Decagonal quasicrystal growth in the undercooled Al72Ni12Co16 alloy

High undercooling of the single-phase decagonal quasicrystal alloy Al72Ni12 Co16 was achieved by cyclically superheating the melt in a containerless el ectromagnetic levitation apparatus. The maximum undercooling of it was up t o 368 K. XRD, TEM, SEM and optical microscopy techniques were adopted to in vestigate the microstructure and identify the phase composition, respective ly. A distinct roughening transition was detected in the undercooled sample s with the undercooling increasing. The decagonal quasicrystal phase in sam ples with small undercoolings showed a facet morphology and grew in a later al growth mode below the roughening transition undercooling point, Delta T = 64 K. Then the decagonal phase took on an equiaxed structure and indicate d a continuous growth pattern. The roughening transition was caused by a su dden change of the growth direction from the two-fold axis normal to plane (0 0 0 0 2) to the ten-fold axis in the undercooled melt. Toner's surface d ynamics models of,growing three-dimensional quasicrystal and BCT growth mod el in the undercooled melt (proposed by Boettinger, Coriel and Trivedi) wer e adopted to explain the roughening transition phenomenon. (C) 2000 Elsevie r Science B.V. All rights reserved.