Study On High Temperature Oxidation And High Temperature Corrosion Properties Of ?-TiAl Based Alloy

?-TiAl-based alloy is a new type of lightweight high-temperature structural material.Because of its low density,high specific strength,good high temperature strength and creep resistance.Therefore,it has wide application potential in the field of light and high temperature structural components.Nonetheless,the high temperature oxidation resistance and thermal corrosion resistance of?-TiAl alloys were poor,which restrict their application as high temperature structural materials.Adding Nb to?-TiAl based alloy can not only improve its high temperature deformation performance,but also improve its high temperature oxidation resistance to a certain extent.In this paper.Al elemental powder was mixed with refined Tipowder and Nb powder.Equiaxed,fine-grained?-TiAl and?₂-Ti₃Al double phases Ti-45Al-x Nb(x=5?7?9)alloys were prepared by vacuum hot pressing sintering.The high temperature oxidation behavior and hot corrosion behavior of the alloy were systematically investigated.The microstructure and phase characteristics of oxidation and corrosion layers were analyzed by X-ray,transmission electron microscope,scanning electron microscope and energy spectrum analyzer.The following conclusions are drawn:(1)The fine grain equiaxed duplex(?+?₂)?-TiAl alloy was prepared by mixing the refined Tiand Nb powder with Al powder by vacuum hot pressing sintering process.Nb element can promote the formation of?₂-Ti₃Al phase in?-TiAl-based alloy.With the increased of Nb element content,the?₂-Ti₃Al phase increased.and the?₂-Ti₃Al phase plays the role of strengthening the second phase,which improves the microstructure hardness of?-TiAl alloy.(2)The oxidation products in pure oxygen environment were TiO₂ and Al₂O₃.Due to the difference of expansion coefficient between oxide layer and matrix.As a result,the weight gain rate of the alloy was different in the heating stage,cooling stage and heat preservation stage.Among them,the weight gain was the fastest in the heat preservation stage and the slowest in the cooling stage.The addition of Nb element reduced the oxidation weight gain rate and oxide layer thickness of the alloy.It effectively inhibited the formation and growth of oxides on the surface of the alloy.It can improve the oxidation resistance of?-TiAl alloy.(3)Long-term air oxidation experiments showed that the oxide layer of?-TiAl-based alloy consists of four parts.There were TiO₂/Al₂O₃/(Ti,Nb)O/Nb-rich layer from outside to inside.Due to the diffusion of Tiand Al and their combination with oxygen to form oxides,Nb was enriched in the transition region and Nb-rich layer was formed.The(Ti,Nb)O region contained Nb,where high valence Nb replaces Ti⁴⁺in TiO₂,thereby reducing the defect concentration of Tiions.The oxidation reaction of matrix was effectively inhibited.(4)The hot corrosion experiment in the Na₂SO₄ melting environment at 900?shows that the Nb activity increased in the Na₂SO₄ melting environment,and the Nb reacts to form the Nb₂O₅ phase and the Na Nb O₃ phase.The corrosion layer after hot corrosion for100 h was a mixed corrosion layer of various products of TiO₂,Al₂O₃,Nb₂O₅ and Na Nb O₃.The increase of Nb content led to the increase of corrosion products.The alloy cannot form a uniform corrosion layer.The structure of the corrosion layer is loose and cannot effectively protect the substrate from further corrosion.