Effect Of Water Vapor On High-temperature Oxidation Of NiAl Alloy

NiAl alloy is one of the most promising materials used in the field of high temperature and corrosion resistance environment among aluminum-containing intermetallic compounds.In recent years,the high temperature oxidation resistance of NiAl alloy has become a hot topic.At present,there have been a lot of researches on the oxidation of NiAl alloys in the atmosphere or oxygen atmosphere,but the research of alloys under high-temperature water vapor is still lacking.This is because the presence of water vapor makes the oxidation process very complicated,and the current research has not been able to explain its oxidation mechanism.Although some studies have found that water vapor can accelerate the oxidation of alloys,the atomic level mechanism is still unclear.In this paper,NiAl alloy is taken as the research object and the oxidation behavior of the alloy at 950℃,20%O₂+Ar and 20%H₂O+Ar for 1 h,4 h,12 h,24 h and 48 h is compared.The main research contents are as follows:（1）SEM,FIB,TEM,are used to analyze the microstructure of the original alloy and the alloy after high temperature oxidation.The results show that the original alloy isβ-NiAl phase.The metastableγ-Al₂O₃ forms at the early stage of oxidation in the atmosphere of oxygen and water vapor,and thenα-Al₂O₃ begins to nucleate and grow at the interface ofγ-Al₂O₃/alloy.The growth ofα-Al₂O₃ is the result of the continuous transformation ofγ-Al₂O₃ toα-Al₂O₃ phase.After the alloy is oxidized in two atmospheres,the oxide film both has a two-layer structure,that is,the upper layer isγ-Al₂O₃,the lower layer isα-Al₂O₃.The difference between them is that the upper layer in oxygen is an independent blade-likeγ-Al₂O₃,while the upper layer in water vapor is adhered to each otherγ-Al₂O₃.The lower layer of oxygen inα-Al₂O₃ is flat and dense,while the lower layer of water vapor inα-Al₂O₃ has a lot of tiny pores.Compared with oxygen,the oxide film contains a large amount of metallic nickel particles in the oxide film after being oxidized in water vapor for 24h.These nickel particles are formed because the oxide film in water vapor contains a large number of pores,which makes the nickel element in the matrix easy to diffuse outward and form nickel particles.The thickness of the oxide film on the surface of the alloy in water vapor is always greater than the thickness of the oxide film in the oxygen,which shows that water vapor can accelerate the oxidation of the alloy.（2）The first principle calculations are used to reveal the mechanism of water vapor accelerating alloy oxidation.The calculation results show that the H protons generated by the decomposition of water vapor can penetrate intoα-Al₂O₃ lattice to form interstitial H protons.The interstitial H proton can reduce the vacancy formation energy inα-Al₂O₃ and promote the formation of vacancies;meanwhile,H proton can also reduce the vacancy aggregation energy inα-Al₂O₃ and promote the vacancy aggregation.The vacancies are easier to generate and more likely to aggregate,which leads to the formation of holes in the protectiveα-Al₂O₃.These holes can accelerate the diffusion of ions through the oxide film and accelerate the oxidation of the alloy.