Growth stages and hexagonal-rhombohedral structural arrangements in spheroidal graphite observed in ductile iron

Abstract

Crystallographic structures of spheroidal graphite particles (graphite nodules) were examined using transmission electron microscopy (TEM). Structures of graphite nodules were investigated relative to different stages of nodule growth in ductile iron. Curved graphene layers were observed during the early growth of the graphite nodules. Thin layered stacking faults give rise to streaking in the basal reflections, which give rise to curvature of the nodule and growth steps on the surface. Columnar substructures consisting of parallel peripheral subgrains were found in the outside region of graphite nodules that were formed during the intermediate growth stage. Straight planar graphene layers were observed in the columnar substructures. Crystallographic orientation of graphite showed little change through the subgrain and the c-axes of multiple subgrains in a single columnar substructure were parallel. A method for characterizing the crystal structures of graphite based on the selected area diffraction pattern was introduced. Both hexagonal structure and rhombohedral structure were found in the spheroidal graphite particles. Possible crystallographic defects associated with hexagonal-rhombohedral structure transition were discussed. 1. INTRODUCTION The authors have previously reported on quenching experiments where an inoculated liquid ductile iron was examined at various stages of solidification. [1-3]. It was shown that the size of a graphite nodule reflects its growth stage [1-3]. A graphite nodule with a diameter less than ten microns is in contact with liquid phase and regarded as being retained at an early growth stage [1-2]. Smooth surfaces with circumferentially grown steps were observed in the nodules at early growth stages, and an example of a graphite nodule (seven microns diameter) at early growth stage is shown in Fig 1(a). A growth step observed on the surface of the graphite nodule is highlighted by arrow in Fig. 1(a).

Crystallography of Growth Blocks in Spheroidal Graphite

“…As the observed nodule is very small because of being in its early growth stage, these large disorientations may be indicative of a competition process: if the nodules had been left to grow to larger size, the best oriented blocks would have given rise to welldeveloped sectors. On the other hand, the thickness of the blocks in the radial direction is well in line with values reported by Miao et al [4] and Qing et al [11].…”

“…Qing et al have shown that evenly distributed point defects are observed in such blocks [11] which may account for the apparent small irregularities in the graphite stacking. Fig.…”

“…Qing et al [11] also observed graphite blocks with well-defined c orientation on a large nodule taken out from a sample that had been quenched at intermediate solidification stage. They reported the presence of numerous point defects in such blocks but did not observe any long range structure that would support a screw or spiral-like growth mechanism of these blocks.…”

“…This suggests that neighbouring blocks formed while developing some strong relationships between each other. More or less complex twin relations such as those investigated by Qing et al [11] may be put forward here. This somehow confirms that every block seen in Fig.…”

A 2-D nucleation-growth model of spheroidal graphite

“…Qing et al [14] report observation of defects and dislocations in graphite, but do not seem to have observed long range ordered arrangements of these defects that would support the screw dislocation or the cone-helix mechanism. Amini and Abbaschian [12] observed both plate-like and spheroidal growth of graphite in Ni-C samples.…”