmosphere profoundly affects the oxidation behavior of the alloy,but the effect of CO₂gas on NiAl oxidation is poorly understood.In this thesis,the oxidation behavior of NiAl alloys under CO₂environment was investigated using a first-principles approach based on density function theory.The main research and results of this thesis are as follows.:（1）CO₂is stably chemisorbed on the NiAl（110）surface,and the decomposition of CO₂into stably adsorbed CO molecules and O atoms on the NiAl（110）surface is an exothermic 0.59 e V process,and the energy barrier required for decomposition is 1.62e V;The CO from CO₂decomposition is more stably adsorbed on the NiAl（110）surface than CO₂,and the decomposition of stably adsorbed CO molecules into stably adsorbed C and O atoms absorb 1.55 e V energy,and the energy barrier for decomposition is 6.76 e V.According to Deformation Charge Density,the decomposition of CO₂and CO is the result of surface Ni and Al atoms giving charge to C and O,respectively.In addition,the adsorption of individual C and O atoms on the NiAl（110）surface was calculated and it was found that both adsorbed C and O atoms gain charge from the Ni atoms on the NiAl（110）surface.（2）The stable adsorption configuration of CO₂on the perfect surface ofγ-Al₂O₃（100）is a linear molecule tilted to the surface,and dissociation of CO₂on the perfect surface is not feasible.In this paper,by calculating the diffusion of oxygen vacancies（V_O）and aluminum atoms from the interior ofγ-Al₂O₃to the（100）surface separately,we find that the energy barrier for upward migration of V_Ois lower than that for downward migration of V_Al,indicating that V_Omigrates more easily.Therefore,the effect of V_Oon the surface ofγ-Al₂O₃（100）on the decomposition of CO₂adsorption is further considered in this paper.The results show that the introduction of surface V_Oleads to the creation of new stable chemisorption configurations(CO₂adsorbed on Al(Al _NN)around V_O).Depending on the activation mode of CO₂,two decomposition modes of CO₂on the surface ofγ-Al₂O₃（100）-V_Oare defined in this paper.（1）Stepwise decomposition:CO₂is adsorbed on Al _NN,and CO₂molecules or C-O rotate to the V_Oposition and decompose;（2）Direct decomposition:vibrationally activated CO₂does not undergo adsorption on Al atoms,but decomposes directly on V_O.For the two decomposition modes,the charge transfer between CO₂andγ-Al₂O₃（100）surface atoms during decomposition was obtained by calculating the Bader charge,which explains the decomposition mechanism of CO₂in different decomposition modes.（3）CO is stably chemisorbed on the perfect surface ofγ-Al₂O₃（110）,and the dissociation of perfect surface is not feasible.By calculating the diffusion of oxygen vacancies and Aluminum atoms from the interior ofγ-Al₂O₃to the（110）surface separately,it is found that the inward diffusion of V_Alis a thermodynamically favorable process,indicating that the Al-rich surface ofγ-Al₂O₃（110）is easily formed.Therefore,this paper further considers the effect of the presence of oxygen vacancies（Aluminum-rich）γ-Al₂O₃（110）surface on the adsorption and decomposition of CO,and finds that the Al _NNaround V_Opromotes the adsorption of CO.On the V_Osurface,the decomposition of free CO into O and C atoms was calculated with a heat release of 1.18 e V.To further investigate the behavior of C inγ-Al₂O₃,the diffusion of C atoms from the surface ofγ-Al₂O₃（110）to the interior through the interstitial mechanism was calculated.The results show that C at the subsurface is more stable and C diffuses from the surface to the subsurface emitting 1.46 e V of heat and diffusing from the subsurface to the subsurface absorbing 1.03 e V of energy.