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

Abstract

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→βo+α2+γ→βo+γ+α2/γ→βo+γ+α2/γ+βo,sec, with the phase transformation α→β temperature (Tβ) = 1311 °C, phase transformation γ→β temperature of (Tγsolv) = 1231 °C, phase transformation α2→α or βo→β temperature (Tα2→α/Tβo→β) = 1168 °C, eutectoid temperature (Teut) = 1132 °C and Tα2/γ→βo,sec≈1120 °C. In comparison with Ti-42Al alloy, the Teut and Tγsolv are slightly increased while both the Tβ 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 Tγsolv (1231 °C), the γ grains would nucleate directly from the β phase. For the temperature slightly lower than Teut (1132 °C), the dotted βo,sec phases could nucleate in the lamellar colonies besides the γ lamellae precipitated within α2 phase. Finally, at room-temperature (RT), the alloy exhibits (βo+α2+γ) triple phase with microstructure of βo+lamellae+γ, of which the lamellar structure consists of α2, γ and βo,sec phases. The phase transformation mechanisms in this alloy, involving β→α′, β→γ, α2→α2/γ and α2→βo,sec were discussed.