Research On Preparation And High Temperature Deformation Of TiAl-based Powder Metallurgy Alloy

The new solidified β-TiAl alloy is a kind of high temperature structure materials, which has wide application, due to its high specific strength, high rigidity, excellent oxidation resistance and well deformalility at elevated temperature. In this thesis,43.5Al-4Nb-1Mo-0.3B (at.%) alloy was designed and prepared by the powder metallurgy using element powders. The microstructure of TiAl alloys prepared under different sintering process and the effect of ball-milling on the alloy were studied. The effect of the heat treatment on the transformation behavior and characteristics of β phase in the TiAl alloy was explored, and the high temperature deformation behavior and the microstrucuture of the TiAl alloy were investigated.The ball-milling process of the alloy powders shows that the addtion of a small amount of stearic acid, as milling additives, powder flour yield can be improved; Ball-milling time have important influence on the impurity content. Through ball-milling 8 h, adding 1.2-1.5% stearic acid, fine alloy powder can be obtained, which contains a small amount of impurities. Research on sintering process show that full dense organization can be obtained with adding 20 MPa sintering pressure. Sintering temperature is the dominant factor affecting the TiAl alloy microstructure. Near lamellar of microstructure can be gotten at 1350 ℃, in the upside area of (a+P) two phase region; duplex structure at 1300 ℃, in the middle area of (a+β) two phase region; near y microstructure can be obtained at 1250 ℃ in the downside area of (α+(3) two phase region.TiAl alloy, prepared by mechanical blending and then sintered at 1350 ℃ for 3 h under 20 MPa pressure, was used for heat treatment test. The results show that heat treatment can significantly improve the microstructure uniform, which makes the B2/y bulk microstructure gradually transform into fine α2/γ lamellar. The heat-treated temperature is the dominant factor affecting microstructure evolution. During the heat treatment near the eutectoid transformation temperature,1100～1160 ℃/24 h, the initial β phase can be partly dissolved, while after the short time heat treatment above Ta temperature and subsequent stabilization heat treatment, i.e. 1400 ℃/24 h/AC+850 ℃/6 h/AC, the initial beta phase is completely eliminated without obviously grain coarsening.TiAl alloy sintered at 1300 ℃ for 3 h under 20 MPa pressure was compressed under the condition of 0.1-0.002 s-1,1050-1150 ℃, for hot deformation study.Test research indicated that the TiAl alloy is sensitive to deformation temperature and strain rate, the flow stress decreases with the deformation temperature increased and strain rate reduced. By using hyperbolic sine function equation, the hot deformation constitutive equation was obtained, with the thermal deformation activation energy Q being 410.376 kJ/mol and stress index n being 3.13. The deformation mechanism mainly refers to the kinking, bending, crushing and dynamic recrystallization of α2/γ lamellar grains, while the rheological behavior of B2 phase and dynamic recrystallization of y phase are good supplements to the deformation. The existence of block B2 and y phase is advantageous to the harmonious deformation of α2/γ lamellar grains.