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 Delta T. 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 Delta T 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 alpha-Ti (alpha) to gamma-TiAl (gamma) 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 alpha 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. (C) 2019 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.