Chemical State of Phosphorus Segregated at Grain Boundaries of Iron

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Abstract
Determination of the chemical state of phosphorus segregated at grain boundaries of iron is important to discuss mechanisms of the grain boundary decohesion by phosphorus. X-ray photoelectron spectroscopy (XPS) analysis and the scanning Auger electron spectroscopy (AES) analysis were performed to determine the chemical state of segregated phosphorus. Specimens of iron containing various amount of phosphorus (0.05 to 15.6 wt%) were fractured under a vacuum better than 6 × 10-11 Torr. The clean ultra-high vacuum allows a prolonged XPS analysis (10h) which is necessary for a small amount of phosphorus segregated at grain boundaries. The spectra were compared with those from an Fe3P specimen, and the chemical state of the segregated phosphorus was determined to be the same as phosphorus in Fe3P. This result supports the grain boundary decohesion mechanism proposed by Losch, who considers the segregated phosphorus atoms to form a strong binding with surrounding iron atoms and the iron-iron binding neighboring the phosphorus to be weaken. The shape of AES of phosphorus in solid solution varies with the concentration of phosphorus, but that of phosphorus at grain boundaries is independent of the phosphorus concentration and same as that of phosphorus in Fe3P. Importance of the variation of the shape in the AES quantitative analysis of segregated phosphorus is discussed.

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