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Kornqrenzen Metalle / Segregation / StoffeiqenschaftenThe segregation of sulfur, nitrogen. and carbon at surfaces and grain boundaries of x-iron was studied by Auger Electron Sp~c"lr",("p( AES). Specimens of pure iron, doped with nitrogen 0, carbon by equilibria with gas mixtures were introduced into the UHV-chamber and fractured by impact at about -130 'c. Enrichment of nitrogen, carbon, or sulfur was observed on the intergranular fracture surfaces.Displacement equilibria exist between nitrogen or carbon and sulfur, which was present in the bulk material only as a trace impurity. The presence of sulfur decreases the cohesion of the grain boundaries, so that mainly intergranular fracture occurs. Obviously nitrogen or carbon reinforce the cohesion of grain boundaries, so that transgranular fracture prevails. ( 1) obtain the temperature dependence the equation can be written as follows where t1H s e gr . is the segregation enthalpy and t1S se gr . is the segregation entropy. In the case of segregation the activity of the adsorbed species X is given by a function of the concentration, in the case of adsorption the activity is given by the partial pressure of X in the gas phase [4]. We have studied the surface segregation Xtdissolved)~X(adsorbed) for carbon, nitrogen and sulfur, on iron surfaces in dependence on the temperature and on the bulk concentration of the solid solutions of the dissolved atoms in iron [5,6]. For those measurements, small iron specimens were introduced in an UHV chamber, the surface was cleaned and the samples were heated to elevated temperatures. The studies were performed only in the temperature and concentration range in which the solid solutions are stable, so that formation of threedimensional compounds was avoided. The surface concentration was followed by Auger-Electron-Spectroscopy (AES). The peakto-peak height in an (differential) Auger spectrum is a measure for the surface concentration, this was measured for the element in question, related to the peak-to-peak height for one iron peak to be free of apparative variables and plotted vs. the temperature. Such a plot is shown in Fig. 1 for the surface segregation equilibria of sulfur on polycrystalline iron specimens with different bulk concentrations of sulfur.The surface concentration decreases with increasing temperature. At lower temperatures a saturation coverage is approached, which corresponds to a coverage of one S-atom per 2 Fe-atoms. This was derived from LEED studies with single crystal samples and from a calibration of the Auger signal by radio-active sulfur [7]. Using the extrapolated data for the saturation coverage, the degree of coverage (J can be

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Herstellung von Eiseneinkristallproben mit bestimmten Konzentrationen an Kohlenstoff und Stickstoff, Einstellung der Anreicherungsgleichgewichte bei erhöhter Temperatur in der Ultrahochvakuumapparatur, Messung der Oberflächenkonzentrationen von Kohlenstoff oder Stickstoff in Abhängigkeit von Volumenkonzentration und Temperatur im Stabilitätsbereich des α‐Mischkristalls mittels Auger‐Elektronen‐Spektroskopie, Beobachtung der Adsorptionsstrukturen mittels LEED.