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

This report is the first analysis of the coexistence and microstructure of the equilibrium phases in the Fe-Pd L10 + L12 eutectoid region. Coexistence of L10 + L12 is observed at higher temperatures (650 degrees C\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$<^>\circ {\text{C}}$$\end{document}), resulting in L10 polytwin plates with internal boundaries that are decorated by L12. For higher Pd content, the L10 plates are embedded in an extended L12 matrix, but the L12 wetting layers still persist. For aging at low temperatures (525 degrees C\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$<^>\circ {\text{C}}$$\end{document}), L1' + L12 coexistence is observed, but the microstructure is essentially similar, except that L10 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 L10 and 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.