Sulphur Grain Research Articles

Cross-sectional measurement of grain boundary segregation using WDS

A new method is proposed for the quantification of grain boundary segregation using Wavelength Dispersive Spectroscopy (WDS) in a Scanning Electron Microscope (SEM). Analyses are undertaken on a simple metallographically polished section of material. The method is demonstrated for the model system of sulphur segregation to nickel grain boundaries. Quantification was carried out from sulphur concentration profiles acquired across 11 grain boundaries of a nickel specimen containing 5.4wtppm of sulphur in the bulk and equilibrated at 550°C. The average sulphur grain boundary concentration determined is µ=35.2ngcm−2=6.6×1014 atoms cm−2≈0.5 monolayer, which is in good agreement with a previous quantification obtained from SIMS (Secondary Ion Mass Spectrometry) on the same material. However this is lower by a factor of two than the quantification obtained using “surface” techniques on fractured specimens of the same material. With the conditions of analysis used in this study, the limit of detection of the method developed is found to be better than 10% of a sulphur monolayer.

Ultra-fast sulphur grain boundary segregation during hot deformation of nickel

Sulphur grain boundary segregation during hot-compression of nickel (5.4wt.ppmS) is monitored using Auger electron spectroscopy and wavelength dispersive X-ray spectroscopy. The deformation conditions (temperature/deformation rate) investigated are: 550°C/0, 550°C/3×10−5s−1, 550°C/3×10−4s−1 and 450°C/3×10−5s−1. It is shown that plastic deformation accelerates the kinetics of sulphur grain boundary segregation by a factor of ∼103 to a few 105, depending on the deformation conditions. Very high levels of segregation (∼0.8 monolayer of sulphur) are obtained after very low deformation (∼5%). In addition a linear dependence of the segregation level with time and deformation is demonstrated. The segregation kinetics during plastic deformation is proportional to the deformation rate and almost independent of temperature. Several metallurgical mechanisms are discussed and confronted with the experimental results: dislocations dragging, pipe diffusion, dislocation collection/diffusion and acceleration by excess vacancies. It appears that the models developed in this work on the basis of the two latter mechanisms (dislocation collection/diffusion and acceleration by excess vacancies) predict the experimental data correctly.

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.

Recent Developments in the Study of Grain Boundary Segregation by Wavelength Dispersive X-Ray Spectroscopy (WDS)

It was recently shown [1] that EMPA-WDS (Electron Probe MicroAnalysis by Wavelength Dispersive X-ray Spectroscopy) can be used to detect and to accurately quantify monolayer surface and grain boundary segregation. This paper presents the last developments of this application. It focuses on the measurement of sulphur grain boundary segregation in nickel on fractured surfaces. A special attention was paid to the quantification of the sulphur coverage, taking into account the non-normal incidence of the electron beam on a fracture surface. Sulphur grain boundary segregation kinetics was measured at 750°C in nickel to document the quantitative possibilities of the technique.

Measuring grain boundary segregation using Wavelength Dispersive X-ray Spectroscopy: Further developments

A technique for the quantification of surface and grain boundary segregation using Wavelength Dispersive X-ray Spectroscopy (WDS) in the Scanning Electron Microscope (SEM) is proposed. As an example, the case of sulphur segregation in nickel is considered. The sulphur segregation can be quantified using a single voltage measurement (20 kV) of the sulphur Kα line intensity. The quantification is made from a simple proportionality equation derived from the XPP microanalysis model of correction. The result of segregation quantification by WDS is a surface concentration expressed in g.cm -2. Special attention was paid to the quantification of grain boundary segregation on fracture surfaces, taking into account the off-normal incidence of the electron beam on the analyzed surface. A simple technique that allows determination of the tilt angle from the specimen absorbed current is proposed. The influence of the azimuth angle of the analyzed surface with respect to WDS spectrometer is also discussed. The results of the WDS technique are shown to be repeatable within 5% with reasonable counting times (a few minutes per facet). As an example, the kinetics of sulphur grain boundary segregation in nickel at 750 °C was measured using WDS to document the quantitative capabilities of the technique. As already pointed out in a previous paper, it is confirmed that WDS is insensitive to surface contamination, which allows the analysis of ex-situ fractured samples.

Scanning AES and chemical investigation of materials failure due to phosphorus and sulphur grain boundary segregation in type AISI 304L stainless steel

Scanning AES and chemical investigation of materials failure due to phosphorus and sulphur grain boundary segregation in type AISI 304L stainless steel

Scanning AES and chemical investigation of materials failure due to phosphorus and sulphur grain boundary segregation in type AISI 304L stainless steel

Investigations have been performed on the influence of heat treatment on grain boundary segregation in austenitic stainless steel. In the present work segregation behaviour at type AISI 304L austenitic stainless steel containing relatively high amount of P and/or S has been examined after heat treatment at 550°C/1000 hours. In order to make grain boundaries accessible, the preparation conditions for a high intergranular fracture part have been determined. This procedure is based on cathodic hydrogen charging followed by thermal exposure and straining up to the yield stress. The enrichment of P and S at grain boundaries has been detected directly by means of Auger Electron Spectroscopy after in situ fracture in ultra high vacuum and indirectly by the Coriou test. Significant evidence for segregation of P and S at grain boundaries has been found after the heat treatment only. The segregation causes increasing intergranular stress corrosion susceptibility and embrittlement indicated by CERT (Constant Extension Rate Test). AES- und chemische Untersuchung von Materialschaden durch Phosphor- und Schwefel-Korngrenzensegregation an nichtrostenden Stahlen vom Typ 1.4306 Der Einflus einer Langzeitwarmebehandlung auf die Korngrenzensegregation wurde in austenitischem Cr-Ni-Stahl untersucht. In dieser Arbeit wurde die Segregation am Stahl AISI 304 L mit relativ hohem Gehalt von P und/oder S nach einer Warmebehandlung bei 550°C/1000 h charakterisiert. Fur die Untersuchungen mittels Auger-Elektronenspektroskopie konnte durch eine geeignete Vorbehandlung der Proben ein hoher interkristalliner Bruchanteil erzielt werden. Diese Vorbehandlung basiert auf einer kathodischen Wasserstoffbeladung mit anschliesender thermischer Auslagerung, gefolgt von mechanischer Dehnung. Die Anreicherung von Phosphor und Schwefel an den Korngrenzen wurde direkt nach in situ-Bruch im Ultrahochvakuum durch Auger-Elektronenspektroskopie und indirekt mit Hilfe des Coriou-Tests nachgewiesen. An Proben von warmebehandeltem Stahl zeigten die AES-Ergebnisse eine hohere Konzentration von P und S an Korngrenzen im Vergleich zu nicht warmebehandelten Proben. Bei den Zugversuchen mit CERT (Constant Extension Rate Test) konnte eine erniedrigte Bruchdehnung und erhohte interkristalline Korrosionsanfalligkeit als Folge der Segregationsprozesse nachgewiesen werden.

Grain boundary segregation and diffusion in Ag-S solid solution

A study is presented of grain boundary equilibrium segregation and diffusion of sulphur in high purity silver, using the 35S radiotracer. At 550°C the segregation coefficient α = C gb C b does not depend on the bulk concentration C b for very low concentrations; it depends only on temperature; the energy for sulphur grain boundary interaction is ΔH = −15,000 cal- mole −1. For higher C b , α decreases while the segregated sulphur varies slightly. The simultaneous study of both equilibrium segregation and diffusion brings out the close relationship between these two phenomena. At a given temperature, the complex variations of αD gb are explained by the increase of D gb for very low bulk concentrations and by a decrease of α for higher ones. For C b constant, the grain boundary diffusion energy Q gb (33,500 cal-mole −1) is derived from the measured activation energy ( Q gb + ΔH = 18,500 cal- mole −1).