Abstract
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.
Highlights
There are several allotropes of carbon based on a hexagonal lattice: 0-D fullerene, 1-D carbon nanotube, 2-D graphene, and 3-D graphite [1]
The basic building blocks of the graphite aggregates are hexagonal faceted graphite platelets generated through the growth of graphene layers
In this and earlier research we found evidence of foliated dendrite growth for LG (Fig. 6), CG and chunky graphite (Fig. 8), and SG (Fig. 10-a,d)
Summary
There are several allotropes of carbon based on a hexagonal lattice: 0-D fullerene, 1-D carbon nanotube, 2-D graphene, and 3-D graphite [1]. Amini and Abbaschian [20] demonstrated that, upon cooling of hypereutectic Ni-3% C alloys, primary graphite can grow either with flake or spherical morphology, depending on solidification rate and supersaturation They argued that morphological transition is the result of changing from growth of the ledges produced by 2-D nucleation to carbon diffusion-limited growth. Lamellar graphite grows from the liquid as crystalline hexagonal parallel platelets, with growth morphology consistent with that of foliated dendrites (Fig. 6-b). The foliated dendrites produce layered faceted crystals with a tiled-roof configuration (Fig. 6a) This confirm early TEM work on lamellar graphite [24]. A good example of foliated dendrites growth was found in the 0.02% Mg iron, where during early solidification, polygonal flat platelets grow in a tiled-roof configuration (Fig. 10-a). SEM images of chunky graphite from an industrial room-temperature sample (composition mass %: 3.72 C, 2.09 Si, 0.12 Mn, 0.012 S, 0.052 Mg, 0.0006 Ce, 0.0056 La, 0.0021 Sb, 0.005 Sn) in Fig. 10-e,f exhibit helical growth and foliated dendrites
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