| A high undercooling was got in bulk Al72Ni12Co16 alloy melt by means of cyclic superheating and a special coating crucible. On this basis, the solidification behavior of decagonal quasicrystal in the undercooled Al72Ni12Co16 alloy melt, such as nucleation and growth mechanism, was systematically investigated. Based on the classical nucleation theory and the time-dependent nucleation theory, the thermodynamics and the kinetics for the nucleation of decagonal quasicrystal have been discussed in detail. The microstructure evolution of the quasicrystal-forming Al72Ni12Co16 alloy under a wide undercooling range was studied. With increasing undercooling, the growth mode of decagonal quasicrystal was investigated and bulk single-phase decagonal quasicrystal materials were prepared. In addition, the thermal stability and basic mechanical properties of single-phase decagonal quasicrystals were researched by scanning microscopy (SEM), X-ray powder diffraction (XRD), differential thermal analysis (DTA) and microhardness technology. The main results are as follows:(1) A maximum undercooling, 180K, was achieved in the Al72Ni12Co16 alloy melt by means of cyclic superheating and the nucleating inhibition crucible. The undercooling of alloy melts was markedly influenced by some technical parameters, such as content of noncrystal in crucible coating, superheating temperature, superheating time and cyclic superheating times.(2) Based on the experimental results about the undercooling of quasicrystal-forming alloy melts, the structure dependent α- factor of primary phases have been calculated within the framework of classical nucleation theory. It indicated that the maximum undercooling of alloy melts is determined by the primary-phase structure only in consideration of homogeneous nucleation and a linear relation exits between the primary-phase structure and the relative undercooling of melts. The more similar the structure of ISRO in the undercooled melt and the primary phase, the lower is the undercoolability of the melt.(3) According to the relation between the primary-phase structure and the relative undercooling of melts, the maximum undercooling of Al72Ni12Co16 alloy was calculated when decagonal quasicrystal was preferentially nucleated. The results indicated that the heterogeneous nucleation mechanism is adopted by thedecagonal quasicrystal in the undercooling range achieved in the experiments and the heterogeneous sites with witting angles smaller than 92?have been effectively eliminated by cyclic superheating and the nucleating inhibition crucible in the Al72N12Co16 alloy melts undercooled by 180K. The interfacial energy and the incubation time for nucleation of decagonal quasicrystal are smaller than those of crystal phases. Besides, the homogeneous nucleation rate of decagonal quasicrystals is several orders of magnitude larger than that of crystal phase.(4) According to the experimental results and theoretical analysis, it indicated that at the undercoolings smaller than 65K, the crystal-phase y would form near the columnar-grain boundaries of decagonal quasicrystal in the solidification process of Al72Ni12Co16 alloy because of the solute lack in front of the L/S interface, which causes the destruction of single-phase microstructure. When the undercooling is larger than 65K, due to the development of solute trap, an equiaxed microstructure of single-phase decagonal quasicrystal could be formed.(5) Thermodynamic calculation indicated that the fine equiaxed microstructure in the Al72Ni12Co16 alloy originates from the break-up of primary dendrites under the action of remeiting in recalescence. Near the undercooling of 70K, the initial dendrite skeleton has the maximum tendency to be remelted. With increasing undercooling, the remeiting in recalescence decreases to a low level and is not severe enough to make the initial structure disintegrate into fine grains. So the relatively coarse microstructures are formed.(6)...
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