Local Carbon Concentration Research Articles

Concentration-Dependent Carbon Diffusivity in Austenite

The diffusion coefficient of carbon in austenite depends on the local carbon concentration. This concentration dependence is particularly noticeable during low-temperature “paraequilibrium” carburization. A critical review of the extensive literature on this topic reveals that an early paper by Asimow provides an excellent description of this substantial concentration dependence. The present analysis suggests that the marked concentration dependence of carbon diffusivity is most likely due to interstitial carbon decreasing the activation energy for carbon jumps from one interstitial site to its neighbor.

Spatial scale of land‐use impacts on riverine drinking source water quality

Key Points Microbial water quality most closely related to local landuse Organic carbon concentrations only associated with entire watershed landuse Work supports watershed scale management to protect drinking water sources

Effect of Cooling Rates on the Second‐Phase Precipitation and Proeutectoid Phase Transformation of a Nb–Ti Microalloyed Steel Slab

AbstractDuring the continuous casting of low‐carbon Nb–Ti microalloyed steel, control of the slab surface microstructure and the behavior of the second‐phase precipitation are significantly influenced by the cooling rate. Through confocal laser scanning microscopy, the effect of the cooling rate on the behavior of ferrite precipitation both at the grain boundary and within the austenite was observed in situ and analyzed. The relationship between the cooling rate and precipitation of the microalloying elements on the slab surface microstructure was further analyzed by transmission electron microscopy. The results showed that the effect of microalloying element precipitation on proeutectoid ferrite phase transformation is mainly manifested in two aspects: (i) the carbonitrides of microalloying elements act as inoculant particles to promote nucleation of the proeutectoid ferrite and (ii) the carbon near the grain boundary is depleted when the microalloying elements precipitate into carbonitrides, inducing a decrease in the local carbon concentration and promoting ferrite precipitation.

A Study of Dual Phase Steel Damage Evolution with Microstructure

A detailed analysis of the evolution of industrial Dual Phase (DP) steel microstructures is carried out as a function of various annealing and tempering conditions. Advanced characterization techniques such as Parallel Electron Energy Loss Spectroscopy (PEELS) in the TEM and high spatial resolution Secondary Ion Mass Spectrometry (NanoSIMS) are employed in order to provide qualitative and quantitative measurements of local carbon concentration in the martensite. For certain annealing and tempering conditions, it is observed that local variations in carbon levels have occurred inside the individual martensite islands. These carbon variations strongly influence the damage behaviour of the steel. During tensile tests, a clear dependence of the damage mode on the local martensite carbon content is observed. Better knowledge of the relationship between the microstructure evolution at the sub-grain level and the damage behaviour can facilitate the design of DP steels with improved damage resistance.

In situ synchrotron study on the interplay between martensite formation, texture evolution and load partitioning in low-alloyed TRIP steels

Abstract We have studied the micromechanical behaviour of two low-alloyed multiphase TRIP steels with different aluminium contents by performing in situ high-energy X-ray diffraction experiments at a synchrotron source under increasing tensile stress levels. A detailed analysis of the two-dimensional diffraction data has allowed us to unravel the interplay between the martensite formation, the texture evolution and the load partitioning, and to correlate the observed behaviour to the macroscopic response of the material. The high aluminium content TRIP steel grade presents a higher volume fraction of retained austenite at room temperature that transforms more gradually into martensite under deformation, providing a larger uniform elongation. The comparison between the observed transformation behaviour and the texture evolution indicates that the 〈1 0 0〉 component along the loading direction corresponds to a low critical stress for the transformation. The evolution of the elastic strains revealed the occurrence of a significant load partitioning before reaching the macroscopic yield strength, which becomes more pronounced in the plastic regime due to the progressive yielding of the different grains in the polycrystalline material. This opens the door to tailor the austenite stability by altering the distribution in grain size, local carbon content, and grain orientation in order to produce the optimal load partitioning and work hardening for improved combinations of strength and formability in low-alloyed TRIP steels.

Silicide-induced multi-wall carbon nanotube growth on silicon nanowires

A novel multi-walled carbon nanotube (MWNT) growth process is reported based oncarbon incorporation in a nickel catalyst layer deposited via plasma-enhancedatomic layer deposition (PEALD) on silicon nanowires and silicon wafer substrates.As-deposited PEALD Ni films containing relatively high amounts of carbon (>18 at.%) were observed to promote the growth of MWNTs upon post-deposition rapid thermalannealing. For these films the carbon originated from the ALD precursor ligand andMWNT growth occurred in the absence of a vapor-phase carbon feedstock. MWNT growthrelied on the formation of nickel silicide at the PEALD Ni/Si interface whichincreased the local carbon concentration in the Ni film sufficiently to promotecarbon saturation/precipitation at Ni catalyst grains and nucleate MWNT growth.Similar MWNT growth from annealed PEALD Ni films was not observed onSiO2-coated Si wafer substrates, consistent with the role of silicidation in the observedNi-catalyzed MWNT growth on Si. This MWNT growth mode requires neither thecatalytic decomposition of a gaseous hydrocarbon source nor the high-temperaturepyrolysis of metallocene materials and purposely avoids a catalyst diffusion barrier at the Sisubstrate, commonly used in MWNT growth processes on Si.

Parametric study of multiphase TRIP steels undergoing cyclic loading

The behavior of multiphase steels assisted by transformation-induced plasticity (TRIP steels) undergoing low cycle, fully-reversed strain-controlled deformations is studied by means of numerical simulations based on micromechanical models. The ferritic phase is simulated using a single-crystal elasto-plasticity model for BCC crystals whereas the austenitic phase, which may transform into martensite, is simulated by a crystallographic phase transformation model coupled to a single-crystal elasto-plasticity model for FCC crystals. The influence of the TRIP mechanism on the overall behavior of the steel is investigated for selected variations of microstructural properties such as phase morphology, local carbon concentration in the austenite and austenitic grain size. The results of the simulations show a strong initial hardening associated to the martensitic transformation in accordance with experimental results. The simulations indicate an asymmetric hardening behavior under extension and contraction, particularly for large austenitic volume fractions and lower carbon concentrations.

Effects of vacancy-solute clusters on diffusivity in metastable Fe-C alloys

Diffusivity in defected crystals depends strongly on the interactions among vacancies and interstitials. Here we present atomistic analyses of point-defect cluster (PDC) concentrations and their kinetic barriers to diffusion in ferritic or body-centered-cubic (bcc) iron supersaturated with carbon. Among all possible point-defect species, only monovacancies, divacancies, and the PDC containing one vacancy and two carbon atoms are found to be statistically abundant. We find that the migration barriers of these vacancy-carbon PDCs are sufficiently high compared to that of monovacancies and divacancies. This leads to decreased self-diffusivity in bcc Fe with increasing carbon content for any given vacancy concentration, which becomes negligible when the local interstitial carbon concentration approaches twice that of free vacancies. These results contrast with trends observed in fcc Fe and provide a plausible explanation for the experimentally observed carbon dependence of volume diffusion-mediated creep in ferritic (bcc) Fe-C alloys. Moreover, this approach represents a general framework to predict self-diffusivity in alloys comprising a spectrum of point-defect clusters based on an energy-landscape survey of local energy minima (formation energies governing concentrations) and saddle points (activation barriers governing mobility).

The effect of aluminium and phosphorus on the stability of individual austenite grains in TRIP steels

We have performed in situ synchrotron X-ray diffraction experiments to assess the influence of aluminium and phosphorus on the austenite stability in low-alloyed transformation-induced plasticity steels during the high-temperature bainitic holding and the subsequent martensitic transformation during cooling to temperatures between room temperature and 100 K. Although the addition of aluminium increases the chemical driving force for the formation of bainitic ferrite plates significantly, the phosphorus exerts a larger influence on the bainitic transformation kinetics. Consequently, the addition of phosphorus leads to a higher degree of carbon enrichment and a narrower grain volume distribution of the metastable austenite. The stability of the individual austenite grains with respect to their martensitic transformation depends on both the local carbon content and the grain volume for austenite grains smaller than 20 μm 3. The presence of aluminium and phosphorus further stabilizes the austenite grains.

Absolute intensity calibration of the Wendelstein 7-X high efficiency extreme ultraviolet overview spectrometer system

The new high effiency extreme ultraviolet overview spectrometer (HEXOS) system for the stellarator Wendelstein 7-X is now mounted for testing and adjustment at the tokamak experiment for technology oriented research (TEXTOR). One part of the testing phase was the intensity calibration of the two double spectrometers which in total cover a spectral range from 2.5 to 160.0 nm with overlap. This work presents the current intensity calibration curves for HEXOS and describes the method of calibration. The calibration was implemented with calibrated lines of a hollow cathode light source and the branching ratio technique. The hollow cathode light source provides calibrated lines from 16 up to 147 nm. We could extend the calibrated region in the spectrometers down to 2.8 nm by using the branching line pairs emitted by an uncalibrated pinch extreme ultraviolet light source as well as emission lines from boron and carbon in TEXTOR plasmas. In total HEXOS is calibrated from 2.8 up to 147 nm, which covers most of the observable wavelength region. The approximate density of carbon in the range of the minor radius from 18 to 35 cm in a TEXTOR plasma determined by simulating calibrated vacuum ultraviolet emission lines with a transport code was 5.5x10(17) m(-3) which corresponds to a local carbon concentration of 2%.

Determination of local carbon content in austenite during intercritical annealing of dual phase steels by PEELS analysis

Parallel electron energy loss spectroscopy has allowed to analyse and quantify local variations in the carbon concentration of austenite islands transformed during the intercritical annealing treatment of commercial dual-phase steels. These changes in the carbon content of different austenite regions are responsible for the different volume fractions of tempered martensite, martensite and retained austenite obtained after intercritical annealing and overaging treatment. This technique reveals how carbon distribution in austenite evolves as the transformation process advances.

Microwave Emission from Spinning Dust in Circumstellar Disks

In the high-density environments of circumstellar disks dust grains are expected to grow to large sizes by coagulation. Somewhat unexpectedly, recent near-IR observations of PAH features from disks around Herbig Ae/Be stars demonstrate that a substantial amount of dust mass in the surface layers of these disks (up to several tens of percent of the local carbon content) can be locked up in small nanometer-sized particles. We investigate the possibility of detecting the electric dipole emission produced by these nanoparticles (sizes ~1 nm) as they spin at thermal rates (tens of GHz) in cold gas. We show that such emission peaks in the microwave range and dominates over the thermal disk emission at ν 50 GHz typically by a factor of several if 5% of the total carbon abundance is locked up in nanoparticles. We test the sensitivity of this prediction to various stellar and disk parameters and show that if the potential contamination of the spinning dust component by the free-free and/or synchrotron emission can be removed, then the best chances of detecting this emission would be in disks with small opacities, having SEDs with steep submillimeter slopes (which minimize thermal disk emission at GHz frequencies). Detection of the spinning dust emission would provide important evidence for the existence, properties, and origin of the population of small dust particles in protoplanetary disks, with possible ramifications for planet formation.

Solid-state phase transformations involving solute partitioning: modeling and measuring on the level of individual grains

A simplified grain growth model is presented for the transition from non-overlapping to overlapping diffusion fields of growing neighboring grains during partitioning solid-state transformations in polycrystalline materials. The model is based on unique observations on the austenite decomposition into ferrite and pearlite in medium-carbon steel with the three-dimensional X-ray diffraction microscope. The model explains three types of observed pro-eutectoid ferrite grain growth and austenite grain decomposition, and the indirectly observed carbon exchange between decomposing austenite grains. A direct comparison of the model and the experiment at the level of individual grains shows that the growth of ferrite grains is strongly related to the local carbon concentration and the local density of nuclei. Unusual observations of a non-stationary austenite grain size prior to the transformation, and oscillatory ferrite growth are reported.

Study of the carbon concentration distribution in iron and titanium after low-temperature carbon ion implantation

Carbon ion distribution profiles after implantation in iron and titanium were extensively investigated using Rutherford Backscattering Spectrometry (RBS) and Elastic Recoil Detection Analysis (ERDA). Ion energy and fluence were varied over large ranges using target temperatures of less than −70°C. In both systems a retention of a gaussian-like implantation profile was found up to very high fluences. Unexpected high values of local carbon concentration were obtained for high fluences. Results were compared to Monte Carlo simulations using the dynamical code TRIDYN. Additionally the influence of subsequent annealing on the carbon distribution was studied. The diffusive behaviour was found to be very different in both systems. The iron–carbon system shows strong rearrangement of carbon atoms and tries to achieve local concentrations of 25 at.% upon thermal treatment. In the corresponding temperature regime no significant carbon diffusion in titanium takes place.

Decomposition of hypo‐eutectoid Fe‐C austenites; a numerical diffusion model

A numerical model is presented which describes the growth rate of ferrite during the decomposition of austenite in Fe‐C alloys. The growth rate is modelled assuming carbon diffusion in austenite as the rate determining mechanism. The effect of the definition of the diffusion coefficient of carbon in austenite on the growth rate is shown. The model is used to examine the effects of initial carbon concentration, the local carbon concentration in austenite at the interface and the initial austenite grain size on the growth rate. Good agreement between theoretical results and both new and existing experimental data was observed.

Effect of graphitic carbon films on diamond nucleation by microwave-plasma-enhanced chemical-vapor deposition

Diamond nucleation on very smooth (100) silicon substrates coated with thin films of a colloidal graphite suspension was investigated with a microwave-plasma-enhanced chemical-vapor-deposition system. Nucleation densities of the order of 106 cm−2 were obtained by coating the substrates with carbon films of thicknesses less than 1 μm. However, very low nucleation densities were obtained with carbon film thicknesses greater than 1 μm. The effect of the carbon film thickness on diamond nucleation was examined by measuring the etching rate of carbon films exposed to a hydrogen plasma and was further interpreted on the basis of scanning electron microscopy and Raman spectroscopy results. Etching of the original carbon may lead to the formation of a thin residual carbon film when the initial film thickness is less than a critical value. Results demonstrated that the high nucleation densities of good quality cubo-octahedral diamond crystals obtained with relatively thin carbon films were primarily due to the formation of a porous ultrathin residual carbon film. The critical initial film thickness was a function of the plasma etching and deposition rates of carbon which, in turn, affected the effective local carbon concentration. Thick carbon films yielded insignificant nucleation densities and poor quality diamond because of the high local carbon content resulting from the partial etching of carbon and the increased carbon concentration in the plasma. The local carbon concentration and the residual carbon film are the proposed principal factors for the obtained high diamond nucleation densities on unscratched silicon substrates.

Interaction of hydrogen with carbidic carbon on Ni(100)

The interaction of hydrogen with carbidic carbon deposited on Ni(100) by decomposing methane has been studied by isothermal hydrogenation experiments and by temperature-programmed hydrogen desorption (H 2−TPD). The hydrogenation experiments were made in the hydrogen-pressure range 0.5–10 Torr and the temperature range 450–550 K. The hydrogenation curves indicate the presence of carbon states with different reactivities. A kinetic model with carbon-state reactivities, which depend on the local carbon concentration, is compatible with the hydrogenation results. H 2−TPD spectra were recorded for carbon coverages in the range 0–0.34 ML. Application of a simple kinetic expression shows that the “effective” hydrogen binding energy of the model depends on both the carbon and the hydrogen coverage.

Interaction of hydrogen with carbidic carbon on Ni(100)

The interaction of hydrogen with carbidic carbon deposited on Ni(100) by decomposing methane has been studied by isothermal hydrogenation experiments and by temperature-programmed hydrogen desorption (H2−TPD). The hydrogenation experiments were made in the hydrogen-pressure range 0.5–10 Torr and the temperature range 450–550 K. The hydrogenation curves indicate the presence of carbon states with different reactivities. A kinetic model with carbon-state reactivities, which depend on the local carbon concentration, is compatible with the hydrogenation results. H2−TPD spectra were recorded for carbon coverages in the range 0–0.34 ML. Application of a simple kinetic expression shows that the “effective” hydrogen binding energy of the model depends on both the carbon and the hydrogen coverage.