Korngrenzensegregation des Phosphors im Austenit und Einflu? von Kohlenstoff

Summary Equilibrium segregation of phosphorus at the grain boundaries of austenite has been studied in Fe-P-, Fe-0.16%C-P- and Fe-0.35% C-P-alloys. The samples were equilibrated at temperatures of 850–1200 ° C and analysed after rapid quenching using Auger-Electron Spectroscopy. The results in Fe-P-alloys may be evaluated according to the Langmuir-McLean equation. The following expression for the free energy of P-segregation in austenite was obtained: -gDG _p ^o = –21300 –25.8* T (J/mole). For the Fe-P-C-alloys the carbon concentration increases at the grain boundaries with increasing carbon concentration in the bulk and phosphorus segregation decreases. The observed carbon segregation does not show any dependence on austenitising temperature and the temperature dependence on P-segregation is also insignificant. Reasons for this behaviour are discussed.     

Grain boundary segregation of P and micro-structural changes in 12% Cr-steels

Phosphorus occurs as impurity in 12% CrMoV-steels and tends to enrich at grain boundaries. The phosphorus grain boundary segregation in a laboratory cast of the steel X 20 CrMoV 12 1 with a relatively high phosphorus content (0.057% P) was studied by Auger electron spectroscopy and the segregation kinetics and equilibria were described. Using the results phosphorus segregation could be predicted and compared to measured phosphorus concentrations in long-term annealed steels. Investigations by transmission electron microscopy showed recrystallisation and an agglomeration of carbides with aging time and the occurrence of new phases (Laves-phase Fe₂ Mo). All these processes lead first to an increase in toughness as established in notch-impact fracture tests for the laboratory cast, however, after long-term use a reduction in toughness is expected.     

AES study of underclad cracking

The present work deals with the occurrence of underclad cracks in three types of steels differing in the contents of the alloying elements (Cr, Ni, Mn) and in the impurities level (As, Sb, Sn). The presence of the intercrystalline fracture typical for the underclad cracks was proved to be dependent on the temperature and on the ageing time. It follows from the results of Auger electron spectroscopy, that the sulphur and phosphorus segregations accompanied by a higher number of carbide (possibly sulphide) particles cause a decrease of grain boundary strength. Segregations of other impurity elements were not detected at grain boundaries.     

Phosphorus as sintering activator in powder metallurgical steels: characterization of the distribution and its technological impact

Powder metallurgy is a highly developed method of manufacturing reliable ferrous parts. The main processing steps in a powder metallurgical line are pressing and sintering. Sintering can be strongly enhanced by the formation of a liquid phase during the sintering process when using phosphorus as sintering activator. In this work the distribution (effect) of phosphorus was investigated by means of secondary ion mass spectrometry (SIMS) supported by Auger electron spectroscopy (AES) and electron probe micro analysis (EPMA). To verify the influence of the process conditions (phosphorus content, sintering atmosphere, time) on the mechanical properties, additional measurements of the microstructure (pore shape) and of impact energy were performed. Analysis of fracture surfaces was performed by means of scanning electron microscopy (SEM). The concentration of phosphorus differs in the samples from 0 to 1% (w/w). Samples with higher phosphorus concentrations (1% (w/w) and above) are also measurable by EPMA, whereas the distributions of P at technically relevant concentrations and the distribution of possible impurities are only detectable (visible) by means of SIMS. The influence of the sintering time on the phosphorus distribution will be demonstrated. In addition the grain boundary segregation of P was measured by AES at the surface of in-situ broken samples. It will be shown that the distribution of phosphorus depends also on the concentration of carbon in the samples.     

SAM investigations of temperature programmed surface segregation of impurities in α-iron

The time development of the surface concentration of impurities such as sulphur and nitrogen in high-purity polycrystalline α-iron samples has been investigated by means of Auger Electron Spectroscopy (AES) and Scanning Auger Microscopy (SAM) during linearly increasing the temperature from about 100 to 850°C. The subsequent segregation of N and S revealed strong differences in the segregation kinetics of these elements — especially with respect to the contribution of the dominating transport mechanism (grain boundary diffusion and bulk diffusion). Calculating the effective diffusion coefficient of S from two subsequent segregation runs, strongly differing values were found. An interpretation of the change in the diffusion properties of sulphur is given based on SIMS (Secondary Ion Mass Spectroscopy) investigations performed before and after the thermal treatment.     

Thermoelectric properties of single- and polycrystalline RuGa3

The thermoelectric properties of the intermetallic semiconductor RuGa₃ are investigated. Large single crystals were grown to study intrinsic properties. To investigate the influence of grain boundaries in this system, the single crystals were ground to powder and densified using spark plasma sintering treatment. The initial chemical composition is maintained with the introduction of grain boundaries. Electrical resistivity data show semiconducting behavior for single- and polycrystalline samples. The high thermal conductivity (>500 W K⁻¹ m⁻¹) obtained for single crystals at low temperatures is reduced by a factor of 10 in the polycrystalline specimen. The thermopower shows a change between n-type and p-type behavior with a sharp minimum of about −700 μV K⁻¹ at 38 K for single crystals, which is completely suppressed by the introduction of grain boundaries. A comparison with RuIn₃ shows the potential of RuGa₃ as a thermoelectric material in its single- and polycrystalline form.     

Physicochemical properties of rare earth doped ceria Ce0.9Ln0.1O1.95 (Ln = Nd,Sm, Gd) as an electrolyte material for IT-SOFC/SOEC

Nanosized crystallites of rare earth doped ceria Ce_0.9Ln_0.1O_1.95 (Ln = Nd, Sm, Gd) a promising electrolyte material for Intermediate Temperature – Solid Oxide Fuel Cells/electrolysis cells have been synthesized by standard ceramic route. Detection of impurities in the samples was done by FTIR spectroscopy. X-ray diffraction studies were used for the determination of phase purity, crystal structure and average crystallite size of the samples. Kinetics involved in phase formation has been discussed. Raman study showed a major band around 465 cm⁻¹ in all the samples, which is attributed to the cubic fluorite structure of ceria. It was also found that for samples Ce_0.9Ln_0.1O_1.95 (Ln = Nd, Sm, Gd) the frequency of F_2g shifts to lower value. Electrochemical impedance spectroscopy has been used to measure the ionic conductivity of the samples at elevated temperatures. The Gd doped sample showed the highest grain boundary and total conductivity in comparison to Sm and Nd doped sample. Bulk thermal expansion behavior, sintered densities and micro structural features of the samples have also been studied.     

Quantification of grain boundary equilibrium segregation by NanoSIMS analysis of bulk samples

A technique for the quantification of equilibrium grain boundary segregation by high resolution secondary ion mass spectroscopy (NanoSIMS) on simple metallographically polished surfaces has been demonstrated for the model system of sulphur segregation to nickel grain boundaries. Samples of nickel containing 5.4 wt ppm of sulphur were annealed at different temperatures to achieve different equilibrium sulphur grain boundary concentrations, ranging from less than 1% to about 50% of a monolayer. Quantification was carried out from sulphur concentration profiles acquired across about 20 grain boundaries in each sample. An internal standard (nickel containing a known concentration of sulphur in solid solution) was used for calibration. It is found that, depending on the annealing temperature, the average grain boundary sulphur concentration ranges from 0.9 to 25.8 ng cm^–2 (or 1.7 10¹³ to 4.8 10¹⁴ atoms cm^–2), i.e. ~0.015 to ~0.43 monolayer. Thermodynamic analysis gives a segregation free energy of ?97.8 kJ mol^–1 and a grain boundary sulphur concentration at saturation of 26.7 ng cm^–2 (or 5.0×10¹⁴ atoms cm^–2), i.e. ~0.44 monolayer, in good agreement with previous measurements on this system. The limit of detection of the technique is shown to be as low as 0.24 ng cm^–2 (or 4.5×10¹² atoms cm^–2), i.e. ~0.004 monolayer, with a counting time of only 10 min. Copyright © 2012 John Wiley & Sons, Ltd.     

Surface analytical study of hydrothermally treated zirconia ceramics

Summary The hydrothermal instability of yttria-stabilized tetragonal zirconia polycrystals (Y-TZP) imposes serious problems on their range of applications as ceramic materials. It is well known that a generally uniform distribution of yttrium cations prevents the transformation of the metastable tetragonal into the monoclinic crystallographic modification. The induced crack propagation is widely considered to be a major source of mechanical failure in zirconia ceramics. The segregation was investigated of yttrium and other cations on the surface of hydrothermally treated ceramic samples with angle resolved XPS (X-ray photoelectron spectroscopy). The combination of this method with the high lateral resolution of SAM (scanning Auger microprobe) yields three dimensional distribution profiles of Y³⁺, Al³⁺ and Si⁴⁺. The relative content of the two crystallographic modifications of zirconia at the surface has been determined with XRD (X-ray diffraction). Additional Raman spectroscopic measurements partially confirm these results but suffer from quantification problems. A possible mechanism for the enrichment of yttrium in the surface layer and its promotive function for the phase transformation of zirconia is proposed. Preliminary results from experiments with temperature variable XPS measurements also indicate the formation of either yttrium- or zirconium hydroxide which are both suspected to contribute to crack propagation along the grain boundaries.     

Surface oxide development on aluminum alloy 6063 during heat treatment

We report on the influence of oxygen partial pressure for the development of surface oxides covering the industrial aluminum alloy standard 6063 at temperatures ranging from room temperature to 500° C. Using an array of synchrotron-based techniques, we followed the change in oxide thickness, chemical composition, and the lateral distribution of alloying elements. The impact of the oxygen chemical potential is most visible at high temperatures where the oxide composition changes from mostly Al based to mostly Mg based. This is in stark contrast to the ultra-high vacuum (UHV) conditions where only a partial compositional transition is observed. The microscopy data demonstrate that in the UHV case, Mg segregation onto the surface occurs firstly at grain boundaries at 300° C and secondly at sites over the entire surface at 400° C. Further, the initial oxide thickness is 45 Å, as determined by XPS and XRR, decreases in all observed cases after heating to 300° C. At higher temperatures, however, the oxygen partial pressure highly influences the resulting oxide thickness as evident from our X-ray reflectivity data.     

Specific properties of fine SnO<Subscript>2</Subscript> powders connected with surface segregation

The effect of surface segregation in Sb- and In-doped SnO₂ fine-grained powders has been analyzed in comparison with single-crystalline samples. The kinetics and thermodynamics of the Sb and In segregation processes were studied as a function of annealing temperature by X-ray photoelectron spectroscopy (XPS) after annealing in an oxygen-containing atmosphere. Significant differences between diffusion and segregation were revealed for doped powders and single crystals, obviously because of simultaneous diffusion and particle-growth processes proceeding during annealing of powders. For doped single crystals the thermodynamic equilibrium is approached after 24 h annealing above 850 °C and at 1000 °C for Sb and In, respectively. Higher effective activation energies of diffusion are observed for doped powders and the thermodynamic equilibrium is not achieved under technologically relevant annealing conditions. On the basis of dopant profile measurements anomalies in the electrical resistivity at 300 °C of Sb-doped SnO₂ powders annealed at 700 and 900 °C were attributed to an Sb-depleted zone formed beneath the segregated surface during the kinetic regime. To achieve optimum resistivity behavior for commercial application, inhomogeneous doping of powders must be avoided by appropriate preparation steps.     

Grain boundary segregations in nanocrystalline alloys

The dependence of grain boundary segregations and alloy composition in the bulk of the grain on the grain size was considered. At moderate temperatures, there is a critical grain size for which the integrated degree of alloy decomposition reaches its maximum; this size increases as the average (over the sample) impurity concentration decreases. In complex alloys, the type of the component that segregates on the boundaries depends on the grain size. For segregation kinetics, an approximate analytical solution was obtained based on the assumption that the grain boundary is an ideal drain for the impurity. The equilibrium grain size exists below the critical temperature and its temperature dependence is nonmonotonic in the general case. A segregation mechanism of stabilization of nonequilibrium grain boundaries in nanocrystalline alloys obtained by severe plastic deformation was suggested.     

Fast thermal desorption spectroscopy study of H/D isotopic exchange reaction in polycrystalline ice near its melting point

Using fast thermal desorption spectroscopy, a novel technique developed in our laboratory, we investigated the kinetics of HD isotopic exchange in 3 microm thick polycrystalline H2O ice films containing D2O layers at thicknesses ranging from 10 to 300 nm at a temperature of -2.0+/-1.5 degrees C. According to our results over the duration of a typical fast thermal desorption experiment (3-4 ms), the isotopic exchange is confined to a 50+/-10 nm wide reaction zone located at the boundary between polycrystalline H2O and D2O ice. Combining these data with a theoretical analysis of the diffusion in polycrystalline medium, we establish the range of possible values for water self-diffusion coefficients and the grain boundary widths characteristic of our ice samples. Our analysis shows that for the grain boundary width on the order of a few nanometers, the diffusivity of D2O along the grain boundaries must be at least two orders of magnitude lower than that in bulk water at the same temperature. Based on these results, we argue that, in the limit of low concentrations of impurities, polycrystalline ice does not undergo grain boundary premelting at temperatures up to -2 degrees C.     

Quantitative Analyse von Bruchfl?chen mit AES bei der Untersuchung der Korngrenzensegregation von Schwefel in Nickel

Summary In a study of the grain boundary segregation of sulphur in nickel, the variation of sulphur enrichment on single boundary planes and on several boundaries as well as the distribution of sulphur on the two fracture surfaces of one specimen was determined. Two quantification procedures were applied, one using the APPH-ratio in the derivative spectrum and the other using the peak-to-background (P/B)-ratio in the E*[N(E)]-spectrum. Angle dependent measurements at grain boundary planes and at polished nickel surfaces show a lower sensitivity of the P/B-ratio to geometric and crystallographic effects, and the restricted region of applicability of the P/B ratio. The parallel use of both procedures for an optimum quantification, especially in studies on the structural dependence of grain boundary segregation, is proposed.     

Oxygen diffusion in nanocrystalline yttria-stabilized zirconia: the effect of grain boundaries

The transport of oxygen in dense samples of yttria-stabilized zirconia (YSZ), of average grain size d approximately 50 nm, has been studied by means of 18O/16O exchange annealing and secondary ion mass spectrometry (SIMS). Oxygen diffusion coefficients (D*) and oxygen surface exchange coefficients (k*) were measured for temperatures 673相似文献   

Segregation in zirconia: equilibrium versus non‐equilibrium segregation

The present work considers the differences between the following two phenomena: equilibrium segregation, which is a thermodynamic phenomenon where the driving force is excess interfacial energy; and non‐equilibrium segregation, which can be produced by phenomenological shocks applied to the solid surface. Fully stabilized cubic yttria‐stabilized zirconia (YSZ) is used as the exemplar for these considerations. Equilibrium segregation in YSZ results in enrichment of the surface and near‐surface layers in constituent elements, typically yttria and impurities. This segregation is an intrinsic material property and has an impact on the performance of zirconia at elevated temperatures. On the other hand, non‐equilibrium segregation leads to a complex distribution of properties (structure and concentration gradients) that are determined by the experimental procedures used rather than being a material property. However, such non‐equilibrium segregation can result in localized structural changes. The present paper also considers the effect of the gas phase on surface properties of metal oxides, including YSZ, and the surface dynamics of YSZ at temperatures below that required to reach equilibrium. Copyright © 2005 John Wiley & Sons, Ltd.     

TOF‐SIMS studies of yttria‐stabilised zirconia

The surface of an as‐polished and an as‐sintered yttria‐stabilised zirconia pellet was analysed with XPS and TOF‐SIMS (depth profiling and imaging) in order to study the distribution of impurities. The polished sample was slightly contaminated with Na, K, Mg and Ca. The sintered sample showed a thin surface film of segregated species, especially Na, Si and Al. Below the surface film, it was found that the grain boundaries were filled with impurities. The chemical compositions of the as‐polished and as‐sintered surfaces are very different and the surface state should be considered when performing electrochemical measurements. Copyright © 2006 John Wiley & Sons, Ltd.     

Competitive segregation of Si and P on Fe96.5Si3.5 (100) and (110)

Annealing an Fe_96.5Si_3.5 (100)/(110) bicrystal, containing 90 ppm P, leads immediately to a strong segregation of silicon. The Si atoms, however, desegregate subsequently and are displaced by P, whose segregation enthalpy is larger than that of silicon. The corresponding surface structures formed on both faces have been studied using complementary methods: Scanning tunneling microscopy (STM) to obtain atomically resolved geometrical information and Auger electron spectroscopy (AES) for the determination of the surface composition. Si substitutes surface Fe atoms on both faces and forms ordered surface alloys, whereas P occupies hollow sites on the surface. Si and P form c(2 × 2) superstructures on the (100) surface, whereby each segregated phosphorus atom blocks in the average one silicon segregation site. The (110) surface, on the other hand, is characterized by a c(1 × 3) Si superstructure. Due to the anisotropy of this surface the P/Si exchange proceeds by the formation of silicon coverage decreasing domain boundaries within the silicon structure, which are simultaneously occupied by P atoms. Furthermore the comparison of the AES and STM derived phosphorus coverages indicates a P multilayer segregation on the (110) surface.