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KGROUND

IN recent years, significant work was devoted to the understanding of the nucleation, crystalline structure and growth mechanisms of graphite in iron-carbon-silicon alloys. The crystalline structure of spheroidal graphite (SG) includes up to four distinctive regions.[1–4] The regions result from changes in the growth kinetics of the graphite as it transits through four distinct stages: stage I—growth of graphite in direct contact with the liquid; stage II—growth of graphite during the eutectic reaction through carbon diffusion from the melt through the austenite shell; stage III—growth in the solid state between the eutectic and eutectoid temperature, because of decreased carbon solubility in austenite; stage

D.M. STEFANESCU is with the The Ohio State University, Columbus, OH, and also with the The University of Alabama, Tuscaloosa, AL. Contact e-mail: [email protected] A. CRISAN is with the Transilvania University of Brasov, Brasov Romania. G. ALONSO and P. LARRAN˜AGA are with IK4-Azterlan, Durango, Bizkaia, Spain. R. SUAREZ is with IK4-Azterlan, Durango, and also with the Veigalan Estudio 2010, Durango, Bizkaia, Spain. Manuscript submitted November 2, 2018.

METALLURGICAL AND MATERIALS TRANSACTIONS A

IV—growth in the solid state between the eutectoid and room temperature, because of decreased carbon solubility in ferrite. Usually, the growth in stage IV cannot be distinguished from that in stage III. Two regions are typical if the sample is obtained through interrupted solidification or rapid cooling, and three regions may be visible on room temperature samples. In Figure 1(a), graphite platelets, growing concentric to the (MgCa)S nucleus, are seen. This core region is formed during early solidification, when graphite growth occurs in direct contact with the liquid. The growth mechanism in this stage is, most of the time, curved-circumferential crystal or foliated growth.[3,5,6] The graphite platelets in the core are rather disorganized (Figures 1(a) and 2(a)). For reasons yet to be explained, a growth kinetics transition produces stacking of the graphite platelets into clusters of polyhedral blocks and highly curved graphite aggregates resulting in conical (pyramidal) sectors made of platelets parallel to one another. While for well-shaped spheroidal graphite this is believed to be the product of divorced eutectic solidification during stage II,[3,6] a recent synchrotron radiation X-ray study by Yamane et al. [7] found that early solidification of imperfect SG from the liquid can also proceed with columnar conical sectors. This is also consistent with degenerated graphite found in flotation zones in large

Fig. 1—Graphite spheroids exhibiting regions with different crystallography: (a) plasma-etched image of graphite spheroid obtained through interrupted solidification from the liquid (1288 C), from Ref. [1] with permission; (b) confocal scanning laser image of a graphite spheroid after cooling to room temperature, from Ref. [2] with permission.

Fig. 2—SEM images showing nucleation of graphite platelets: (