Raising the Curie point in the iron-cobalt-(aluminum) system

The ternary alloy system Fe-Co-Al exhibits a little-noticed anomaly in that some alloys containing Al have higher Curie points than their undiluted counterparts. New results present the evolution of Curie point and saturation magnetization as a function of Al content, especially below 7 wt % (∼13.5 at. %) in the midrange of the Fe-Co composition field. Maxima of T_c in the range of 1000-1025 °C are found for Al contents of about 2-5 wt % (4-10 at. %) for alloy series with 30- , 40- , and 50-wt % Co (∼27- , 37- , and 47-at. % Co). These values are some 25-45 °C higher than the corresponding zero-Al alloys. Correlation of low-field thermal hysteresis and saturation-field thermomagnetic curves indicate first-order Curie point behavior at low-Al content, followed by classical second-order transitions at and above the Al content corresponding to T_c maxima. The former follow the α-bcc to the γ-fcc phase transition. For the latter class the α phase has been stabilized to higher temperatures with a relatively uncommon Curie point enhancement behavior.