Carbon Supersaturation Research Articles

Solution Growth of SiC from the Crucible Bottom with Dipping under Unsaturation State of Carbon in Solvent

SiC crystals are grown using a Si-Cr-based solvent by a top-seeded solution growth (TSSG) method by changing the dipping time after when the growth temperature is reached. Step-flow-like curve morphologies were observed for a dipping time after 15 min, while polycrystallization occurred at the periphery for that after 120 min, which corresponded to the dipping under unsaturated and supersaturated carbon in the solvent, respectively. Furthermore, the solution growth of SiC with dipping under unsaturated carbon was easily realized by the growth from the crucible bottom, step-flow-like growth was achieved. Using this technique, dominant polytypes of SiC in various growth conditions after stable seed dipping under the unsaturation in the solvent can be demonstrated.

Effects of microstructure and crystallography on mechanical properties of cold-rolled SAE1078 pearlitic steel

The evolution of the microstructure and crystallography in SAE1078 pearlitic steel sheets under different cold-rolling reductions of up to 90% were quantified using transmission electron microscopy (TEM), electron backscatter diffraction (EBSD), and X-ray diffraction (XRD). The mechanical properties were determined by tensile testing at room temperature. TEM analysis showed that the pearlite structure was obviously refined with the interlamellar spacing decreasing to about 57nm at the rolling reduction of 90%. EBSD investigations indicated that the ferrite exhibited a {001} <110> texture in the 90% cold-rolled pearlitic steel. The dislocations were mainly concentrated during cold rolling between the 10% and 70% reduction ratios as the average kernel average misorientation (KAM) angle increased from 0.75° to 1.20°. XRD examination revealed that a transformation from bcc to bct crystal structure of ferrite occurred at 90% rolling reduction due to the supersaturation of carbon. Significant augmentation in the ultimate tensile strength during cold rolling results from the boundary, dislocation, and solid solution strengthening mechanisms.

Плазменная поверхностная цементация с использованием графитового покрытия

This paper is considered a new technology of plasma surface carburization using graphite pastes made from liquid glass and graphite. The use of this type of coating together with a new composition of the plasma-forming gas (a mixture of argon and carbon dioxide) makes it possible to saturate the metal surface with carbon without surface melting. It is established that during the time of plasma action 0.1-1с the surface layer is saturated to the level of white iron concentration. The main parameters of the cemented layer are determined: the depth of the cemented layer is 35-250 μm with microhardness of up to 12000 MPa. Depending on the composition of the coating and the cooling rate, γ → α - the conversion may result in the formation of a wide range of structural components. The morphological composition obtained in the process of plasma surface carburization is very diverse and specific, which is due to the distribution of carbon in the volume of material in the plasma treatment zone, the speed parameters of heating and cooling, and the composition of the graphite coating. Each phase and structure in the hardened layer (retained austenite, cementite, martensite, ledeburite) has its own varieties. The cemented layer consists of two zones, the first zone with significant carbon supersaturation (white iron structure) is formed due to frontal diffusion of carbon (ledeburite,retained austenite, martensite). And the second zone is formed due to intensive or reactive diffusion, in this zone the carbon concentration gradually approaches the initial one.

Controlled growth of ultrathin Mo2C superconducting crystals on liquid Cu surface

Exhibiting thickness-dependent change in the critical temperature (Tc) for the onset of superconductivity, Mo2C has emerged as an important new member in the family of two-dimensional atomic crystals. Controllable growth in terms of morphology and thickness is necessary to elucidate its intrinsic properties at the 2D limit. Here we demonstrate the chemical vapor deposition of ultrathin Mo2C crystals on liquid Cu surface where the morphology of the crystals can be controlled by tuning the carbon supersaturation. A unique staggered carbon vacancy ordering is discovered in Mo2C crystals having particular geometries. Thickness engineering of the crystal can be achieved by controlling the thickness of the Cu catalyst layer, which affords a facile route to grow ultrathin 2D samples. Ultrathin Mo2C crystals so obtained, have been characterized using aberration corrected scanning transmission electron microscopy annular dark field imaging, where the co-existence of both AA and AB stacking modes is observed. The high crystallinity of the Mo2C crystals synthesized in this work is attested by its characteristic sharp superconducting transition.

Predicting the Transition between Upper and Lower Bainite via a Gibbs Energy Balance Approach

The transition temperature between upper bainite and lower bainite is calculated with an extended Gibbs energy balance model, which is able to quantitatively describe the evolution of carbon supersaturation within bainitic ferrite sheaves during the entire thickening process. The nucleation rate of intra-lath cementite precipitation on a dislocation is calculated based on of the degree of carbon supersaturation. Upper bainite and lower bainite are thus distinguished by the effective nucleation density and therefore a numerical criterion can be set to define the transition. The model is applied to Fe-xC-1Mn/2Mn/1Mo ternary alloys. Results show that the transition temperature increases with bulk carbon content at lower carbon concentration but decreases in the higher carbon region. This prediction agrees very well with the experimental observations in Mn and Mo alloyed systems. Moreover, the highest transition temperature and the carbon content at which it occurs in the Fe-xC-2Mn system are in good agreement with reported experimental data. The inverse “V” shaped character of the carbon concentration–transition temperature curve indicates two opposite physical mechanisms operating at the same time. An analysis is carried out to provide an explanation.

Physico-chemical causes of the formation of chunky graphite in austenitic ductile iron

Chunky graphite (CHG) is a well-known phenomenon which occurs in heavy section spheroidal graphite iron (SGI) castings or SGI castings with higher alloying content. Chunky graphite results in a decrease of mechanical properties, in particular a decrease in elongation. Despite numerous studies, the root cause of the CHG formation remains unidentified. In publications analyzing the different stages of CHG formation based on quenched samples, it is assumed that carbon diffusion through the austenite halo is inhibited due to the size of the eutectic grain. Furthermore, due to the carbon supersaturation CHG nucleates near the liquid-austenite interface of the eutectic grain. Additionally, other studies show that partly incomplete graphite spheres can be interconnected with CHG where no nucleation is required. Using thermodynamic-kinetic material simulation two morphologies, the decoupled SGI growth and the coupled CHG growth, were modeled to simulate their impact on the solidification kinetics. In the present study, it was shown for the first time that with increasing Ni content conditions arise in the austenite halos which strongly reduce the diffusive flux of carbon towards the graphite spheroid and thereby decelerate the kinetics of SGI growth. With a content above 5 wt% Ni, the competing kinetics of CHG growth exceeds the SGI growth. Therefore, the preference in kinetics and the present thermodynamic constraints are critical to the morphology change from SGI to CHG.

Unusual growth macrolayers on {100} faces of diamond crystals from magnesium-based systems

We studied unusual growth macrolayers on the {100} faces of diamond crystals grown in Mg-C and Mg0.9Ge0.1-C systems at 7.0GPa and in the temperature range of 1800–1900°C. The layers were shaped as rectangles highly elongated in one of the [110] directions. The layer elongation (length to width ratio) was different, but did not exceed 23. We found that the ends of the macrolayers in most cases were composed of {111} microfacets, and their growth occurred by layers along these microfacets. Using selective etching, we found that the macrolayers were not associated with outcrops of dislocations and any planar defects on the {100} faces. It is supposed that the formation of these highly elongated layers occurs by two-dimensional nucleation that becomes possible at high carbon supersaturation under conditions of kinetically limited diamond growth in Mg-based systems.

Total pressure‐controlled PVT SiC growth for polytype stability during using 2D nucleation theory

A total pressure‐controlled physical vapor transport growth method that stabilizes SiC polytype is proposed. The supersaturation of carbon during SiC growth changed as a function of the growth time due to changes in the temperature difference between the surfaces of the source and the grown crystal. Supersaturation also varied as a function of the pressure inside the furnace. Therefore, modification of the pressure as a function of growth time allowed for constant supersaturation during growth. The supersaturation was calculated based on classical thermodynamic nucleation theory using data for heat and species of Si2C and SiC2 transfer in a furnace obtained from a global model. Based on this analysis, a method for polytype‐stabilized SiC growth was proposed that involves decreasing the pressure as a function of growth time. The 4H‐SiC prepared using this pressure‐controlled method was more stable than that of 4H‐SiC formed using the conventional constant‐pressure method.

EIS of carburised CoCrMo: Evolution of parameters characterising the metal-electrolyte interface

Abstract Corrosion-wear is of major concern for Cobalt-Chromium-Molybdenum (CoCrMo) hip implants as it leads to premature failure of the prosthesis and treating the CoCrMo surface using low temperature carburising treatment is a plausible investigation to mitigate such problem. This diffusion treatment introduces a supersaturated carbon solid solution, termed S -phase, which hardens the CoCrMo without detriment to the corrosion resistance. There are only basic studies on the electrochemical characteristics of S -phase surface and therefore, from a scientific and industrial point of view, there is a lot of scope to investigate it further. This research uses electrochemical impedance spectroscopy (EIS) to study the metal/electrolyte interface and how it changes after carburising and over a time span of 24 h. The technique reveals that the CoCrMo/Ringer's solution interface contains a highly resistant passive film and an electrical double layer when under equilibrium conditions. Comparison of the impedance results attained when testing untreated and carburised CoCrMo reveals that the treated metal has a passive film which has higher polarisation resistance compared to that of the untreated alloy. This may have been a consequence of the fact that the treated metal's passive film has lower time constant dispersion and faster film growth kinetics compared to the untreated alloy's passive film. In addition, both untreated and carburised interfaces suffer a considerable loss in their polarisation resistance after 20 h of testing which indicates a non-monotonic relationship between passive film impedance and electrochemical potential.

The role of silicon, vacancies, and strain in carbon distribution in low temperature bainite

We investigated the phenomenon of carbon supersaturation and carbon clustering in bainitic ferrite with atom probe tomography (APT) and ab-initio density functional theory (DFT) calculations. The experimental results show a homogeneous distribution of silicon in the microstructure, which contains both ferrite and retained austenite. This distribution is mimicked well by the computational approach. In addition, an accumulation of C in certain regions of the bainitic ferrite with C concentrations up to 13 at % is observed. Based on the DFT results, these clusters are explained as strained, tetragonal regions in the ferritic bainite, in which the solution enthalpy of C can reach large, negative values. It seems that Si itself only has a minor influence on this phenomenon.

Potential resistance to transgranular fatigue crack growth of Fe–C alloy with a supersaturated carbon clarified through FIB micro-notching technique

The threshold stress intensity factor range (ΔKth) of water-quenched Fe–0.017C (wt.%) fully ferritic steel was determined using room-temperature fatigue tests on micro-notched specimens. The experimentally determined ΔKth was approximately 40.5% higher than conventionally predicted results. This extraordinary resistance to transgranluar fatigue crack propagation likely results from the strain–age hardening.

Influence of Mg on Solidification of Hypereutectic Cast Iron: X-ray Radiography Study

Radiography using a synchrotron radiation X-ray source was performed to examine solidification and melting behaviors in hypereutectic cast iron specimens containing 0.002 and 0.05mass pctMg. The solidification sequence in the alloy containing 0.002mass pctMg was (1) nucleation and growth of graphite particles of which transformed to a flake-like shape, (2) growth of γ-Fe dendrites, (3) nucleation of graphite particles ahead of the interface just prior to the eutectic solidification, and (4) the eutectic solidification. In contrast, (1) and (2) occurred nearly at the same time in the specimen containing 0.05 mass pct Mg. The addition of 0.05mass pctMg significantly reduced the temperature range in which the graphite particles grew as the primary phase. Image-based analysis of melting behavior showed that even 0.05 mass pct addition was sufficient to modify the phase equilibrium of the liquid, γ-Fe, and graphite phases. Thus, the observed influence of Mg on the solidification sequence was attributed to the modification of the phase equilibrium. The influence was consistently explained by considering the shift of the eutectic composition to the carbon side in the pseudo-ternary system. It was also suggested that supersaturation of carbon in the melt increased as the temperature decreased even though the primary graphite particles existed. The supersaturation may cause the nucleation of the graphite particles just before the eutectic solidification.

Low temperature bainitic ferrite: Evidence of carbon super-saturation and tetragonality

Experimental evidence indicates that bainitic ferrite formed by transformation at low temperatures (200–350°C) contains quantities of carbon in solid solution far beyond those expected from para-equilibrium. A change in the conventional symmetry of the bainitic ferrite lattice from cubic to tetragonal explains the abnormal solid solubility detected. This carbon supersaturation was measured by atom probe tomography, and the tetragonality of the bainitic ferrite, was characterized by means of X-ray diffraction analysis and high resolution transmission electron microscopy.

A mixed-mode model for the ferrite-to-austenite transformation in a ferrite/pearlite microstructure

A concise semi-analytical mixed-mode model is proposed to describe the ferrite-to-austenite transformation kinetics. The initial microstructure for the model consists of a ferrite matrix with supersaturated austenite grains. The carbon supersaturation of austenite grains resulted from the rapid dissolution of pearlite colonies present in the initial microstructure. A similar approach to the one used for describing the ferrite growth kinetics (Bos and Sietsma, 2007) was used, employing a sharp interface between the phases. In comparison to ferrite growth, the carbon concentration profile of the growing austenite contains an additional parameter, which is the carbon concentration at the centre of the grain. This extra parameter is accounted for by assuming a parabolic carbon profile in the austenite. A comparison with a numerical solution of the differential equations shows that the developed semi-analytical mixed-mode model gives a good description of the ferrite-to-austenite transformation, significantly more accurate over the entire course of the transformation than the classical interface-controlled and diffusion-controlled models. Significant deviations from the numerical solution, as a result of the parabolic approximation, are limited to the initial stage of the transformation.

Effect of pressure on the growth of boron and nitrogen doped HFCVD diamond films on WC–Co substrate

Boron and nitrogen compounds are added in the acetone/hydrogen gas mixture to deposit hot filament chemical vapor deposition (HFCVD) diamond films on the cobalt cemented tungsten carbide (WC–Co) substrate under the pressure of 1–4 kPa. The as‐deposited diamond films are characterized by field emission scanning electron microscope (FESEM), atomic force microscopy (AFM), X‐ray diffraction (XRD) spectroscopy and Raman spectroscopy. The results reveal that the surface morphology, growth rate, structure and quality of the diamond films vary with the pressure and the type of the impurity addition. The diamond grains tend to develop into the nanometer scale with the decrease of the pressure. However, adding of boron or nitrogen impurities in the gas mixture will weaken the nanocrystallization effect by reducing the carbon supersaturation. Density functional theory (DFT) calculations indicate that co‐adsorption of B and N containing radicals can favor the adsorption of CH3 on diamond (100) surface. Thus, at low pressure of 1 kPa, large grained cubic (100) facet diamond rather than typical nanometer diamond is produced for B–N co‐addition gas mixture. The present results appear to be useful to efficiently synthesize high quality doped diamonds with desirable properties for mechanical application. Copyright © 2015 John Wiley &amp; Sons, Ltd.

“Colossal” interstitial supersaturation in delta ferrite in stainless steels—I. Low-temperature carburization

Low-temperature carburization has been successfully used to surface harden 17-7 precipitation-hardening (PH) and 2205 duplex stainless steels. After carburization, the delta ferrite grains in both alloys near the free surface show a uniform weak contrast under conventional transmission electron microscopy (TEM). Spatially resolved compositional analysis shows that these delta ferrite grains possess enormous carbon contents (as high as 18at.%) in solid solution, but structurally there is no detectable tetragonality (<5%) or evidence of carbide formation. Near the interface between the interstitially hardened layer and bulk material, weak-contrast plates with significant carbon concentrations were observed in ferrite grains in 17-7 PH stainless steel. A carbon-induced spinodal-like decomposition of delta ferrite to the nanometer-scale Cr-rich and Fe-rich alpha ferrite phases is observed. Carbon is enriched in Cr-rich ferrite due to the high affinity between C and Cr, which introduces lattice mismatch between the Cr-rich and Fe-rich regions. The weak contrast is believed to be the result of overlapping strain fields of these Cr-rich and Fe-rich phases. As the binding energies of carbon interstitials to dislocations in body-centered cubic Fe-based alloys are greater than the binding energy of C to Fe in possible carbides, segregation to dislocation cores is expected. The extremely high dislocation density we observe in high-resolution scanning TEM is consistent with the hypothesis that carbon segregation to dislocation cores effectively delays carbide precipitation and makes possible the “colossal” carbon supersaturation.

White Etching Crack Root Cause Investigations

White etching crack (WEC) failure is distinct to classical fatigue and driven by the composition of lubricants under special loading conditions; for example, slippage and electricity. The white etching area (WEA) within WEC contains carbon supersaturated ferrite (bcc-iron) and carbides, with a size of a few nanometers. This article presents investigations supporting the hypothesis that WEC processes start within a failure-free period by successive accumulation of a structural distortion. This can be measured by acoustic emission. Failure statistics show a steep ascent in the Weibull diagram (ß values beyond 1) leading to the assumption that WEC processes start unsuspicious, as one would see as a failure-free period, but imply a hidden subsurface accumulation of material defects. It is suggested and supported by the evidence presented within this article that WEC is neither related to the presence of nonmetallic inclusions nor related to other impurities in the steel. Instead, the failure is a sequence and accumulation of plastic deformations in the microstructure. Within the SAE 52100 material as discussed in this article, this accumulation is located in the microstructure around cementite, seen in a turn of hard magnetization toward soft magnetization proven by Barkhausen noise measurements. This decay is caused by the plastic deformation of such domains. Distortions in the vicinity of a cementite first would lead to carbon supersaturation by diffusion processes and later to a plastic deformation of the carbides. In the end, the complete distorted region will release the accumulated energy by downsizing the microstructure toward WEC.

Thermodynamic evaluation of the C–Cr–Si, C–Ti–Si, and C–Fe–Si systems for rapid solution growth of SiC

Solution growth of SiC is currently regarded as a promising process to produce high-quality SiC crystals. To date, Si–Cr, Si–Ti, and Fe–Si solvents have been used for rapid solution growth of SiC. However, optimization of the solvent system and composition is still needed to maximize the growth rate of high-quality SiC crystals. In this paper, to clarify the features of respective solvents from the viewpoint of the solubility of carbon, the C–Cr–Si, C–Ti–Si, and C–Fe–Si systems were evaluated by thermodynamic analysis. Phase relations in the respective ternary systems were investigated by the calculation of phase diagrams method. In addition, the solubility of carbon in Si–Cr, Si–Ti, and Fe–Si alloys at saturation with SiC was measured and its consistency with the estimated results was examined. Correlation of the experimental growth rate of SiC using Si–Cr, Si–Ti, and Fe–Si solvents was analyzed in terms of the supersaturation of carbon in each solution evaluated from the estimated temperature dependence of carbon solubility. Rate determining step for solution growth was presumed to be mass transfer in the liquid phase.

Enhanced Corrosion Resistance of 316L Stainless Steel Interstitially Hardened with Nitrogen or Carbon

Case hardening by traditional carburization or nitridation methods has long been recognized to produce wear resistant surfaces in steels. When these methods are applied to stainless steels, corrosion performance is degraded because of carbide and nitride formation. However, low temperature gas-phase and plasma interstitial hardening (IH) processes relying on paraequilibrium kinetics have been developed (1-5) that allow substantial amounts of carbon or nitrogen to be introduced into stainless steels without formation of carbides or nitrides. Paraequilibrium refers to the concept that diffusion kinetics of substitutional solutes, such as Cr and Ni, diffuse slower than interstitial solutes, such as carbon or nitrogen. Substitutional solutes are effectively immobile, whereas interstitial solutes can diffuse into the alloy to depths of 10-30 µm. Surface carbon or nitrogen concentrations on the order of 12 at% or greater can be obtained, resulting in a hardened surface. This modified case-hardened region has been referred to as S-phase or expanded austenite. The industrial implementation of the IH treatment with carbon (IH-C) or nitrogen (IH-N) is straightforward, is a non-line of sight process, maintains sample dimensions, and is relatively inexpensive. A growing body of work on IH-C treated stainless steel alloys shows that gas phase IH-C treatment improved localized corrosion, fatigue, and wear resistance of the treated materials compared to the untreated alloys (5-9).The origin of the hardening and the improved corrosion resistance for IH-C treated 316L stainless steels is the “colossal” supersaturation of interstitial carbon. The corrosion resistance of stainless steel involves a Cr2O3-rich passive film. In previous work, grazing incidence X-ray photoelectron spectroscopy (GI-XPS) was used to determine the composition and measure the thickness of these Cr-rich passive films developed during anodic polarization at selected potentials for both IH-C and non-treated 316L stainless steel samples. Since no Cl-was observed in the passive oxide film of both the IH-C and non-treated samples at any of the potentials examined, and the chemical composition was the same for IH-C and non-treated samples at each potential, we suggest that passive film breakdown is of chemo-mechanical origin [8,9].In this work, exploration into the nature of passive oxide film breakdown was extended to gas phase IH-N. The experimental work conducted follows that of IH-C in References 7 and 9. This includes determining the polarization behavior in 0.6 M NaCl solutions, performing potentiostatic experiments at selected potentials below the pitting potential with subsequent XPS, and impedance measurements on the electronic response of the oxide film. XPS will be used to determine the oxide film thickness and composition of the potentiostatically polarized samples. These results will then be compared to the results reported in References 7 and 9. Acknowledgements The authors gratefully acknowledge the Office of Naval Research and the Naval Research Laboratory for financial support of this work. References (1) Z.L. Zhang and T. Bell, Surf. Eng., 1,131 (1985).(2) P.C. Williams and S.C. Marx: U.S. Patent 6,093,303, July 25, 2000.(3) Y. Cao, F. Ernst, and G.M. Michal, Acta Mater., 51, 4171 (2003).(4) T. Christiansen and M.A.J. Somers, Surf. Eng., 21,445 (2005).(5) G.M. Michal, F. Ernst, H. Kahn, Y. Cao, F. Oba, N. Agarwal, and A.H. Heuer, Acta Mater., 54, 1597 (2006).(6) A.H. Heuer, H. Kahn, J. O'Donnell, G.M. Michal, R.J. Rayne, F.J. Martin, and P.M. Natishan, Electrochem. and Solid State Lett., 13 (12) C37-C39 (2010).(7) F.J. Martin, E.J. Lemieux, T.M. Newbauer, R.A. Bayles, P.M. Natishan, H. Kahn, G.M. Michal, F. Ernst, and A.H. Heuer, Electrochem. and Solid State Lett., 10 (12) C76-C78 (2007).(8) A.H. Heuer, H. Kahn, P.M. Natishan, F.J. Martin, and L.E. Cross, Electrochimica Acta, 58,157 (2011). (9) A.H. Heuer, H. Kahn, F. Ernst, G.M. Michal, D. Hovis, R.J. Rayne, F.J. Martin, and P.M. Natishan, Acta Materialia, 60, 716 (2012).

Colossal Carbon Supersaturation of Delta Ferrite in 17-7 PH Stainless Steel

Colossal Carbon Supersaturation of Delta Ferrite in 17-7 PH Stainless Steel