Roughening transition of decagonal quasi-crystal in undercooled Al72Ni12Co16 alloy

High undercooling of the single-phase decagonal quasi-crystal alloy Al72Ni12Co16 was achieved by cyclically superheating the melt in a containerless electromagnetic levitation apparatus. The maximum undercooling of the said material was up to 400 K. Transmission electron microscopy (TEM), scanning electron microscopy (SEM), and optical microscopy techniques were used to investigate the microstructure and identify the phase composition. The decagonal quasi-crystal phase in the samples with small undercoolings showed a facet morphology and sew in a lateral growth mode below the roughening transition undercooling point, Delta T=64 K. With increasing undercooling, the decagonal phase assumed an equiaxed structure and indicated a continuous growth pattern. Toner's surface dynamics model of growing three-dimensional quasi-crystal was adopted to explain the roughening phenomena in the undercooled melt. (C) 2000 Elsevier Science B.V. All rights reserved.