Structure and anelasticity of ordered and disordered Fe–Ge alloys

Abstract

Internal friction effects in a series of Fe–Ge–C alloys with 3–27 at% Ge and about 0.01–0.04 at% C have been investigated and related to possible mechanisms of anelasticity according to the different alloy structures, which were examined by X-ray diffraction, calorimetry, vibrating sample magnetometry, optical and electron microsopy. While the alloys with 3, 8, 12 and 19 at% Ge have bcc structure (for 3 and 8 at% without ordering and for 12 and 19 at% with ordering of Ge atoms) and show distinct Snoek-type relaxations (with peaks for Fe–C–Fe and for Fe–C–Ge neighbours), in alloys with 22 and 27 at%Ge, where hexagonal phases dominate, no Snoek relaxation is observed as expected. A pronounced Zener relaxation at elevated temperatures occurs in Fe–12–27 at%Ge, with an activation enthalpy of H = 1.78 eV (preexponential time τ 0 = 2 × 10 −17 s −1) for Fe–27at%Ge and near 2 eV for Fe–12at%Ge, which is consistent with diffusion data. The mixed order/disorder structures of Fe–12–19 at% Ge give rise to a further peak, even found also in nominally disordered Fe–8%Ge, which is due to ordering involving quenched-in vacancies. In the temperature range below room temperature two neighbouring peaks are detected which are supposed to be related to dislocation motion.