ELEMENTARY DIFFUSION JUMP OF IRON ATOMS IN INTERMETALLIC PHASES STUDIED BY MOSSBAUER-SPECTROSCOPY .2. FROM ORDER TO DISORDER

We determine the elementary diffusion jump mechanism of Fe atoms in B2 ordered Fe-Al single crystals (Fe contents 50.5, 55 and 66 at.%) by means of quasielastic Mossbauer spectroscopy at Fe-57. The mechanism obviously follows the same logics irrespective of the alloy composition: it is a jump into an antistructure site on the Al sublattice, followed by a jump back to a regular site on the Fe sublattice. Close to stoichiometry Fe50Al50 the Fe atom resides on the antistructure site only for a small fraction of the time, but with increasing Fe concentration and at sufficiently high temperatures this fraction increases until the residence time on the Al sublattice approaches the residence time on the Fe sublattice. In Fe66Al37 there is no discontinuity in the Mossbauer spectra at the B2 --> A2 phase transition. This proves that short-range order decreases continuously in the region of the phase transition.