Investigation of the epsilon phase in the Fe-Al system by high-temperature neutron diffraction

In the central part of the Fe-Al system between about 58 and 65 at.% Al, a high-temperature phase denoted as epsilon occurs with a hitherto unknown crystallographic structure. The phase is stable between 1231 degrees C and 1095 degrees C. In order to study the crystallographic structure of the epsilon phase, in situ high-temperature neutron time-of-flight diffraction experiments have been performed at the HIPPO instrument at the Los Alamos Neutron Science Center (LANSCE). The epsilon phase was found to have the formula Fe(5)Al(8) with a body-centred cubic structure of the Hume-Rothery Cu(5)Zn(8) type (1 (4) over bar 3m (No. 217), Z = 4, cI52) and 52 atoms in the unit cell. Its lattice parameter is a = 8.9756(2) angstrom at 1120 degrees C, which is 3.02 times that of cubic FeAl (B2) at the same temperature. We report here the evolution of the crystallographic parameters over the temperature range between 1080 degrees C and 1120 degrees C.