Segregation phenomena on surfaces of the ordered bimetallic alloy FeAl

The segregation and ordering behavior of the (100), (110), (210), (310) and (111) surfaces of the ordered intermetallic compound FeAl were investigated by means of AES and LEED. Starting from preferentially sputtered, i.e. Al-depleted surfaces sequential annealing leads to segregation of aluminum for all five surfaces. For intermediate annealing temperatures several ordered superstructures are found for the low index surfaces, which can partly be attributed to metastable Fe3Al films developing at the surfaces. All well-annealed surfaces exhibit Al concentrations beyond bulk stoichiometry. The (100) surface is found to be Al terminated, additionally the second layer is enriched in Al. For all other FeAl surfaces no 1x1 diffraction patterns are observed in the whole temperature range up to 1000 degrees C, indicating the absence of bulk terminated surfaces, in contrast to the equivalent surfaces of NiAl. An incommensurate surface alloy of FeAl2 stoichiometry finally forms on the most densely packed FeAl(110) surface. The other more open surfaces are characterized by multilayer segregation of Al. While (111) and (210) surfaces develop long-range surface reconstructions (3 x 3 and 1 x 3), the (310) surface is unstable and facets. The differences in the segregation behavior between NiAl and FeAl can qualitatively be explained by a combination of different segregation models using the heat of formation of the alloy. (C) 1998 Elsevier Science B.V. All nights reserved.