Effects Of Alloying And Heat-treatment On The Microstructrue Of The So-called β/γ-Tial Alloys

y-TiAl alloys due to their low density, high strength, good corrosion resistance, and many other advantages become a hot research topic in nearly30years, but the low plasticity and poor processability hinder the practical application of y-TiAl alloys. To solve the problem of coarse and anisotropic micro structure in conventional y-TiAl alloys, in this paper, some so-called β/γ-TiA alloys were designed and the effects of alloying and heat-treatment on the microstructrue of the so-alled β/γ-TiA alloys were investigated.According to the composition range of the so-called β/γ-TiAl alloy, different alloys with different compositions were designed. The alloys were melted by the water cooled copper crucible arc melting and water-cooled copper crucible maglev induction melting. The optical microscope, scanning electron microscopy, X-ray diffraction, etc were used as experimental methods. Alloying elements Nb, Cr, Mo were added singly or together, and the effects on the microstrcure and the phase transition temperature of the alloys were investigated. According to the special phase transition process in the β/γ-TiAl alloys:β+αâ†'β/γ. The alloys were heated to1250℃, keep for0.5hours following quenching, and then tempering at900℃for4hours. The influence of heat treatment on microstructure was investigated.The investigation reveals that the microstructrue of the so-called β/γ-TiAl alloys is fine, homogenous, no obvious composition segregation, no obvious casting texture, amount of β phase are reserved in the boundry of the colonys. the alloys contains Nb element tend to form lamellar structure, the alloys contain Cr element doesn’t form lamellar structure and different phases distribute evenly, the alloys contain Mo element tend to form widmannstatten structure. Combined addition of Nb and Cr creates greater influence than singly addition of them on stabilizing β phase. The effects of these three alloys on the temperature of γâ†'α transition (Ta) is not evident, but the influence on the eutectoid temperature (Te:α2+γâ†'α) is obvious. The element Mo improves alloy’s eutectoid temperature most obviously. After heat-treatment, the β phase in the Ti-43Al-4Nb-2Cr-0.2B alloy’s grain boundary was completely eliminated, but there still be a small amount of beta in the lamella interface. The residual β phase in the colony boundary of the alloy Ti-43Al-4Nb-1Cr-1Mo-0.2B can’t be completely eliminated. but because of the reaction:αâ†'β+γ, the boundary position formed some laminated structure similar to pearlitic, consists of β phase and y phase.