Facility for the undercooling and solidification experiments with the combination of electromagnetic levitation and glass fluxing treatment. 

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... samples, and obtained the tensile strength and the yield strength of the alloy with 3 and 1.3 times respectively as high as those with the conventional as-cast structure. The Cu 80 Ni 20 alloy was prepared from a mixture of pure elements in an arc melting furnace under a Ti- gettered Ar atmosphere, and was subsequently sucked into a copper mold with water cooling. The sample was cut into 5 mm in diameter and 10 mm in length. Then the sample was put into a crucible and wrapped with dehydrated B 2 O 3 . Afterwards, the sample was heated by the conical coil as seen in Fig. 1. It is noted that this design of the coil can generate a temperature gradient through the sample due to the longitudinally changing magnetic field strength along the sample, and this is quite different from the traditional one, which generally offers a uniform temperature through the sample. The generation of the temperature gradient favours the unidirectional solidification at a certain undercooling of the melts. The temperature was measured by a two-colour infrared pyrometer at the bottom end of the sample. After several cycles of heating and cooling, the sample was solidified at different degrees of undercooling. Lastly, the samples were cut along the longitudinal and cross directions, etched, and the microstructures were analysed by an optical microscope (OM) and a scanning electron microscope (SEM). After several cycles of heating and cooling treatment, a series of undercooling at the bottom end were obtained, in the range of 30 K to 220 K. No matter how large is the undercooling, the temperature gradient kept nearly constant around 8 to 10 K/cm. The samples solidified from the undercooled melts were measured by x-ray diffraction. It is found that there was only one phase, CuNi solid solution, was formed independent of the degrees of undercooling, as seen in Fig. 2. Four typical microstructures were observed along the longitudinal section of the samples, as seen in Fig. 3. At the undercooling of 30 K, the microstructure consists of mainly coarse dendrites which exhibits well-developed first arms and second-arms. When the undercooling reaches 110 K, the short and coarse dendrites change into a worm-like form. There is a tendency to form fine granular grains, without ob- vious second arms. At undercooling of 140 K, the microstructures are quite different from those men- tioned above, and changes to well-arranged unidirectional dendrites, the first arms are long and parallel and the second-arms are short or not obvious, as seen in Fig. 3(c). When the undercooling went up to 220 K, the directional dendrites were changed into very tiny equiaxed grains, which were different from the microstructures of Fig. 3(b). Repeated experiments show that only in the undercooling range of 120–170 K the directional microstructure can be obtained. Below 120 K, the release of solidification latent heat leads to the reduction of undercooling degree as well as the temperature gradient influence on the solidification, which in turn affects the growth and make it difficult to realize preferential growth direction. Above 170 K, due to the compe- tition of nucleation and growth, quite large amount of nuclei formed prior to growth, and this results in the microstructure transformation of dendrites into spherical crystals. In the undercooling range of 60– 120 K, the microstructures are granular grains. Guo et al . [18] argued that the type of microstructure might be formed by remelting of the dendrite formed in rapid solidification and recrystallization of the dendrite frag- ments. It is therefore concluded that the formation of the unidirectional microstructures is closely associated with the degree of undercooling and the temperature gradient in the melts. The central area in the cross section of the sample solidified at the undercooling of 140 K is also analysed, as shown in Fig. 4. It is found that the dendrites of the microstructures are homogeneous and well dis- tributed. In conclusion, we have designed a conical rf coil to realize the undercooling of rod-like CuNi alloy melt with a combination of fluxing method. At the temperature gradient of 8–10 K/cm, with the increasing degree of undercooling in the range of 35–220 K, the microstructures of the solidified alloy are found to evolve from the coarse dendrites to granular grains, unidirectional dendrites, and eventually to equiaxed ...