Ordered-Phase Equilibria in the Eutectoid Region of Bulk Fe-Pd

Abstract This report is the first analysis of the coexistence and microstructure of the equilibrium phases in the Fe-Pd L1 0 + L1 2 eutectoid region. Coexistence of L1 0 + L1 2 is observed at higher temperatures (650 $$^\circ {\text{C}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mmultiscripts> <mml:mrow/> <mml:mrow/> <mml:mo>∘</mml:mo> </mml:mmultiscripts> <mml:mtext>C</mml:mtext> </mml:mrow> </mml:math> ), resulting in L1 0 polytwin plates with internal boundaries that are decorated by L1 2 . For higher Pd content, the L1 0 plates are embedded in an extended L1 2 matrix, but the L1 2 wetting layers still persist. For aging at low temperatures (525 $$^\circ {\text{C}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mmultiscripts> <mml:mrow/> <mml:mrow/> <mml:mo>∘</mml:mo> </mml:mmultiscripts> <mml:mtext>C</mml:mtext> </mml:mrow> </mml:math> ), L1’ + L1 2 coexistence is observed, but the microstructure is essentially similar, except that L1 0 is 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 L1 0 and L1 2 , 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 L1 0 .