Equilibrium vacancy concentrations in FeAl and FeSi investigated with an absolute technique

Absolute vacancy concentrations in FeAl and FeSi alloys are being determined by measuring simultaneously the relative changes in macroscopic length Delta L/L and X-ray lattice parameter Delta a/a as a function of temperature in the same specimen. With the high-resolution differential dilatometer vacancy concentrations of about 10(-5) to a few percent can be measured, so that thermal vacancies can be investigated within a wide temperature range of about 700-800 K. The temperature dependence of vacancy formation is correlated with the phase fields according to the phase diagrams referenced by Kubaschewski (Kubaschewski O. Iron-binary phase diagrams. Berlin: Springer, 1982). In the B2' and B2(1) phase of FeAl and the D0(3)(1) phase of FeSi, vacancy formation obeys an Arrhenius law with effective formation enthalpies about 1 eV and effective formation entropies about 4-5 k(B). For higher temperatures, in the B2(h) phase field of FeAl and the D0(3)(h) phase field of FeSi, vacancies are no longer formed as easily as before. This results in a flatter line, effective formation enthalpies about 0.3-0.5 eV may be fitted. In the disordered A2 phase of FeAl the vacancy concentration increases again, effective formation enthalpies about 1.5 eV may be fitted. (C) 1999 Elsevier Science Ltd. All rights reserved.