Solidification pathway and phase transformation behavior in a beta-solidified gamma-TiAl based alloy

The phase transformation behavior of an as-cast Ti-42Al-5 Mn (at.%) alloy after subsequent quenching from 1380 °C to 1000 °C was investigated based on the differential thermal analysis (DTA), electron probe micro analyzer-backscattered electrons (EPMA-BSE), transmission electron microscope (TEM) and X-ray diffraction (XRD). The results show that, the solidification path can be summarized as follows: Liquid→Liquid+β→β→β + α→β + α+γ→β_o+α₂+γ→β_o+γ+α₂/γ→β_o+γ+α₂/γ+β_o,sec, with the phase transformation α→β temperature (T_β) = 1311 °C, phase transformation γ→β temperature of (T_γsolv) = 1231 °C, phase transformation α₂→α or β_o→β temperature (T_α2→α/T_βo→β) = 1168 °C, eutectoid temperature (T_eut) = 1132 °C and T_{α2/γ→βo,sec}≈1120 °C. In comparison with Ti-42Al alloy, the T_eut and T_γsolv are slightly increased while both the T_β is decreased obviously by 5% Mn addition. When quenched from the temperature of 1380-1260 °C, the martensitic transformation β→α′ could occur to form the needlelike martensite structure in β area. This kind of martensitic structure is much obvious with the increase of temperature from 1260 °C to 1380 °C. When the temperature is below T_γsolv (1231 °C), the γ grains would nucleate directly from the β phase. For the temperature slightly lower than T_eut (1132 °C), the dotted β_o,sec phases could nucleate in the lamellar colonies besides the γ lamellae precipitated within α₂ phase. Finally, at room-temperature (RT), the alloy exhibits (β_o+α₂+γ) triple phase with microstructure of β_o+lamellae+γ, of which the lamellar structure consists of α₂, γ and β_o,sec phases. The phase transformation mechanisms in this alloy, involving β→α′, β→γ, α₂→α₂/γ and α₂→β_o,sec were discussed.