Most analysis of graphite morphology in cast iron-carbon alloys is performed on samples cooled to room temperature. This raises the concern that the crystallization of graphite is obscured by ...subsequent recrystallization and growth in solid state. To bring clarity to this issue, the authors used Field Emission Gun Scanning Electron Microscopy on deep-etched interrupted solidification (quenched) specimens to reveal the morphology of graphite growing in contact with the liquid at the very beginning of solidification.
To understand the complexity of graphite crystallization in iron alloys, the analysis included evidence from the crystallization of materials with analogous hexagonal structure, such as of snowflakes and metamorphic graphite, and from the crystallization of diamond cubic structure silicon crystals in aluminum-based alloys. Information from research discussing graphite produced through gas-solid (chemical vapor deposition) and solid-solid (graphite in steel) transformations was also exploited.
The large variety of graphite solidification morphologies described in this and earlier papers derives from the complexities of its faceted growth during crystallization, a diffusion-limited crystal growth process, in the presence of anisotropic surface energy and anisotropic attachment kinetics. It was confirmed that 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 through two-dimensional or screw dislocation nucleation. Depending on bulk composition, local supersaturation and undercooling, the platelets aggregate through a variety of mechanisms including tiled-roof and foliated crystals and dendrites, curved-circumferential, cone-helix, helical (macro-spiral), and polyhedral pyramidal (or conical) sectors growth. The final graphite shape of graphite spheroids is affected by the crystallography of the nucleus, as it affects the initial growth of the graphite platelets.
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Lithium-sulfur batteries possess high theoretical energy density. They are promising in future application. Practical electrochemical performance of lithium-sulfur batteries need to be improved. In ...this study, a facile approach has been proposed to improve electrochemical properties of lithium-sulfur batteries. Lamellar graphite coatings with different thickness have been prepared on the surface of sulfur electrodes. Morphology of materials has been characterized and electrochemical properties for batteries have been tested. Results indicate that there are cracks on the surface of sulfur electrodes. Lamellar graphite coatings with thickness of 6 μm and 12 μm have uniformly covered surface of sulfur electrodes. Graphite coatings effectively hinder the dissolution of lithium polysulfides and contribute to increase in discharge capacity and retention rate. Thickness of graphite coating affects electrochemical properties of lithium-sulfur batteries. Electrode with 12 μm graphite coating possesses higher discharge capacity and cycle stability than that with 6 μm graphite coating. After 150 cycles, discharge capacity of electrodes with 6 μm and 12 μm graphite coating is 693 mAh g
−1
and 821 mAh g
−1
respectively.
The effects of cooling rate and carbon equivalent on the tensile strength of pearlitic lamellar graphite cast irons were investigated. The cooling rate was varied from 6°C/s to 35°C/s for values of ...the carbon equivalent equal to 4.17% and 3.83%. The increase in the cooling rate promoted the refining of the eutectic cell size, primary dendrite modulus, interdendritic hydraulic diameter and pearlite interlamellar spacing. The increase in the cooling rate also refined their graphite flakes, changing the morphology from B to E type. The reduction in the carbon equivalent increased the proportion of primary dendrites from 25% to 40%. The maximum tensile strength increased from 274 to 524 MPa with the increase in the cooling rate and the reduction in the carbon equivalent. These results were used to test two tensile strength predictive models based on modified versions of the Griffith and Hall-Petch equations.
Fatigue limit and microstructure in lamellar graphite iron Fourlakidis, Vasilios; Schmidt, Pål; Jarfors, Anders E.W. ...
Materials science & engineering. A, Structural materials : properties, microstructure and processing,
01/2021, Letnik:
802
Journal Article
Recenzirano
Odprti dostop
Demanding environmental legislation and improve performance specifications requires increasing fatigue strength for the engine components that are made of lamellar graphite iron (LGI). Components ...design, metallurgy and casting conditions define the microstructure formation and mechanical properties. The graphite inclusions embedded in the metallic matrix acting as defects and have a detrimental effect on the fatigue strength of LGI. The cooling conditions determine the coarseness of the microstructure and also have, a great impact on the fatigue resistance. The experimental material was an LGI alloy produced with three different solidification times. A fully reversed fatigue test was performed, and various microstructure features were quantitatively estimated by utilizing the Gumbel's statistics of extremes. The stereological parameter of Hydraulic Diameter of the Inter-dendritic Phase and the graphite Feret size found to be the most suitable microstructure parameters to be correlated with the fatigue limit. The results also indicate the sizeable effect of the solidification time on the fatigue limit. Several other microstructure features that have been reported to influence the tensile strength were also found to be related to the fatigue limit. The obtained endurance ratio ranges from 0.25 to 0.30, a value that is in line with previous investigations.
The aim of this study was to provide insight on thermal conductivity of three cast iron groups, namely lamellar, compacted and spheroidal graphite irons at elevated temperatures up to 673 K (400°C) ...in as-cast and austempered states. Austempering treatments increased mechanical properties of all the studied materials while decreasing thermal conductivity across the line. The effects of austempering on conductivity were lower for grey and compacted graphite iron than for spheroidal graphite irons. The results indicate that heat treating can be a viable option in increasing cast iron performance in thermally stressed applications. One ferritic low-silicon spheroidal graphite iron surpassed lamellar graphite iron in conductivity at elevated temperatures, while high-silicon spheroidal graphite irons exhibited low conductivities.
This study focuses on abrasion resistance of Lamellar Graphite Iron (LGI) using microscratch test under constant and progressive load conditions. The interactions between a semi-spherical abrasive ...particle, cast iron matrix and graphite lamellas were physically simulated using a sphero-conical indenter. The produced scratches were analysed using LOM and SEM to scrutinise the effect of normal load on resulting scratch depth, width, frictional force, friction coefficient and deformation mechanism of matrix during scratching. Results showed a significant matrix deformation, and change both in frictional force and friction coefficient by increase of scratch load. Furthermore, it was shown how abrasive particles might produce deep scratches with severe matrix deformation which could result in graphite lamella's coverage and thereby deteriorate LGI's abrasion resistance.
•Interactions between abrasive particles, pearlite and graphite are simulated.•Scratch load effects on deformation mechanism of pearlitic matrix is discussed.•The critical load for the onset of pearlite plastic deformation is determined.
Gray cast iron parts with lamellar graphite in contact with liquid media can be degraded by cavitation erosion. The TIG remelting technique of the surface of these parts is an efficient way to ...improve the lifespan of engineering components working in cavitation environments. This paper illustrates the cavitation erosion behavior of a gray cast iron with a microstructure composed of a pearlitic matrix, a small proportion of phosphorous eutectic and lamellar graphite. The cavitation test was performed using an ultrasonic vibrator, and the liquid chosen was water from the public network. Based on the cavitation curves and the examination of the surfaces tested by optical and scanning electron microscopy, a sequence of damage models is proposed to explain the behavior of cavitation erosion.
Crystallization of graphite during the solidification and cooling of cast iron to room temperature has been the object of relentless, yet often inconclusive research. The importance of the subject ...cannot be underestimated, as graphite morphology is a major player in establishing the mechanical and physical properties of cast iron. Graphite crystallization is a complex phenomenon controlled by melt composition, local melt supersaturation, melt temperature and temperature gradient (cooling rate). All these are wide-ranging variables in the casting process. The results of a major effort to understand the complexity of graphite crystallization in cast iron is presented in this comparative study of crystal growth in materials with crystal morphologies similar to that of graphite. The analysis includes that of analogous materials such as eutectic aluminum–silicon and nickel carbon alloys, growth of other hexagonal or tetragonal crystals such as ice crystals and Al
3
Ti in aluminum–titanium alloys, growth of graphite through other processing routes such as chemical vapor deposition (a gas-to-solid transformation), and heat treatment of carbon steel (a solid-to-solid transformation), and the previous information on the crystallization of carbon in cast irons. An exhaustive analysis of the most widely accepted models for graphite growth is also presented.
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