Microstructure Of NiAlHF Alloy Under High Temperature Oxidation Environment

The thermal barrier coating with high temperature resistance and high thermal insulation can improve the performance and thermal efficiency of the engine.It is recognized as the most feasible method to raise the working temperature of aero-engine greatly at present.?-NiAl is the most potential material for the bond coat of thermal barrier coating because of its high melting point,low density and high Young's modulus.Although NiAl alloy has strong oxidation resistance at high temperature,the poor brittleness at room temperature and the poor cyclic oxidation property at high temperature of NiAl alloy limit its application in practice.A large number of studies have shown that the use of active elements can reduce the oxidation rate and significantly improve the adhesion between the oxide film and the alloy substrate.However,few studies have been done to consider the effect of active element addition on the microstructure of the oxidation process.Therefore,the microstructure evolution at the interface between the oxide and the alloy after oxidation at 950? for 1 h,8 h,16h and 24 h is investigated by means of ICP-AES,XRD,SEM,EDS and TEM.The main contents are as follows:?1?The microstructure evolution of the original alloy and the oxidized material is analyzed by ICP-AES,XRD and SEM.The results show that the primary alloy is mainly?-NiAl,and the main composition of the oxidation film is the blade-like?-Al2O3after isothermal oxidation at 950?.Moreover,there are large pores between the oxide film and the alloy interface.With the prolongation of oxidation time,a thin layer of oxide appears on the inner surface of some pores.Hf in the alloy diffuses to the interface between the oxide and the metal substrate to form a nail-like oxide.The oxide is a core-shell structure with HfO₂ in the center and Al₂O₃ in the outer layer.The reason for the core-shell structure is that the high diffusion rate of O around HfO₂,which leads to the formation of short circuit diffusion channels at the interface between the oxide film and alloy.O preferentially reacts with Al in the diffusion path to form Al2O3.At high temperature,Hf is mainly distributed in the form of HfO₂ at the interface between the oxide film and the alloy.Moreover,there is no hafnium in the upper layer of the oxide film.?2?The microstructure of NiAlHf alloy after oxidation at 950? for 24 h is characterized by TEM and EDS.The oxidation mechanism is further analyzed.The results show that?-Al2O3 is first formed on the surface of the alloy due to the high content of Al in the early stage of oxidation.With the increase of oxidation time,?-Al₂O₃ grows gradually and Hf diffuses to the interface to form HfO₂.Due to the Kirkendall effect,there are some pores in the interface between the oxide film and the alloy.Some oxide appears gradually on the inner surface of the pores due to the diffusion of O inward the interface between the oxide and the alloy.With the increase of oxidation time,the layered structure composed of?-Al2O3,NiAl2O4 and HfO2 is formed on the inner surface of pores.The formation mechanism of oxide in pores is as follows:due to the low oxygen pressure in the pores,the selective oxidation of Al takes place first to form?-Al2O3,followed by the enrichment of Ni in alloys near the interface,resulting in the reaction between Ni and?-Al₂O₃ to form NiAl₂O₄,less Hf also disperses in spinel phase with the outward diffusion of Ni and reacts with O to form HfO₂.This process occurs over and over again.At last the oxide in the pores presents a layered structure.