Primary dendrite growth kinetics and rapid solidification mechanism of highly undercooled Ti-Al alloys

The rapid solidification processes of undercooled Ti-(47, 50, 54) at.% Al alloys were investigated by electromagnetic levitation (EML) method combined with a high-speed photoelectric detector. The maximum undercoolings of the three liquid alloys were 376 K, 352 K and 316 K, respectively. Recalescence processes corresponding to the primary dendrite growth and subsequent phase transition were recorded at various undercoolings. The primary dendrite growth velocity V meets a double exponential relationship with the undercooling ΔT. Besides, a novel formula with physical meaning is proposed to explain that the more ordered liquid metal atoms accelerate the primary dendrite growth. Three recalescences are found at all undercoolings for Ti-47 at.% Al alloy and at high undercoolings for Ti-50 at.% Al alloy. The microstructures of solidified Ti-47 at.% Al alloys successively appear as coarse lamellar dendrites and finally evolve to refined parallel lamellar dendrites with the increasing undercooling. When ΔT rises, the microstructures of solidified Ti-50 at.% Al alloys appear from coarse primary dendrites and interdendritic dendrites to refined lamellar dendrites. In the process from low undercooling to high undercooling, the primary phase of undercooled Ti-54 at.% Al alloys changes from α-Ti (α) to γ-TiAl (γ) and the microstructures of solidified alloys evolve from spherical primary dendrites and matrix phases to cellular dendrite phases. Meanwhile, for the Ti-(47, 50) at.% Al, the transformation temperature of metastable intermediate α phase decreases with the increase of undercooling. Moreover, the microhardness of the three solidified alloys reaches the maximum when the undercoolings are 185 K, 270 K and 316 K, respectively.