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Abstract This report is the first analysis of the coexistence and microstructure of the equilibrium phases in the Fe-Pd L1₀ + L1₂eutectoid region. Coexistence of L1₀ + L1₂is observed at higher temperatures (650$$^\circ {\text{C}}$$ $_{}^{\circ }\text{C}$), resulting in L1₀polytwin plates with internal boundaries that are decorated by L1₂. For higher Pd content, the L1₀plates are embedded in an extended L1₂matrix, but the L1₂wetting layers still persist. For aging at low temperatures (525$$^\circ {\text{C}}$$ $_{}^{\circ }\text{C}$), L1’ + L1₂coexistence is observed, but the microstructure is essentially similar, except that L1₀is replaced by L1’. The two-phase region is found to be much narrower than reported in published phase diagrams, of order 0.6 to 1 at pct in extent. There may be a further re-entrant narrowing below the L1’ formation temperature. This work establishes L1’ as a phase distinguishable from both L1₀and L1₂, but does not yet prove that L1’ is an equilibrium phase. The preferred formation of L1’ at lower temperatures may relate both to stability conferred by overall ferrimagnetic interactions, and perhaps by kinetics, where L1’ should have a reduced nucleation barrier from A1 relative to L1₀. 

Direct evidence is provided for the existence of the tetragonal L1’ phase, first predicted by Shockley in 1938, in bulk Fe - 62 at% Pd alloys aged at 525 deg C. L1’ existence as the dominant phase is supported by quantitative x-ray diffraction analysis. This is combined with transmission electron microscopy of the polytwinned microstructure, examining the diffracted intensities in specific superlattice reflections where the complete extinction in L10 is relaxed in L1’. Ordering to L1’ appears to occur directly from the A1 parent phase at 525 deg C, while aging at 650 deg C only produces L10. The possibility of L1’ ordering may have consequences for the ferromagnetic properties of classic and important binary alloy systems where L10 is the assumed equilibrium phase.