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 i nvestigated by means of AES and LEED. Starting from preferentially spu ttered, i.e. Al-depleted surfaces sequential annealing leads to segreg ation of aluminum for all five surfaces. For intermediate annealing te mperatures several ordered superstructures are found for the low index surfaces, which can partly be attributed to metastable Fe3Al films de veloping at the surfaces. All well-annealed surfaces exhibit Al concen trations beyond bulk stoichiometry. The (100) surface is found to be A l terminated, additionally the second layer is enriched in Al. For all other FeAl surfaces no 1x1 diffraction patterns are observed in the w hole temperature range up to 1000 degrees C, indicating the absence of bulk terminated surfaces, in contrast to the equivalent surfaces of N iAl. An incommensurate surface alloy of FeAl2 stoichiometry finally fo rms 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 b e explained by a combination of different segregation models using the heat of formation of the alloy. (C) 1998 Elsevier Science B.V. All ni ghts reserved.