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, C_o, T_m, m_e and k_e 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.