Phase transformations in Fe-Ni Invar alloys

The first- and second-nearest-neighbour paìrwise interaction parameters for Fe-Ni alloys are determined from published neutron diffraction data and found to be ∼ 10 and — 10 meV respectively. The configurational free energy is then determined for the LI0 (FeNi) structure using the Bragg-Williams point approximation with interactions out to the second-nearest neighbour and, with the above values, is found to contain a local maximum at ∼ 25 at. ° Ni. This leads to a miscibility gap and a spinodal beneath the order-disorder solvus. The wide variety of possible microstructure8 that can result may explain the diverse range of microstructures and magnetic states reported for Fe-Ni Invar alloys.