In Situ TEM Study On Initial Oxidation Behavior Of Ni₃Al

With the rapid development of aerospace technology,the requirements for the performance of aerospace engines are increasing.The safety of thermal barrier coatings widely used in aero-engines is a key issue to be solved urgently.The results show that the failure owing to oxidation of the interface between the ceramic coat and the bond coat is an important factor.Controlling the growth of oxide on the surface of bond coat and forming a layer of protective oxide film on the surface is a very effective and desirable method to avoid the failure of interfacial oxidation.As a widely used material for bond coat,Ni-Al alloys have been paid more and more attention.Ni₃Al has been widely applied because of its high melting point and good oxidation resistance.However,the growth of oxide on Ni₃Al surface is greatly affected by crystal orientation,oxygen partial pressure,oxidation temperature and other conditions.Slight change of growth conditions would lead to different oxidation behavior.Therefore,how to control the growth of oxide and form a layer of stable and protective oxide film on the surface of alloy has become the focus of attention.In some studies,it has been found that the early oxidation behavior of Ni₃Al has an effect on the growth of oxides on its surface.In order to discover the structural development of initial oxides on the surface of Ni₃Al,the high oxidation of Ni₃Al thin films prepared by electron beam evaporation at high temperature is observed directly by in situ transmission electron microscopy?TEM?and combined with first-principles calculation.The growth and development of initial oxides of Ni₃Al are studied,which can provide theoretical basis for controlling the oxide growth.The main conclusions are as follows:?1?At the initial stage of oxidation at 800? and P_O2=8.5×10^-5 Torr,?-Al₂O₃ particles are formed on the surface of Ni₃Al?110?,and the interface between oxide particle and substrate is?-Al₂O₃?110?//Ni₃Al?110?.The substrate surface exposed to the incident direction of electron beam is Ni₃Al?001?,and the surface of oxide particles is?-Al₂O₃?011?or?-Al₂O₃?001?.These oxide particles will not only laterally grow outwards the surfaceof the substrate,but also rotate during oxidation.There are two main ways for the rotation of?-Al₂O₃ particles:one is that the oxide particles rotate around the axis parallel to the incident direction of the electron beam in the incident plane;the other is that the oxide particles rotate around an axis parallel to the incident surface.The final result of the rotation is that the interface between the substrate and the oxide remains at Ni₃Al?110?//?-Al₂O₃?121?.Through the first-principles calculation,it is found that the interfacial bonding energy is the largest when the interface orientation between the substrate and oxide particles is Ni₃Al?110?//?-Al₂O₃?121?,.The results show that the oxide particles rotate in order to achieve a more stable interface.?2?In the process of Ni₃Al oxidation,the pores in?-Al₂O₃ develop continuously.The pores near the edge move outward and disappear,so that the number of pores decreases.The pores in the middle of?-Al₂O₃ will move,merge and grow.The movement process of the pores accords with the law of random motion.The smaller pore firstly approaches the larger pore adjacent to it,then the smaller pore merges into the larger one.The pore mainly grows in the way growing to the left or right along its upper and lower edges.The steps at the surface of the pores will grow and regress,and eventually the pores will develop into squares.The calculation results show that the maximum size of the pore is in good agreement with the experimental results.