Reassessment of Crystal Growth Theory of Graphite in Cast Iron

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.

Archives of Foundry Engineering

“…The structure of the surface layer of graphite separations observed in the surface of the contraction cavity suggests that this is a primary graphite that crystallizes in a metal liquid, which is confirmed by authors in the papers [25][26][27][28][29][30].…”

Bifilm Inclusions in High Alloyed Cast Iron

“…An interesting phenomenon was captured in Figure 3 , which showed that besides bifilm inclusions, the contraction cavities also contained spheroidal graphite precipitates. The external surface of the graphite precipitates suggests that these are primary precipitates [ 16 , 17 , 18 , 19 ]. These precipitates were pushed out of the liquid alloy towards the contraction cavity.…”

Catalytic Graphitization of Biocarbon for Lithium‐Ion Anodes: A Minireview

“…There has been some interest in separating graphite from kish as a value‐added coproduct, however, the high levels of impurities, particularly silica, make processing and purification costly [34] . Nickel, cobalt, and magnesium are additional metals that have demonstrated, similar to iron, the ability to incorporate, or dissolve, carbon, creating a metal‐carbide that precipitates graphite in the molten phase (1200–1400 °C) [35–37] . Metal‐catalyzed graphite grows in the vertical c‐axis to a greater extent than mineral graphite, which typically favors the lateral $${\alpha }$$ ‐axis [26,27,38] .…”

“…[34] Nickel, cobalt, and magnesium are additional metals that have demonstrated, similar to iron, the ability to incorporate, or dissolve, carbon, creating a metal-carbide that precipitates graphite in the molten phase (1200-1400 °C). [35][36][37] Metal-catalyzed graphite grows in the vertical c-axis to a greater extent than mineral graphite, which typically favors the lateral a-axis. [26,27,38] Thus, the morphologies of metal catalyzed graphite typically consist of crystallites with high dimension ratios of vertical stacking (L c ) to horizontal expansion (L a ) (Figure 5), making them suitable for lithium-ion intercalation and deintercalation.…”