Ductile and High Strength White Cast Iron of Ultrafine Interconnected Network Morphology
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WHITE cast iron[1] is an Fe-based alloy containing more than 1.7 wt pct of carbon. It has a eutectic microstructure of aFe and Fe3C. aFe is ductile, while Fe3C is hard and brittle. Since Fe3C greatly exceeds aFe in volume fraction, the alloy is hard and abrasion resistant, but brittle. It was found that the alloy can be toughened by adding to it as much as ~30 wt pct of Cr. Although toughness improves, it remains brittle.[2] To introduce ductility, white cast iron is heat treated at elevated temperatures to decompose Fe3C into aFe and graphite. The annealed alloy is called malleable iron,[3] and appreciable ductility is observed; for example, 32510 ferritic malleable iron has a yield strength of 220 MPa and a plastic strain to failure of 10 pct, and 90001 pearlitic malleable iron has a yield strength of 620 MPa and a plastic strain to failure of 1 pct. Thermal annealing, however, undermines the high hardness and high abrasive wear of white cast iron. Moreover, the yield strengths of the malleable irons are low when compared with those of martensitic stainless steels (which contain ~16 wt pct of Cr); for example, 431 martensitic stainless steel has a yield strength of 1030 MPa and a plastic strain to failure of 16 pct, and 440C martensitic stainless steel has a yield strength of 1900 MPa and a plastic strain to failure of 2 pct. Recently,[4] by using a fluxing technique, a molten Fe83C17 alloy can be undercooled to a temperature T, which is ~400 K (400 C) below its liquidus [Tl ~1423 K (1150 C)] just before it solidifies. The molten alloy may also be described as having an undercooling (DT = Tl – T) of ~400 K (400 C). On solidifying, it becomes a
crystalline solid of interconnected network morphology, which is due to the intermixing of mainly Fe3C and aFe. Equivalently, the network morphology can be regarded as made up of two intertwining subnetworks: a brittle Fe3C subnetwork (wavelength: k ~ 300 nm) and a ductile aFe subnetwork (k ~ 150 nm). For convenience, it is called network alloy or network white cast iron. Fe83C17 network alloys have attractive mechanical properties:[4] a yield strength of ~2000 MPa, a tensile strength of ~2500 MPa, a plastic strain to failure of ~18 pct, and a hardness value of ~536 HV. The attractive mechanical properties of the network alloys are attributed to two factors: first, the interplay of the ductile aFe subnetwork and the brittle Fe3C subnetwork; and second, their short wavelength. The chemical formula Fe3C represents that 1 C atom is able to capture 3 Fe atoms. If molten Fe100–xCx alloys with x > 17 can be cast into network white cast iron, the volume fraction of aFe is expected to change. In addition, if k is still in the submicron range, the mechanical properties of these network alloys will depend critically on the ratio
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