Evolution of Microstructure in Directionally Solidified Cast Iron Treated with Cerium and Magnesium
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CAST iron alloys, composed primarily of iron, carbon, and silicon, exhibit a broad and complex array of behaviors during the solidification process. Despite centuries of industrial use, the solidification behavior of these commercially important alloys is still incompletely understood. The iron-carbon system follows a double eutectic diagram, with the possibility for carbon to solidify either in the form of metastable cementite, Fe3C, or stable graphite, C. At a basic level, cementite is formed when the melt solidifies at a high cooling rate, while graphite is formed at slower cooling rates.[1] Fe3C has a high hardness, making the material more brittle and decreasing its toughness. The Fe3C phase is therefore generally avoided in cast iron, which is often employed in applications where fatigue is a concern. To ensure solidification of carbon as graphite instead of
SUBHOJIT CHAKRABORTY, SAURABH GADKARI, PHILIPP STEINMETZ, CHARLES A. MONROE, and AMBER L. GENAU are with the Department of Materials Science and Engineering, University of Alabama at Birmingham, 1150 10th Avenue South, Birmingham, AL 35205. Contact e-mail: [email protected] Manuscript submitted September 27, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
Fe3C, the cooling rate is controlled during casting and Si is added to promote graphitization. Silicon decreases the stability of the Fe3C by decreasing the liquidus temperature of the Fe-Fe3C while increasing the stable zone for Fe-C. When the carbon solidifies as graphite, it can assume various morphologies, namely, flake (FG), compacted (CG), or spheroidal graphite (SG). The flake graphite can be further categorized based on size distribution and alignment. The morphology of the graphite is influenced by both the freezing conditions and the presence of trace elements in the melt. In 1965, Herfurth proposed that flake vs nodular growth was a question of growth in the prismatic (-a-) direction compared to the basal (-c-) direction that promotes flake growth, but when crystallographically isotropic growth is achieved, spheroidal growth is favored.[2] Double and Hellawell later suggested that the growth rate in the -a- direction is naturally so much faster that mono-layered sheets of graphene roll or spiral around themselves, forming spheroids.[3] They further proposed that surface active impurities like sulfur (S) and oxygen (O), when present in the melt, are adsorbed at the unsaturated edges of the graphene platelets, which partially inhibits the growth on prismatic planes (in -a- direction). Thus, growth on basal plane (in -c- direction) becomes more probable, which causes thickening of the graphite platelets and formation of flakes. However, minor additions of
surface reactive elements (also known as nodularizing elements) scavenge the S and O from the melt.[3–5] This again allows growth of graphite sheets in the -adirection, which then bend in a cone shape and grow in a helical manner forming sectors of a nodule.[2] Furthermore, by scavenging the S and O from the unsaturated edges at the growth front, the nodu
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