Misorientations in spheroidal graphite: some new insights about spheroidal graphite growth in cast irons

Lacaze, Jacques; Theuwissen, Koenraad; Laffont, Lydia; Véron, M.

doi:10.1088/1757-899x/117/1/012024

Cited by 7 publications

(9 citation statements)

References 11 publications

(12 reference statements)

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“…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. TEM observations of chunky graphite showed the same thing 30 which appears well in line with the schematic proposed by Loper et al 31 for this degenerate graphite. From the above observations, it may be concluded that graphite growth in cast irons always proceeds along the prismatic direction, whatever is the apparent growth direction that the overall shape suggests.…”

Section: Graphite Growthsupporting

confidence: 88%

Trace Elements and Graphite Shape Degeneracy in Nodular Graphite Cast Irons

Lacaze

2016

Inter Metalcast

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Graphite degeneracy in spheroidal graphite cast iron is a common issue faced by foundries. It is generally associated with the presence of so-called poisoning elements and may in some cases be suppressed by the addition of other elements. Mastering these additions is not simple in practice since industrial alloys do generally contain many elements that can affect graphite shape even when present at low or trace levels. In this work, trace and low-level elements are considered in relation with three steps of microstructure formation: (1) nucleation of graphite; (2) growth of graphite; and (3) solid-state transformations.

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Section: Graphite Growthsupporting

confidence: 88%

Trace Elements and Graphite Shape Degeneracy in Nodular Graphite Cast Irons

Lacaze

2016

Inter Metalcast

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“…In all tested specimens, the traces of the radial crack initiation were observed not only in central disgregations, but also at the G-M interface (Figures 6 and 15-18), what was previously mentioned in the work of Cooper et al [8]. The misorientation of graphene blocks between sectors [25,34], especially in large spheroids, could be not enough to accommodate all network mismatches that resulted from the multiplication of grown ledges [25,34]. Therefore, internal areas of open volume defects, and even small cavities on the matrix-graphite interface (Figures 9 and 10), seem to be preferred initiation sites of radial cracks.…”

Section: Discussion Of the Resultssupporting

confidence: 74%

“…The binding energy of Van der Waals bonds (7 kJ/mol) connecting the basic graphene planes in the elementary cell along the "c" direction, is rather small as compared to the binding energy of covalent bonds (524 kJ/mol) between carbon atoms in the graphene plane along the "a" direction ( Figure 1a) [11,23]. The covalent bonds in the basic plane (along the "a" direction) were less often broken also due to their more random orientation relative to the direction of tensile stress [34,35]. Thus, both types of internal cracks identified in the observed graphite spheroids usually passed through the area of weakened cohesion, the radial cracks along sector boundaries, and the peripheral cracks between individual graphene plates.…”

Section: Discussion Of the Resultsmentioning

confidence: 98%

“…The pure G-M debonding phenomenon was defined as a complete separation of graphite nodule from the alloy matrix along the G-M interface [18,28]. In situ observations of the G-M debonding course [26,28,29,34], showed that this process began when the actual stress exceeded the Figure 18. Internal damage effects in the graphite nodule due to tensile stresses: (a) Radial crack formation between particular sectors, (b) a peripheral crack initiated between basal planes in two neighboring sectors, (c) a displacement of the basal graphite plates combined with shift and rotation of basal graphite plates in particular sectors in the external rim.…”

Section: Discussion Of the Resultsmentioning

confidence: 99%

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Micromechanism of Damage of the Graphite Spheroid in the Nodular Cast Iron During Static Tensile Test

Warmuzek

Polkowska

2020

JMMP

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This work was focused on two particular phenomena contributing to a damage process of nodular cast iron under tensile stress: Internal destruction of graphite nodule and debonding at graphite/matrix (G-M) interface. The G-M debonding was analyzed depending on the phase characteristics of the metal matrix and with the increase in the distance of the observation field from the main crack surface. Typical morphological effects of decohesion in the graphite-matrix microregions related to an internal structure of graphite nodule were revealed and classified. The obtained results of the microscopic observations suggest that the path of both types of internal cracks in the graphite nodule passed through areas of weakened cohesion. Detailed microscopic observations allowed revealing some additional phenomena associated with G-M debonding along the G/M interface. In the most ductile of the tested alloys, with ferritic and ausferritic matrix, the G-M debonding was preceded by the formation of a layer of shifted graphene plates in the external envelope of the spheroid. In the alloys of polyphase pearlitic and ausferritic matrix, the revealed morphology of the G-M interface suggests that G-M debonding might be delayed by the interaction with some phase components as cementite lamellae and austenite plates.

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“…2-a), giving features that would agree with the observation of sectors. However, transmission electron microscopy (TEM) observations have shown that the orientation of the c axis along a sector tilts at random and in either ways [10] which implies that continuous growth around a screw dislocation did not occur. The second type of models ( Fig.…”

Section: Introductionmentioning

confidence: 99%

A 2-D nucleation-growth model of spheroidal graphite

2017

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a b s t r a c tAnalysis of recent experimental investigations, in particular by transmission electron microscopy, suggests spheroidal graphite grows by 2-D nucleation of new graphite layers at the outer surface of the nodules. These layers spread over the surface along the prismatic direction of graphite which is the energetically preferred growth direction of graphite when the apparent growth direction of the nodules is along the basal direction of graphite. 2-D nucleation-growth models first developed for precipitation of pure substances are then adapted to graphite growth from the liquid in spheroidal graphite cast irons. Lateral extension of the new graphite layers is controlled by carbon diffusion in the liquid. This allows describing quantitatively previous experimental results giving strong support to this approach. IntroductionGraphite spheroids in spheroidal graphite cast iron are known to consist of piling up of graphite layers having their c axis oriented parallel to the spheroid radius. To accommodate for the change in orientation in the tangential direction, the spheroids appear to be divided in adjacent sectors which are easily observed by optical microscopy as illustrated in Fig. 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. Two types of models have been suggested in the past to account for this tangential growth: i) those based on screw dislocations [4] or cone-helix growth [5e7]; and ii) those based on the continuous growth of a graphite layer folding around the spheroid [8,9]. In the first type, tangential growth proceeds around screw dislocations or cones emanating from the spheroid's centre (Fig. 2-a), giving features that would agree with the observation of sectors. However, transmission electron microscopy (TEM) observations have shown that the orientation of the c axis along a sector tilts at random and in either ways [10] which implies that continuous growth around a screw dislocation did not occur. The second type of models (Fig. 2-b) would hardly explain the formation of sectors as already stressed by Gruzleski [9]. However, a slight modification where nucleation of new layers proceeds at the step between two neighbouring sectors which was suggested by Double and Hellawell [11] would (Fig. 2-c).There has been a renewed interest these last years for investigating the growth mechanism of spheroidal graphite [12e14]. 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 ...

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Misorientations in spheroidal graphite: some new insights about spheroidal graphite growth in cast irons

Cited by 7 publications

References 11 publications

Trace Elements and Graphite Shape Degeneracy in Nodular Graphite Cast Irons

Trace Elements and Graphite Shape Degeneracy in Nodular Graphite Cast Irons

Micromechanism of Damage of the Graphite Spheroid in the Nodular Cast Iron During Static Tensile Test

A 2-D nucleation-growth model of spheroidal graphite

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