Structural Stability in Fe-Based Alloys

Condensed Matter Theory Group, Physics Department, Uppsala University, S-75121 Uppsala, Sweden 

Experimentally it is found that the Fe-Co and Fe-Ni alloys undergo a structural transformation from the bee structure to the hep or fee structures, respectively, with increasing number of valence electrons, while the Fe-Cu alloy is unstable at most concentrations. In addition to this some of the alloy phases show a partial ordering of the constituting atoms. One may wonder if this structural behaviour can be simply understood from a filling of essentially common bands or if the alloying implies a modification of the electronic structure and as a consequence also the structural stability. In this paper we try to answer this question and reproduce the observed structural behaviour by means of accurate alloy theory and total energy calcul ions. 

We have used the basis set of the Linear-Muffin-Tin-Orbital (LMTO) method in the atomic sphere approximation (ASA). The LMTO-ASA is based on the work of Andersen and co-workers and the combined technique allows us to treat all phases on equal footing. To treat itinerant magnetism we have employed the Vosko-Wilk-Nusair parametrization for the exchange-correlation energy density and potential. In conjunction with this we have treated the alloying effects for random and partially ordered phases with a multisublattice generalization of the coherent potential approximation (CPA). 

We have considered the fee and bee phases for both random and ordered (partially ordered) alloys. The ordered bee phase is based on the B2 structure. In this structure only the FcsoXso (X = Co, Ni or Cu) alloys can be perfectly ordered. For the off-stoichiometry compositions partially ordered alloys have been considered with one 

To further discuss the underlying mechanisms that forces the phase stabilities we also did calculations where the alloying effects were treated within the so-called virtual crystal approximation (VGA) where the real alloy constituents are replaced by an atom with an average (noninteger) atomic number. 

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