Structural evidence for the transition from coupled to decoupled growth in the solidification of undercooled Ni-Sn eutectic melt

Ni-18.7 at. pet Sri eutectic melt was undercooled and spontaneously solidified In the encasement of a glass flux. Structure morphologies of the sample surface as well as inside the sample were systematically examined, and a critical undercooling of 130 K was clearly revealed for the alloy. Below the critical undercooling, coupled lamellar eutectics (alpha-Ni and beta-Ni3Sn) dendritically grow in the undercooled melt. Beyond the critical undercooling, a-Ni dendrites first form during the early recalescence. beta-Ni3Sn nucleates uniformly in the remaining liquid and then separately grows with alpha-Ni. Solidification of the remaining liquid into lamellar eutectics only occurs at the places in the sample surface layer where the space between the a-Ni dendrite arms is large enough. The finding that all the solidification structures at undercooling above 20 K comprise anomalous eutectics indicates that both coupled eutectic growth and decoupled dendritic growth in the rapid solidification can result in the anomalous eutectic formation. The results also indicate that it is feasible to measure the crystal growth velocity by observation of the recalescence front when undercooling exceeds 50 K for this alloy.