From flake to nodular: A new theory of morphological modification in gray cast iron
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10/3/04
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From Flake to Nodular: A New Theory of Morphological Modification in Gray Cast Iron A.N. ROVIGLIONE and J.D. HERMIDA A modified Bridgman technique was used to grow a nickel-alloyed gray cast iron. During the experiments, morphological modifications (MMs) were induced by the change in chemical composition of the melt in front of the solid/liquid interface. This was achieved by means of an “in-situ” addition of commercial modifiers, maintaining the thermal-gradient–to–growth-rate ratio. Optical and scanning electron microscopy (SEM) and X-ray diffraction were used to characterize the resulting microstructures. From the analysis of the evolution of the microstructures during our unidirectional experiments, correlated with results of other researchers, a new explanation for MMs observed in cast iron is proposed; it covers the whole range of morphologies from flake to nodular graphite. The explanation emphasizes the following points. (1) Graphite always grows like sheet crystals. (2) The MM of flake cast iron starts when the triple-contact line between austenite-graphite-liquid disappears, provoking the separation of austenite from graphite during eutectic growth. (3) Vermicular and nodular morphologies arise from the compacting action of solid austenite and liquid, respectively, exerted upon sheet crystals.
I. INTRODUCTION
THE different morphologies of graphite in gray cast iron can be classified into three main types: flake, vermicular, and nodular. During the last 50 years, several theories have been proposed to explain the genesis of each one and how the change between them proceeds. From now on, we will simply refer to this transformation sequence as morphological modification (MM). To our knowledge, in practically all the theories of eutectic growth of gray cast iron, it is considered that graphite phase needs the unavoidable previous existence of different types of crystalline defects for growing;[1] these would be necessary in order to support a lateral growth mechanism.[2] Mainly, two kinds of defects are mentioned: rotation boundaries and screw dislocations with a Burger vector emerging from basal planes. The rotation-boundary steps are proposed as the necessary defects for flake-type evolution, because quasi two-dimensional crystals are expected from the occurrence of preferential growth in the a1 (11.0 and 10.0) directions. On the other side, screw dislocations would be responsible for thickening the graphite crystals, permitting the preferential growth along 1 c ([00.1]) directions to take also place. The main theories for explaining the MM also assume the hypothesis that the growth rate of graphite would be controlled by crystalline defects. Essential concepts of these theories can be summarized as follows. 1. The MM would arise when the mobility in the prismatic a1 directions is reduced. Then, the preferential growth direction changes to be coincident with the 1 c axis. A.N. ROVIGLIONE, Professor, is with the Departamento de Ingeniería Mecánica y Naval, Facultad de Ingeni
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