First Principles Study On Vacancy Formation Mechanism In Oxide Film Of Nickel Based Alloy

Nickel-based alloy accounts for a large proportion in the industrial application of superalloys,owing to its stable structure under the coupled high temperature and stress condition.In the high temperature oxidation environment,a layer of continuous and compact oxide film is formed on the surface of Nickel-based alloy.This layer of oxide film can block the reaction between the external gas and the alloy,so that the alloy has good high temperature oxidation resistance.However,there are still some unsolved problems about the oxidation behavior of Nickel-based alloy.For example,does the water vapor oxidation environment inhibit or promote the oxidation of NiCr alloy compared with the oxygen environment?why is the growth rate of the metastable alumina two orders of magnitude higher than that of stable alumina in the early oxidation stage of NiAl alloy?It is well known that the growth of oxides involves the diffusion of gas molecules and metal ions.When there are more vacancies in the material,the reactant can diffuse rapidly diffused through the vacancies,thus the oxidation rate can be improved.In this thesis,the first-principles DFT calculation is adopted to analyze the effects of oxygen and water vapor on the oxidation rate of NiCr alloy by comparing the vacancy formation and aggregation in Cr₂O₃with and without hydrogen protons,and the effect of vacancy formation and aggregation in?-Al₂O₃alloy on the oxidation rate of NiAl alloy.These studies can provide theoretical basis for understanding the growth of oxide on the alloy surface and controlling the oxidation of the alloy.The main conclusions are as follows:(1)By comparing the formation energies of O vacancies and Cr vacancies in Cr₂O₃formed by NiCr alloy oxidation under oxygen and water vapor environment,it can be seen that the interstitial H proton decomposed by water vapor at high temperature can reduce the formation energy of vacancies,which means that the water vapor environment promotes the formation of vacancies and facilitates the formation of Cr vacancies.Furthermore,by analyzing the vacancy aggregation in two atmospheres,it can be seen that O vacancies can spontaneously aggregate in oxygen environment,while Cr vacancies can spontaneously aggregate in water vapor environment.The vacancy aggregation results in the formation of voids in Cr₂O₃,which destroys the integrity of the oxide film,and makes the anions and cations easily diffuse in the film,leading to the increase of the oxidation rate of NiCr alloy.Since the formation energy of vacancies under water vapor can be greatly reduced,it is easier for vacancies to form and aggregate.Therefore,it can be concluded that water vapor can promote the oxidation of NiCr alloy.(2)By calculating the formation and aggregation of vacancies in?-Al₂O₃formed in the early stage of NiAl alloy oxidation,it is found that the vacancies in?-Al₂O₃is caused by the adsorption of foreign O atoms on the surface of?-Al₂O₃in the oxidizing environment,which make the neighboring Al atoms move away from the lattice positions.In?-Al₂O₃bulk,octahedral Al vacancies are more easily formed than tetrahedral Al vacancies.When the vacancy concentration increases to some certain extents,the vacancy pairs mixed by tetrahedral and octahedral Al vacancies are formed.Further,these mixed vacancy pairs arrange in vacancy chains along the direction of[100].Eventually,these vacancies develop into an ordered structure consisting of vacancy chains on the?-Al₂O₃(100)plane.It can be infered that the existence of the ordered vacancies will provide a fast channel for the outward diffusion of Al ions,leading to the increase of the oxidation rate of NiAl alloy.