Abstract
The continuous-cooling-transformation (CCT) diagram is significant to the microstructural design and the control of mechanical properties. The CCT behavior of a high Nb-containing TiAl alloy was experimentally investigated in the present study. Samples with the nominal composition of Ti-45Al-8.5Nb-(W, B, Y) (at.%) were isothermally held at 1500 °C (in the single β phase field) and subsequently cooled at different cooling rates. Based on the samples quenched from high temperatures, the microstructures during continuous cooling was characterized and the CCT diagram was established and evaluated. With the increase of cooling rate (5, 20, 100, 700 K/min), the phase transformation presented a lagging behavior comparing with the equilibrium phase diagram. Meanwhile, the decomposition of high-temperature β/B2 phase was incomplete at large cooling rate, indicating that a fully lamellar (FL) microstructure alloy can be obtained only below a critical cooling rate. Further, the discontinuously coarsened γ phase, the crystalline orientations of precipitated block γ and ω particles in B2 region were characterized and the phase transformation mechanisms were discussed.
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