The Order — Disorder Transformation in Fe₃Al Alloys

Abstract

Abstract A Mössbauer investigation in FeAl alloys ranging from 20 to 28 at% Al has been performed. In the disordered spectra, which were computer analyzed, a very slight mean electric quadrupole interaction has been measured, which may be due to some directional order, while in the ordered spectra two ordered phases were observed for a composition around 25 at% Al: the DO 3 ‐type of ordered structure, in which each Al atom has eight Fe nearest neighbours and six Fe next‐nearest neighbours, and another phase with a structure similar to Cu 3 Au, in which each Al atom has 12 Fe nearest neighbours and six Al next‐nearest neighbours. Both phases are ferromagnetic. In the Cu 3 Au‐type structure a strong electric quadrupole interaction has been measured, which is the same as in the Fe 3 AlC compound. The quadrupole effect does not appear in the DO 3 ‐type structure probably because of special relationships between the easy axis of magnetization and the electric field gradient tensor. Measurements of quadrupole effects show that Al should be strongly ionized when alloyed with Fe. This gives a clue to understand the physical reasons why order‐disorder transitions appear in such alloys. Below the transition temperature, each Al gets better surrounded exclusively by Fe atoms in order to minimize the Coulomb interaction energy with its neighbours.