A large undercooling level up to 550K （0.386Te） was achieved in eutectic Ni78.6Si21.4melt by the combination of molten-glass and cyclic superheating. A microcrystaUine structure is obtained at large undercooling. Su...A large undercooling level up to 550K （0.386Te） was achieved in eutectic Ni78.6Si21.4melt by the combination of molten-glass and cyclic superheating. A microcrystaUine structure is obtained at large undercooling. Surprisingly, the morphology of ct（Ni） phase transits from the non-faceted phaseto faceted phase at large undercooling of 390K. Based on the classical nucleation theory and transient nucleation theory, the process of microstructure evolution and competitive nucleation was analyzed, and the refinement of crystal structure is determined by the high nucleation rate under large undercooling.展开更多
Phase selection and microstructure evolution of the undercooled eutectic Ti-Si alloy were systematically investigated by the electromagnetic levitation method, and the maximum undercooling achieved was 318 K(0.2 TE). ...Phase selection and microstructure evolution of the undercooled eutectic Ti-Si alloy were systematically investigated by the electromagnetic levitation method, and the maximum undercooling achieved was 318 K(0.2 TE). The migration of the liquidsolid interface was in-situ detected by a high-speed camera system. When the undercooling is smaller than 140 K, the liquid-solid interface is smooth. Once the undercooling arrives at 230 K, the liquid-solid interface is irregular, which reflects the growth transition from the solute control to the combined controls of solute and thermal. The eutectic growth velocity increases as an exponential function of undercooling. The electromagnetic stirring effect makes it difficult to increase undercooling, but plays an important role in accelerating the eutectic reaction velocity at low and moderate undercoolings. Primary dendritic β-Ti phase appears in the solidified alloy from 63 to 176 K undercoolings, and the microstructure is completely composed of eutectic once the undercoolings increase up to 230 K. When the undercoolings exceed 273 K, the microstructure consists of uniformly distributed irregular eutectic. For the drop tube experiments, the microstructures composed of a large amount of dendritic α-Ti phase and eutectic phase are found in a wide range of diameters from 69 to 725 μm. As the decrease of diameter, the solubility of Si in the dendritic α-Ti phase dramatically increases from 6.80% to 10.73%, and the ratio of the area occupied by the dendritic α-Ti on a cross-section of solidified alloy obviously increases from 23.52% to 41.02%, which result from the combined effects of high undercooling and large cooling rate.展开更多
基金the National Natural Science Foundation of China （No 50395103） the Doctorate Foundation of Northwestern Polytechnical University.
文摘A large undercooling level up to 550K （0.386Te） was achieved in eutectic Ni78.6Si21.4melt by the combination of molten-glass and cyclic superheating. A microcrystaUine structure is obtained at large undercooling. Surprisingly, the morphology of ct（Ni） phase transits from the non-faceted phaseto faceted phase at large undercooling of 390K. Based on the classical nucleation theory and transient nucleation theory, the process of microstructure evolution and competitive nucleation was analyzed, and the refinement of crystal structure is determined by the high nucleation rate under large undercooling.
基金supported by the National Key R&D Program of China(Grant No. 2018YFB2001800)the National Natural Science Foundation of China (Grant Nos. 51734008, 51771154, and 52088101)。
文摘Phase selection and microstructure evolution of the undercooled eutectic Ti-Si alloy were systematically investigated by the electromagnetic levitation method, and the maximum undercooling achieved was 318 K(0.2 TE). The migration of the liquidsolid interface was in-situ detected by a high-speed camera system. When the undercooling is smaller than 140 K, the liquid-solid interface is smooth. Once the undercooling arrives at 230 K, the liquid-solid interface is irregular, which reflects the growth transition from the solute control to the combined controls of solute and thermal. The eutectic growth velocity increases as an exponential function of undercooling. The electromagnetic stirring effect makes it difficult to increase undercooling, but plays an important role in accelerating the eutectic reaction velocity at low and moderate undercoolings. Primary dendritic β-Ti phase appears in the solidified alloy from 63 to 176 K undercoolings, and the microstructure is completely composed of eutectic once the undercoolings increase up to 230 K. When the undercoolings exceed 273 K, the microstructure consists of uniformly distributed irregular eutectic. For the drop tube experiments, the microstructures composed of a large amount of dendritic α-Ti phase and eutectic phase are found in a wide range of diameters from 69 to 725 μm. As the decrease of diameter, the solubility of Si in the dendritic α-Ti phase dramatically increases from 6.80% to 10.73%, and the ratio of the area occupied by the dendritic α-Ti on a cross-section of solidified alloy obviously increases from 23.52% to 41.02%, which result from the combined effects of high undercooling and large cooling rate.
