Microstructure selection during the directionally peritectic solidification of Ti–Al binary system

Directionally solidified TiAl microstructures were investigated and it was found that there is competitive growth between the stable phase (β) and metastable phase (α) near the peritectic reaction L+β→α in Ti–Al binary system. The phase selection phenomena of Ti–Al system containing (44–50) at.% Al were theoretically studied based on the criterion of the highest interface temperature and with solidification interface response function model of single-phase alloys. Firstly, according to a thermodynamic model of Ti–Al binary system, a part of the phase diagram with the Al content of (44–50) at.% and above 1740 K was calculated. The peritectic reaction temperature is 1763 K, and the peritectic composition is Ti–47.3 at.% Al. The solidus and liquidus of α and β were described as polynomials. Using these polynomials, Co, Tm, me and ke were determined. Suppose that the solidification of α and β phases conforms to the solidification theory of single phase, the interface temperatures were calculated. For Ti–47 at.% Al, when temperature gradient (G) is 10 000 K/m, the critical growth rate of α phase from planar to columnar is about 3.6×10−6 mm/s. The critical growth rate linearly increases with increasing temperature gradient. With the same computational program, the interface temperatures of α and β phases with Al content from 44 to 50 at.% were calculated. Comparing the interface temperatures of α and β phases, and assuming the phase with the higher interface temperature to grow preferentially from the melt, the phase-selection map with the reference frame of the melt composition and the ratio of temperature gradient to growth velocity (G/V) was constructed. The theoretical results are in good agreement with experimental results.