On the 550°C Change of FeCo Superlattice Alloy

Abstract

In 1943, Kaya and Sato measured the specific heat of a FeCo superlattice alloy and observed a bump in the specific heat versus temperature curve at 550°C in addition to the order-disorder transformation at 730°C. Such a bump cannot be seen in the theoretical specific heat versus temperature curve for ordinary superlattice alloys. Despite numerous works that followed, the reason for this phenomenon has not been fully explained.
In the present study the following procedure was employed: In heating the slowly cooled ordered FeCo alloy, the specimen was kept at a predetermined temperature for a sufficient time, so that the electrical resistivity could be retained constant. Then the specimen was submerged into water several times and its electrical resistivity was measured in liquid nitrogen. This process was repeated several times at different temperatures up to 800°C.
In cooling the specimen, the same procedure as mentioned above was employed in a reverse direction.
In the equilibrium electrical resistivity versus quenching temperature curves thus obtained, the anomaly near 550°C cannot be seen which has been reported by some workers. When the temperature of the specimen is changed, it takes a certain time until equilibrium in the ordered degree is reached. This is considered as the cause of the anomaly in specific heat of FeCo alloy.