  Using special quasirandom structures (SQS's), we perform first-principles
calculations studying the metastable bcc NiCu and FeCu alloys which occur in
Fe-Cu-Ni alloy steels as precipitated second phase. The mixing enthalpies,
density of state, and equilibrium lattice parameters of these alloys are
reported. The results show that quasi-chemical approach and vegard rule can
well predict the energetic and structural properties of FeCu alloys but fail to
yield that of NiCu. The reason rests with the difference of bond energy
variation with composition between NiCu and FeCu alloys induced by competition
between ferromagnetic and paramagnetic state. Furthermore, the calculated
results show that the energetic and structural properties of these alloys can
well explain the local composition of the corresponding precipitates in ferrite
steels.
