White cast iron of network morphology-its formation and properties
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White cast iron of network morphology-its formation and properties Yip Yeuk Lan, Leung Ching Chuen, Fuk Chung Ming, Mok Siu Wah, and Kui Hin Wing Physics, The Chinese University of Hong Kong, Physics department, The Chinese University of Hong Kong, Shatin, N.T., Hong Kong, Hong Kong, China, People's Republic of ABSTRACT A white cast iron of composition Fe81C14Si5 can be cast into a nanostructure with network morphology by a fluxing technique. The conventional morphology of white cast iron, which is brittle, is eutectic. The mechanical behavior of network white cast iron is attractive. Hardness tests indicate that its average hardness value is ~770 HV. The indentations made during hardness tests have no cracks. A stylus surface profiler was employed to study the surface profile at and near the indentations. The studies indicate that there are severe plastic flows, but without cracks. INTRODUCTION White cast iron [1] is a widely used material. It consists of a eutectic structure of Fe3C and B.C.C. Fe (called αFe). αFe is ductile, but Fe3C is very hard and brittle. Since Fe3C is the major phase, white cast iron is very hard and has excellent wear resistance. White cast iron does not have high impact toughness for two reasons. First, there are large pieces of Fe3C phase, with length > 100 µm, that cut off the connectivity of αFe. Second, the microstructure is not fine enough. It is therefore of practical importance if the phases in white cast iron are re-arranged to improve its impact toughness. Recently by a fluxing technique [2], Pd40.5Ni40.5P19 [3] and Pd80Si20 [4, 5], both eutectic alloys, can be cast into nanostructures with network morphology. Wavelength λ of the network morphology can be as short as a few nanometers. Unfortunately, the mechanical properties of these nanostructures are not too attractive, e.g. the tensile strength of Pd80Si20 nanostructure is only ~270 MPA [6, 7]. It turned out the constituent phases in the Pd40.5Ni40.5P19 and Pd80Si20 nanostructured alloys are all brittle. In this paper, we report that the fluxing technique was applied to a cast iron, Fe81C14Si5, which is also a eutectic alloy. An asprepared specimen is also a network nanostructure, but one of the constituent phases is ductile. EXPERIMENT In a fluxing technique, a molten metallic specimen is heated and cooled in a molten oxide flux. The oxide presumably serves to dissolve or deactivate the heterophase impurities, allowing the molten metallic specimen to be undercooled substantial below its liquidus, Tl. In the experiment, a Fe81C14Si5 ingot and B2O3 (enough to immerse the whole specimen) were put together in a dry, clean fused silica tube with an inner diameter of 11 mm. The system was then heated up by a torch to ~1500 K, under a vacuum of 10-2 Torr, for about 4 hr. At the end of the heat treatment, the torch was removed and the system was allowed to cool down in air. Crystallized ingots with diameter 1-2 mm were prepared. Their microstructures were examined by both scanning electron microscopy (SEM) and transmissi
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