The anomalous phase instability near the Curie temperature of the Fe-Cu system

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Abstract
Taking into account the effect of the magnetic free energy, the phase stability (i.e., the onset of Cu-rich precipitation) at the Fe-rich end of the Fe-Cu binary system near the magnetic ordering (Curie) temperature has been evaluated with a regular solution model and experimental verification. The calculated results show that the stability of alpha iron is significantly lowered near the Curie temperature, and the solvus of copper in alpha iron is bent toward the Curie temperature line, hence a protrusion should be added to the known phase diagram. Microhardness indentation tests, lattice constant determination by the careful x-ray diffraction (XRD), and transmission electron microscopy (TEM) were carried out on the solution heat treated and then aged Fe-Cu alloys containing 0.25-, 0.95-, and 1.5-at. % Cu. All the three experimental results confirm the calculated ones. The Cu-rich precipitate is identified by TEM on an alloy quenched from the protruded region.