Absolute vacancy concentrations were determined by simultaneously measuring relative changes in macroscopic length and X-ray lattice parameter as a function of temperature. Vacancy concentrations ranging from about 10-5 to a few percent could be measured by means of high-resolution differential dilatometry. Thermal vacancies could therefore be investigated at 700 to 800K. The temperature-dependence of vacancy formation was correlated with the phase diagram. In the B2' and B2(l) phase of FeAl, and the D03(l) phase of FeSi, the vacancy formation obeyed an Arrhenius law with effective formation enthalpies of about 1eV and effective formation entropies of 4 to 5k. At higher temperatures, in the B2(h) phase of FeAl and the D03(h) phase of FeSi, vacancies were no longer formed as easily as before. This resulted in a flatter line, and effective formation enthalpies of 0.3 to 0.5eV. In the disordered A2 phase of FeAl, the vacancy concentration again increased and effective formation enthalpies of about 1.5eV were found.

Equilibrium Vacancy Concentrations in FeAl and FeSi Investigated using an Absolute Technique. R.Kerl, J.Wolff, T.Hehenkamp: Intermetallics, 1999, 7[3-4], 301-8