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I.

INTRODUCTION

G R A P H I T I C cast irons, in addition to being the most commercially important of as-cast materials, are the bestrecognized and most-studied examples of the complex process of divorced eutectic solidification, also known as "faceted-nonfaceted" i or "abnormal" 2 eutectic growth. This process occurs when the growth rate of one of the solid product phases is controlled by its interfacial reaction rather than by long-range diffusion. In contrast to the success of the well-known diffusional model for cooperative eutectics, 3 the macroscopic modeling of these reactions is in its infancy. 4 Compared with cooperative eutectic microstructures, divorced ones are frequently coarser and the orientation of the minor phase appears random at the level of reflected-light microscopy, so that it is widely held that particular orientation relationships do not recur in these systems. 2 Moreover, the morphology of the minor phase is highly variable, often depending strongly on minor chemical additions. In cast irons the graphite particles range from flakes (gray iron) to spherulites (ductile iron) with intermediate (vermicular) forms in between. In spite of the acknowledged importance of interfacial reaction rates in developing divorced eutectic microstructures, the modem techniques of interface analysis have been surprisingly sparsely applied to these materials. In contrast, interfacial microstructures in both rod-like 5'6 and lamellar 7'8 cooperative eutectics have been analyzed using transmission electron microscopy (TEM) and found to correspond quite closely with expectations based on prior investigations of precipitate interfaces produced in the solid state. In addition to showing recurring orientation relationships between the two phases, these studies report ledges and misfit dislocations in these interfaces. By analogy, the former are thought to provide sites for deposition or removal of material during particle growth, and the latter to accommodate strains due to the difference in lattice parameter between the two product phases.9 Two TEM-based investigations of the structure of cast irons have been published, neither of which addressed the relative orientations of both graphite and the iron phase. Double and Hellawell 1~ SOOHO KIM, formerly with the Department of Metallurgy and Mming Engineering, University of Illinois, Urbana, IL, is with AnalyUcal Chemistry Department, General Motors Research Laboratories, Warren, MI 48090-9055. D. S. PHILLIPS, formerly with the Department of Metallurgy and Mining Engineering, University of Illinois, Urbana, IL, is with Division of Materials Science and Technology. Los Alamos National Laboratory, Los Alamos. NM 87545. Manuscript submitted August 29, 1983 METALLURGICAL TRANSACTIONS A

investigated primarily "secondary" boundaries within the graphite, and confirmed several inferences from prior SEM analyses. For instance, all three major graphite forms have the basal plane normal to the particle surface, and graphite particles in both gray and ductile irons consist of n