Comparison of pyrolytic graphite spheres from propylene with spheroidal graphite nodules in steel

Li, Dongdong; Tan, Ruixuan; Gao, Jingxiang; Wei, Bingqiang; Fan, Zheqiong; Huang, Qizhong; He, Ketai

doi:10.1016/j.carbon.2016.10.018

Cited by 19 publications

(9 citation statements)

References 47 publications

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“…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.…”

Section: Introductionmentioning

confidence: 74%

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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Section: Introductionmentioning

confidence: 74%

A 2-D nucleation-growth model of spheroidal graphite

2017

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“…Crystallization of graphite from Fe-C melts begin with the stacking of the graphene sheets in the c-direction into hexagonal faceted graphite platelets that are the basic building blocks of the graphite aggregates. Such platelets, 10-30 nm thick and hundreds of nanometers in length, have been found in graphite spheroids from carbon steel [28] (Figure 6a), in Mg-treated SG iron [29] ( Figure 6b), and in compacted graphite (CG) aggregates as hexagonal faceted graphite platelets with nanometer-height in the c-direction and micrometer width in the a-direction [30] (Figure 6c). To produce a graphite aggregate as found in cast iron, the platelets must thicken and aggregate.…”

Section: Growth Mechanisms For Crystals Relevant To Graphite Growthmentioning

confidence: 93%

“…It assumes that Figure 6. Images of graphite platelets in graphite of various origins: (a) HR-TEM image of the 002 fringes of a graphite platelet in a graphite spheroid from carbon-steel, reproduced from [28] with permission from Elsevier; (b) TEM dark field image of platelets in a graphite spherulite, reproduced from [29] with permission from Springer; (c) SEM image of growth front of new layers on platelets in a compacted graphite iron sample, reproduced from [30] with permission from Elsevier. Because 2-D nucleation (Figure 7a) is slow and could not explain graphite thickening during graphite growth, a variant of 2-D nucleation, poly-nucleation (PNG), was considered.…”

Section: Growth Mechanisms For Crystals Relevant To Graphite Growthmentioning

confidence: 99%

“…Henning [40] found concentrations of screw dislocations whose Burgers vector parallels the c-axis in single crystals of natural graphite and in pyrolytic graphite. More recently, examples of spiral growth for pyrolytic graphite spheres [28], and for both lamellar [41], and compacted [42] graphite were found.…”

Section: Growth Mechanisms For Crystals Relevant To Graphite Growthmentioning

confidence: 99%

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Recent Developments in Understanding Nucleation and Crystallization of Spheroidal Graphite in Iron-Carbon-Silicon Alloys

Stefanescu

Alonso

Súarez

2020

Metals

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The last decade has witnessed significant research efforts directed to the understanding of nucleation and crystallization of graphite and associated solidification phenomena, driven in part by the ever-growing interest in the use of spheroidal graphite cast iron in the manufacture of large castings, such as wind turbine parts. These applications raised new challenges to the production of sound castings, mostly because of the exceedingly long solidification times imposed by the size of the castings. These solidification conditions resulted in many instances in graphite degeneration with subsequent decrease in mechanical properties. Obviously, the subject of graphite nucleation and crystallization in cast iron is still in need of additional answers. Over the years, many reviews of the subject have been published. The goal of this paper is to provide an update on the advances achieved in comprehending the mechanisms that govern the nucleation and crystallization of spheroidal graphite and related imperfect morphologies from iron-carbon-silicon melts. In this analysis, we examine not only the crystallization of graphite in cast iron, but also that of metamorphic graphite (natural graphite formed through transformation by heat, pressure, or other natural actions), and of other materials with similar lattice structure and crystallization morphologies.

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“…Li et al [23] study of SG obtained by heat treatment of medium carbon steel, also concluded that the graphite aggregate is built of radial sectors, and that the building blocks of the sectors are graphite platelets, 10-30 nm thick and hundreds of nm long, growing nearly parallel, with misorientation of up to 20°. They argue that the lengthening in a-direction is constrained by multiple site nucleation around the nucleus.…”

Section: Graphite Produced Through Solid-solid and Gas-solid Transformentioning

confidence: 99%

Reassessment of Crystal Growth Theory of Graphite in Cast Iron

Stefanescu

Alonso

Larrañaga

et al. 2018

MSF

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The problem of graphite crystallization and growth in cast iron has recently received increased attention. As most of the published literature describe analysis of room temperature graphite, there is a legitimate concern that the crystallization of graphite is concealed by recrystallization and growth in solid state occurring after solidification. To avoid confusion in the interpretation of room temperature graphite morphology, the authors used Field Emission Gun SEM on deep-etched interrupted solidification (quenched) specimens to reveal the morphology of graphite at the very beginning of solidification, when the graphite is in contact with the liquid. Information from related phenomena, such as crystallization of hexagonal structure snowflakes and metamorphic graphite, as well as of diamond cubic structure silicon crystals in aluminum alloys is incorporated in the analysis. Research discussing graphite produced through gas-solid and solid-solid transformations is also examined. Because the faceted growth of graphite is the result of diffusion-limited crystal growth in the presence of anisotropic surface energy and anisotropic attachment kinetics, a variety of solidification morphologies are found. The basic building blocks of the graphite aggregates are hexagonal faceted graphite platelets generated through the growth of graphene layers. As solidification advances, the platelets thicken through layer growth, and then aggregate through mechanisms that may include foliated/tiled-roof crystals and dendrites, curved-circumferential, cone-helix, helical, and columnar or conical sectors growth.

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Comparison of pyrolytic graphite spheres from propylene with spheroidal graphite nodules in steel

Cited by 19 publications

References 47 publications

A 2-D nucleation-growth model of spheroidal graphite

A 2-D nucleation-growth model of spheroidal graphite

Recent Developments in Understanding Nucleation and Crystallization of Spheroidal Graphite in Iron-Carbon-Silicon Alloys

Reassessment of Crystal Growth Theory of Graphite in Cast Iron

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