Abstract
Prior ordering of the lattice has been shown to have a profound effect on the physical properties and phase transformations of iron-platinum-nickel alloys. Using x-ray diffraction and thermal analysis it has been shown that Ll[2] ordering occurs across the quasi-binary section Fe[3]Pt-Fe[3]Ni at continuously lower temperatures as Ni is progressively substituted for Pt, with a postulated critical ordering temperature for the phase, Fe3Ni, of 625+/-5K. The literature has shown the Fe[3]Ni phase to be metastable and dependent on special circumstances for its detection, which is in agreement with this study of the system. The effect of chemical ordering on the martensitic transformation has been shown to be similar to that of the binary Fe3Pt alloy, and a thermodynamic model is put forward in an attempt to explain the thermoelastic nature of the transformation in terms of both the atomic and magnetic order. The anomalous results from the expansion coefficients and disordered lattice parameters have been shown to be consistent with the two gamma state theory for Invar behaviour, however, this theory could not be extended to the binary Fe-Pt alloys.