Au composition dependent order-disorder transitions of Fe−Pt intermetallic compounds: Experiments and thermodynamic analysis

Abstract

The Fe−Pt intermetallic nanocrystals exhibit good chemical stability, unique magnetic and catalytic properties, but suffer from the ordering difficulty. It has been found that the addition of Au can promote the disorder-order transitions, but the mechanism is far from being well understood. In this work, the phase equilibria and phase transformations of the Fe−Pt−Au alloys have been experimentally investigated and thermodynamically analyzed. The results show that the driving forces for both the L10_FePt and L12_FePt3 ordering are so weak that the homogeneous nucleation in the disordered fcc matrix is difficult, while a heterogeneous nucleation mechanism on the pre-existed phase boundary is preferred. On the other hand, there exists a metastable ternary miscibility gap between αAu and fcc-FePt or fcc-FePt3, and accordingly, when Au content is properly designed in the spinodal region, the enrichment of Au can take place spontaneously throughout a fcc Fe−Pt−Au structure during annealing. The segregation of αAu in the Fe−Pt−Au alloys can further accelerate the diffusion of Pt and Fe atoms, facilitate the nucleation of the ordered L10_FePt and L12_FePt3 phases, and stimulate the ordering process. These results correlate the Au addition with the ordering of Fe−Pt intermetallics and further the magnetic and catalytic properties, and it is thus expected to provide guidance for a rational alloy design of Fe–Pt–Au materials.