Structure of graphite precipitates in cast iron

Abstract

This study presents microstructural investigations of graphite precipitates found in cast irons. Binary Fe eC, FeeCeSb and FeeCeCe alloys containing different graphite morphologies (flake and spheroidal) were produced and compared to commercial irons in an attempt to rationalize the effect of Sb and Ce on graphite growth. The extensive use of transmission electron microscopy (mainly electron diffraction and high resolution lattice fringe imaging) enabled further understanding of graphite growth mechanisms. It was found that the inner structure of graphite precipitates consists of growth blocks stacked upon each other, for all investigated morphologies. This suggests that graphite crystals develop mainly by a 2D nucleation and growth mechanism, and that the final shape of the precipitates is associated to the occurrence of crystallographic defects in the graphite lattice (such as twins, misorientations and rotations) during growth.

A 2-D nucleation-growth model of spheroidal graphite

“…Considering the model shown in Fig. 2-c and recent TEM observations [2,15] suggest to extend to spheroids the 2-D nucleation and growth model developed by Amini and Abbaschian [12] for plate-like growth of graphite from the liquid. Such an approach has been in fact already suggested based on a simple model [16] and is presented here in a much more formal and quantitative way.…”

“…1. The change in orientation of the c axis at the boundary between two sectors may amount from 10 to several tens of degrees, while the orientation changes within sectors are more limited [1,2].…”

“…1. The change in orientation of the c axis at the boundary between two sectors may amount from 10 to several tens of degrees, while the orientation changes within sectors are more limited [1,2].It has long been recognized that graphite layers within spheroids are arranged in growth blocks which are elongated along the prismatic a direction of the graphite structure [3]. This would imply that graphite grows along the prismatic (tangential) direction during spheroidal growth even though the overall (apparent) growth direction is the radial one.…”

Trace Elements and Graphite Shape Degeneracy in Nodular Graphite Cast Irons

“…Exotic or interesting graphite precipitates could thus be obtained, such as exploded nodules illustrated in Figure 5 which were observed in a Fe-C-Ce alloy. 25 In these precipitates, the sectors of the graphite nodule appear well formed but separated from one another. This goes in line with Ce adsorbing on the prismatic planes along the graphite/liquid interface and slowing down the growth along the prismatic direction.…”

“…After doping a Fe-C melt with Sb, it was found that graphite lamellas became wavy and transmission electron microscopy (TEM) observation showed this to be associated with a marked shortening of the SBUs without noticeable change of their thickness. [23][24][25] Other forms of graphite in cast irons may be seen to be similarly made of stacking of SBUs. This was long ago postulated for spheroidal graphite by Mitsche et al 26 and demonstrated with transmission electron microscopy (TEM) by Miao et al 27 The schematic of compacted graphite proposed by Den Xijun et al 28 and the scanning electron microscopy (SEM) observations by Geier et al 29 may be similarly understood.…”