Thermodynamic study of Fe-Ta and Fe-Nb alloys by means of the Knudsen cell mass spectrometry.

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The activity of iron in the Fe–Ta and Fe–Nb alloys at 1 873 K was determined by means of the Knudsen cell mass spectrometry. A modified internal standard method was employed, in which a piece of pure iron foil placed on the sample surface in the Knudsen cell served as the internal standard material. The ion intensity of iron from the foil was first measured at several temperatures before the alloy sample and the foil melted together. The ion intensity from pure solid iron thus obtained was extrapolated to give ion intensity from pure liquid iron at higher temperatures. The temperature of the sample was then raised to form uniform liquid alloy and the ion intensity of iron from the alloy was measured. The activity of iron in the alloy was determined by comparing the ion intensities from pure iron and from alloy.The activities of Ta and Nb were derived from that of iron by integrating the Gibbs-Duhem equation. The activity coefficients of Ta and Nb in the infinitely dilute solution with liquid pure metal as the standard state were also calculated. The interaction parameters were determined. These results were compared with those of elements adjacent to Ta and Nb in the periodic table.Invariant reactions appeared in the Fe–Ta binary alloy system at high temperature were investigated by employing the ion intensity thermal analysis, in which the ion intensity represented the temperature of the sample. Heat treatment and metallographic study was also carried out for the identification and composition determination of phases.