Determination of lattice and grain boundary diffusion coefficients in protective alumina scales on high temperature alloys using SEM,TEM and SIMS

Grain and grain boundary diffusion coefficients in alumina scales on FeCrAl-based ODS alloys have been determined. The boundary diffusion-coefficients have been derived by combining gravimetrically determined growth rate data with SEM and TEM analyses of the oxide scale microstructure. The diffusion coefficients determined have been used as input parameters for a computer model describing the oxygen isotope exchange between grain and grain boundary in the alumina scale which forms during a two-stage oxidation using (18)O-tracers. This comparison of the calculated tracer profiles with profiles determined experimentally by SIMS allows the estimation of the lattice diffusion coefficient of oxygen in the alumina scale.     

TEM and SNMS studies of protective alumina scales on NiCrAlY-alloys

The effect of Si addition on the oxidation behaviour of NiCrAlY alloys in the temperature range 950 bis 1100° C has been investigated. During isothermal oxidation oxide growth rates were practically independent of the Si-content. However during cyclic oxidation Si additions were beneficial. Si additions of 1–2^m/₀ appeared to shift the onset for spallation to longer times. It was found that Si stabilizes the -phase and probably it suppresses the formation of metastable Al₂O₃ modifications.     

TEM and SNMS studies of protective alumina scales on NiCrAlY-alloys

The effect of Si addition on the oxidation behaviour of NiCrAlY alloys in the temperature range 950 bis 1100 degrees C has been investigated. During isothermal oxidation oxide growth rates were practically independent of the Si-content. However during cyclic oxidation Si additions were beneficial. Si additions of 1-2(m)/(0) appeared to shift the onset for spallation to longer times. It was found that Si stabilizes the beta-phase and probably it suppresses the formation of metastable Al(2)O(3) modifications.     

Analysis and modelling of transport processes in alumina scales on high temperature alloys

Summary The oxidation behaviour of a number of FeCrAl-based oxide dispersion strengthened (ODS) alloys of the type MA 956 was investigated at temperatures between 1100°C and 1200°C. The main emphasis was placed on the effect of alloy yttria content on scale composition and structure. Studies of three model alloys with various alloy yttria contents in the range 0.02–0.7% showed an increase in the oxide growth rate with increasing yttria content. Alumina scales on alloys with 0.17% and 0.7% yttria grow almost exclusively by oxygen diffusion, whereas the scale on the alloy with 0.02% yttria exhibits a significant contribution from cation diffusion. By using SIMS depth-profiling and SIMS-imaging it was found that the oxygen diffuses through the alumina scale via oxide grain boundaries. TEM-studies revealed that the yttria is incorporated in the scale grain boundaries as precipitations and as a segregation layer. This behaviour of the yttrium is believed to be responsible for its influence on scale growth mechanisms. A mathematical model has been developed which describes the alumina scale growth by oxygen diffusion through grain boundaries. The model accounts for the observed increase in aluminium oxide grain size in scale growth direction.     

Stress measurement in alumina scales on high temperature alloys using X-ray stress evaluation and laser Raman spectroscopy

The effect of silicon and titanium on the spallation resistance of alumina scales grown on NiCrAlY-type alloys has been investigated using model alloys with different additions of Si or Ti. For this purpose cyclic oxidation experiments have been carried out at temperatures between 950 and 1100 degrees C. After various times stresses in selected Si-doped samples have been determined by X-ray stress evaluation (XSE) at ambient temperature. The compressive stresses in the scales have been found to increase with an increasing oxidation time tending to become constant for long times. The development of stress is affected by the presence of Si. Laser Raman spectroscopy (LRS) has been calibrated for strain measurement using XSE results. Then LRS has been applied for strain measurement at higher temperatures.     

Stress measurement in alumina scales on high temperature alloys using X-ray stress evaluation and laser Raman spectroscopy

The effect of silicon and titanium on the spallation resistance of alumina scales grown on NiCrAlY-type alloys has been investigated using model alloys with different additions of Si or Ti. For this purpose cyclic oxidation experiments have been carried out at temperatures between 950 and 1100° C. After various times stresses in selected Si-doped samples have been determined by X-ray stress evaluation (XSE) at ambient temperature. The compressive stresses in the scales have been found to increase with an increasing oxidation time tending to become constant for long times. The development of stress is affected by the presence of Si. Laser Raman spectroscopy (LRS) has been calibrated for strain measurement using XSE results. Then LRS has been applied for strain measurement at higher temperatures.     

SIMS investigations on the growth mechanisms of protective chromia and alumina surface scales

The oxidation behaviour of the oxide-dispersion strengthened (ODS) high-temperature alloys MA 956 (an aluminium oxide former) and MA 754 (a chromium oxide former) has been compared with that of two model alloys, Fe-20Cr-5Al and Ni-25Cr. The morphology and composition of the oxide scales were investigated by metallography, X-ray diffraction analysis and scanning electron microscopy. For analysis of the oxide layer growth mechanisms, twostage oxidation experiments with¹⁸O as tracer were used, the distribution of the oxygen isotopes in the oxide scale being determined by SIMS. The ODS alloys show a more selective oxidation than the two model alloys; moreover, the protective oxides on the ODS alloys have a lower growth rate and better adhesion than those on the two model alloys. From the SIMS investigations it can be deduced that the improved properties of the layers on the ODS alloys result from a change in the transport processes in the protective layer; whereas the aluminium and chromium oxide films on the conventional alloys grow by cation and anion transport, the scales on the ODS alloys grow almost exclusively by anion transport. It is shown that the observed properties of the oxide scales on the ODS alloys can be explained by this change in transport mechanism.Dedicated to Professor Günther Tölg on the occasion of his 60th birthday     

Analysis of corrosion layers on protective coatings and high temperature materials in simulated service environments of modern power plants using SNMS,SIMS, SEM,TEM, RBS and X-ray diffraction studies

In three different examples, the effects of the oxidation behaviour as well as the microstructural stability of high temperature materials and protective coatings was determined by combining the results of kinetic studies with extensive analytical investigations using, among other techniques, SNMS, SIMS, SEM, TEM, Rutherford back scattering (RBS) as well as X-ray diffraction. 1). The effect of water vapour on the oxidation behaviour of 9% Cr steels in simulated combustion gases has been determined. The effects of O2 and H2O content on the oxidation behaviour of 9% Cr steel in the temperature range 600-800 degrees C showed that in dry oxygen a protective scale was formed with an oxidation rate controlled by diffusion in the protective scale. In the presence of water vapour, after an incubation period, the scales became non-protective as a result of a change in the oxidation limiting process. The destruction of the protective scale by water vapour does not only depend on H2O content but also on the H2O/O2-ratio. 2). The increase of component surface temperature in modern gas turbines leads to an enhanced oxidation attack of the blade coating. Improvements in corrosion resistance and longer lifetime thermal barrier coatings in gas turbines have been achieved by improvement of the high temperature properties of MCrAlY coatings by additions of minor alloying elements such as yttrium, silicon and titanium. 3). The use of oxide dispersion strengthened (ODS) alloys provides excellent creep resistance up to much higher temperatures than can be achieved with conventional wrought or cast alloys in combination with suitable high temperature oxidation/corrosion resistance. Investigation of the growth mechanisms of protective chromia and alumina scales were examined by a two-stage oxidation method with 18O tracer. The distribution of the oxygen isotopes in the oxide scale was determined by SIMS and SNMS. The results show the positive influence of a Y2O3 dispersion on the oxidation resistance of the ODS alloys and its effect on growth mechanisms.     

Analysis of composition and growth mechanisms of oxide scales on high temperature alloys by SNMS,SIMS, and RBS

Composition and growth phenomena of alumina scales on a conventional wrought alloy Fe-20Cr-5Al and an yttria containing ODS alloy with the same base composition were investigated. The alloys were oxidized in a twostage oxidation process at 1000 °C in air and air containing¹⁸O-tracer. By using secondary neutrals and secondary ion mass spectrometry in combination with RBS, the in-depth concentration of main and minor alloying elements as well as the oxygen isotope distribution in the alumina scale could be determined. It is shown, that the difficulties in deriving the exact growth processes of the scales from the oxygen tracer profiles can be overcome by using the time dependance of the iron and chromium distribution in the alumina. From these results it could be derived that growth of the oxide scales formed on the ODS and the conventional alloy mainly differ in the relative contribution of aluminium diffusion. The mechanisms which lead to these differences in scale growth phenomena will be discussed.     

Study of the phosphorus embrittlement in heavy metal alloys by a combination of SIMS and TEM

Summary The embrittlement of heavy metals of the type W96FeNi4 at phosphorus concentrations above 100 g/g is a phenomenon of considerable scientific and technical interest. In order to obtain information on the cause of embrittlement the behaviour of phosphorus as a function of technological parameters, particularly the cooling rate after sintering, has to be investigated. By application of SIMS and TEM it could be shown that phosphorus segregates to the grain boundary tungsten-binder during cooling, and that segregation increases with decreasing cooling rate. By quantitative micro trace analysis with SIMS the solubility limit of P in W could be determined. TEM showed that microprecipitates of a typical diameter of 5 nm are formed in the binder phase during cooling. Larger precipitates in the range 100–1000 nm occur at the grain boundaries tungsten-binder. By electron diffraction with computerized evaluation of the patterns these precipitates could be identified as NiP₂ phases. A major methodological result is that the combination of SIMS and TEM exhibits a great potential for the study of the influence of trace elements on material properties.

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Untersuchung der Phosphor-Versprödung von Schwermetallen mittels SIMS and TEM

Zusammenfassung Die Versprödung von Schwermetallen des Typs W96FeNi4 bei Phosphorkonzentrationen von größer als 100 g/g ist von beträchtlichem technischen und wissenschaftlichen Interesse. Um die Ursache für die Versprödung aufzuklären, müssen Informationen über das Verhalten von Phosphor als Funktion technologischer Parameter, insbesondere der Abkühlgeschwindigkeit nach dem Sintern gewonnen werden. Durch kombinierten Einsatz von SIMS und TEM konnte gezeigt werden, daß Phosphor bei der Abkühlung an die Korngrenze Wolfram — Binderphase segregiert und daß die Segregation mit langsamerer Abkühlgeschwindigkeit zunimmt. Weiterhin konnte die Löslichkeit von Phosphor in den Wolframphasen durch quantitative Mikrobereichsanalyse mit SIMS bestimmt werden. Mittels TEM wurde festgestellt, daß beim Abkühlen Mikropräcipitate mit einem typischen Durchmesser von 5 nm gebildet werden. Größere Präcipitate von ca. 100–1000 nm Durchmesser finden sich an den Korngrenzen. Mittels Elektronenbeugung und Computerauswertung der Beugungsmuster können diese Präcipitate als NiP₂ identifiziert werden. Ein weiteres methodisch orientiertes Ergebnis dieser Untersuchungen ist, daß die Kombination SIMS und TEM ein großes Potential für die Untersuchung des Einflusses von Spurenelementen auf Werkstoffeigenschaften aufweist.

     

Determination of molecular diffusion coefficients in liquid-crystalline cholesteryl myristate using gas chromatography

The molecular diffusion coefficients of xylene and picoline isomers have been determined at different temperatures in the three phases (smectic, cholesteric and isotropic) of cholesteryl myristate by gas chromatography. The results obtained are discussed in terms of molar volume, polarity and length-to-breadth ratio of the isomers.     

Determination of molecular diffusion coefficients in liquid-crystalline cholesteryl myristate using gas chromatography

Abstract

The molecular diffusion coefficients of xylene and picoline isomers have been determined at different temperatures in the three phases (smectic, cholesteric and isotropic) of cholesteryl myristate by gas chromatography. The results obtained are discussed in terms of molar volume, polarity and length-to-breadth ratio of the isomers.     

Investigation of high concentration effects of Sb and P in silicon by combination of SIMS and TEM

Summary The combination of SIMS, electrical resistance measurements and TEM was used for investigation of high concentration effects of Sb and P in silicon. For antimony implantation and annealing combined with and without preannealing were studied. The distributions and the precipitated fraction were determined by SIMS. TEM investigations showed that by preannealing at a temperature of 1150°C no dislocations but only precipitates are formed. Thus the precipitation kinetics for an isothermal case (1000°C) could be studied. The size distributions of precipitates were determined by image processing of plan view TEM micrographs. The kinetics of the precipitation process was simulated for the distribution function by a computer model and compared with experiments. A good agreement between experiment and simulation was obtained.For phosphorus, specimens implanted with medium and high doses were investigated. With medium doses small extrinsic dislocation loops (size 20–40 nm) were observed near the amorphous/cristalline boundary. The dislocation loops are formed by agglomeration of interstitial silicon atoms which are supersatured in the long range of the implantation profile.The coupled diffusion of high doses of P and low doses of Sb and B was studied. In the SIMS distributions enrichments of boron were observed. TEM measurements showed that they were correlating to a severe band of defects.SiP precipitates and large perfect dislocation half loops [mean radius (195±112) nm] were observed after annealing at 900°C for 120 min of implants with a dose of 5×10¹⁶ cm^–2 P. Because precipitation is occurring, simulation of phosphorus diffusion is at present not possible.

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Untersuchung von Hochkonzentrationseffekten von Sb und P in Silicium durch Kombination von SIMS und TEM

     

Relaxation times,diffusion coefficients and high frequency NMR measurements of methane dissolved in MBBA

The measurements of proton T₁, T₂ and diffusion coefficients of CH₄ and deuterium T₁ in CD₄, dissolved in MBBA are reported as a function of the temperature in the nematic and solid phases. Also reported are cw measurements of CH₄ in MBBA at 250 MHz which definitely prove that it is oriented. The results are discussed in terms of their relation to previously published cw NMR measurements on these systems.     

Determination of sulfonamides and trimethoprim using high temperature HPLC with simultaneous temperature and solvent gradient

A fast HPLC method for the analysis of eight selected sulfonamides (SA) and trimethoprim has been developed with the use of high temperature HPLC. The separation could be achieved in less than 1.5 min on a 50 mm sub 2 microm column with simultaneous solvent and temperature gradient programming. Due to the lower viscosity of the mobile phase and the increased mass transfer at higher temperatures, the separation could be performed on a conventional HPLC system obtaining peak widths at half height between 0.6 and 1.3 s.     

Investigation of the formation and properties of protective oxide layers on high purity chromium with SIMS imaging techniques

Spallation of the protective oxide layer formed during hot gas oxidation is the main reason for the corrosion of high purity powder metallurgically produced chromium[1]. To explain the formation and occasional spallation of the oxide layer a chromium sample subjected to two consecutive oxidation processes in¹⁶O and¹⁸O atmosphere at high temperature was characterised by 2D and 3D SIMS.The formation of the protective oxide can be described by the diffusion of chromium from the bulk to the surface and the reaction of the chromium atoms with the oxygen from the gaseous phase. The very low solubility of nitrogen in chromium oxide indicates its inability to diffuse through the growing oxide layer and explains the enrichment of nitrogen (same mechanism applies for chlorine) in the interface metal/oxide. The accumulation of trace elements within the interface during the oxidation process explains the reduced adhesion power of the passivation layer and its spallation.Dedicated to Professor Dr. rer. nat. Dr. h.c. Hubertus Nickel on the occasion of his 65th birthday     

Distribution analysis of trace elements in low alloy steel by means of EPMA,SIMS, and TEM

A combination of chemical analysis, EPMA, SIMS, and TEM was applied to gain information on the bulk values, the microstructure and the microdistribution of contaminants. The limitations of classical chemical analysis were overcome by the application of highly sophisticated analytical techniques. Optimized measurement conditions were worked out for SIMS analysis and basic data such as relative sensitivity factors were gained for further investigations. A correlation between the microdistribution of contaminants and mechanical data could not be established so far, since only two samples were investigated.List of acronyms used BAS British Association of Standardisation (UK) - CGHE carrier gas hot extraction - DL detection limit - ED energy discrimination - EPMA electron probe micro analysis - E ₀ energy of primary ions - HMR high mass resolution - I _B primary ion beam current - NBS National Bureau of Standards (USA) - OES optical emission spectroscopy - PI primary ions - RSF relative sensitivity factor - SI secondary ions - SIMS secondary ion mass spectrometry - SKF SKF Analytica Taeby (Sweden) - TEM transmission electron microscopy - XRFA X-ray fluorescence analysis - d _A diameter of analyzed area     

Transport properties of vanadium ions in cation exchange membranes:: Determination of diffusion coefficients using a dialysis cell

Diffusion coefficients of vanadium ions in cation exchange membranes are of interest because they allow to calculate the ion exchange across the membrane in an all vanadium redox flow battery which leads to undesired cross contamination and energy losses in the battery system. Diffusion coefficients of V²⁺, V³⁺, VO²⁺ and VO⁺₂ ions in CMS, CMV and CMX cation exchange membranes have been determined by measuring the ion exchange fluxes of these ions with H₃O⁺ ions using a dialysis cell. The experimental data are evaluated on the basis of integrated flux equations which require also ion exchange sorption equilibria obtained already in previous work. The lowest diffusion coefficients are observed in the CMS membrane for all vanadium ions. This membrane turns out to be the most suitable one for being applied in a vanadium battery since it is expected to prevent most effectively cross contamination of vanadium ions.     

Measurements of diffusion coefficients in 1-D micro- and nanochannels using shear-driven flows

The present paper describes a method for measuring the molecular diffusion coefficient of fluorescent molecules in microfluidic systems. The proposed static shear-driven flow method allows one to perform diffusion measurements in a fast and accurate manner. The method also allows one to work in very thin (i.e. submicron) channels, hence allowing the investigation of diffusion in highly confined spaces. In the deepest investigated channels, the obtained results were comparable to the existing literature values, but when the channel size dropped below the micrometer range, a significant decrease (more than 30%) in molecular diffusivity was observed. The reduction of the diffusivity was most significant for the largest considered molecules (ssDNA oligomers with a size ranging between 25 to 100 bases), but the decrease was also observed for smaller tracer molecules (FITC). This decrease can be attributed to the interactions of the analyte molecules with the channel walls, which can no longer be neglected when the depth of the channel reaches a critical value. The change in diffusivity seems to become more explicit as the molecular weight of the analytes increases.