Phase and its morphologies of Ti-45%Al alloy directionally solidified at different growth rates

The microstructures of Ti-45%Al (molar fraction) alloy directionally solidified at different growth rates in alumina tube by electromagnetic heating zone melting were studied. The measured temperature gradient of the system is about 10⁴ K/m. The microstructures show that the primary solidified phase is β phase at different growth rates. The growth at low rates from 1.94 × 10^-6 m/s to 4.16 × 10^-6 m/s results in a transient solid/liquid interface structure from planar to shallow cellular. This transient rate is larger than the theoretical value of v_c = 6.94 × 10^-7 m/s. Compared with v_t = v_c/k = l.01 × 10^-6 m/s, the cellular-dendritic transient rate of experiment is observed in the range of 1.67 × 10^-5 - 2.50 × 10^-5 m/s. The primary arm spacing decreases with increasing growth rate.