STRUCTURE EVOLUTION OF DIRECTIONALLY SOLIDIFIED Ti--43Al--3Si ALLOY I. Microstructure Evolution in the Transition Region

Ti--43Al--3Si (atomic fraction, %) alloys, seeding material used in preparation of TiAl base alloy, were prepared by directional solidification with growth rate range of 3---100 μm/s. The influences of growth rate on interfacial morphology and microstructure evolution in transition region were studied. Cellular growth appears in a wide range of growth rate of 3---60 μm/s, and the cellular spacing decreases with increase of growth rate. When growth rate is 90 μm/s, a dendritic growth is observed. During the initial solidification, a thermal transition region can be observed clearly. The distribution of Ti5Si3 phase along transition region and relevance of microstructures between the transition region and directional solidification region have obvious effects on the seeding. When the growth rate is lower than 10 μm/s, a good distribution of Ti5Si3 phase and a good relevance appear, which is favourable in seeding for the present alloy.