| TiAl based alloys are widely used in the aerospace and automotive industrial applications, because of their low density, high specific strength and modulus, excellent creep resistance and oxidation resistance and structural stability, and widely used in the field of aerospace and automotive industries. It is important to investigate the effects of composition, processing method and heat treatment on the microstructure of Ti Al based alloys.In this paper, the CALPHAD method was used to design the low Nb-Ti Al and high-Nb alloy compositions based on the Ti-Al-Nb-Cr-B system. The alloy samples were prepared by vacuum hot pressing sintering method and arc melting method. The phase constitution and microstructures of the samples in the as-fabricated state and after heat treatment were characterized by optical microscope(OA), X ray diffraction(XRD), scanning electron microscopy(SEM) and electron probe micro region(EPMA).The effect of alloy composition, preparation method and heat treatment on the phase constitution and microstructure of Ti Al based alloy were studied. Following results were obtained..(1) CALPHAD method was adopted to establish the thermodynamics database of the Ti-Al-Nb-Cr-B quinary system. The vetical sections of the Ti-Al-Nb-Cr quaternary system along different Al content and by using thermodynamic calculation on the basis of the different Nb content were calculated. Low Nb-Ti Al alloy composition and high Nb-Ti Al alloy composition were designed based on the thermodynamic calculation results. The phase fractions as a function of temperature for the designed alloys were predicted.(2) Two groups of alloy samples, low Nb-Ti Al alloy and high Nb-Ti Al alloy, with different Cr content were prepared by vacuum hot pressing sintering method. The effect of heat treatment of 900 ℃ /4h on the phase constitution and microstructure of the alloy was studied. The phase befor heat treatment are mainly γ-Ti Al and α2-Ti3 Al phase, as well as the Nb-rich particle and Cr-rich particles. The major phases after heat treatment were γ-Ti Al and α2-Ti3 Al phase, but the relative amount differed greatly with those presented in the samples before heat treatment. The density was higher and the porosity was less than that in the samples before heat treatment.(3) Two groups of alloy samples, low Nb-Ti Al alloy and high Nb-Ti Al alloy, with different Cr content were prepared by vacuum non consumable arc melting method. The alloys before heat treatment were mainly γ-Ti Al and α2-Ti3 Al phase as well as B2 phase. After heat treated at 900℃ for 4hrs, the amount of B2 decreased greatly. The lamellar structure become finer with the increase of Cr content. |
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