Grain boundaries in nanocrystalline tungsten probed by 57Fe Moessbauer spectroscopy

Iron enriched in the isotope 57Fe was dispersed in a nanocrystalline tungsten matrix by mechanical alloying. Moessbauer experiments were performed using as-milled, isochronally and isothermally annealed samples, both at room temperature and as a function of measurement temperature. The spectra were interpreted in terms of quadrupole splitting distributions p(QS). By careful analysis of the p(QS) distributions it is possible to deconvolute the spectral signatures of 57Fe atoms on substitutional sites, in the vicinity of lattice defects and in grain boundaries. The Debye-Waller factor of 57Fe atoms in grain boundaries is found to be enhanced with respect to iron atoms in the crystalline matrix. An interpretation of this result is given that is consistent with the observed hyperfine parameters. (orig.)