Order-Disorder Transformation of Fe-Co Alloys in Fine Particles

Abstract

The specific heat and the lattice constant of Fe-Co alloys in the b.c.c. range have been measured. The specimens are prepared by the reduction of solid solution Fe_1−xCo_xC₂O₄·2H₂O in hydrogen at 800°C. It has been confirmed that FeCo (CsCl type) superlattice exists over a wide range (30∼75 at%Co), and the so-called “550°C anomaly” of Fe_0.5Co0.5 superlattice has been observed also in the alloys of 21∼60 at%Co. This anomaly appears at almost the same temperature in the composition range of 21∼50 at%Co (555°∼560°C), and its critical temperature falls rapidly when cobalt content is greater than 50 atomic per cent. There is a broad peak at about 555°C in specific heat curves of 21, 23, 25, 27 and 27.5 at%Co alloys. Formerly these peaks have been imagined due to the transition of Fe₃Co superlattice. Judging from the concentration dependence of its critical temperature and the shape of the specific heat curves, it is reasonable to consider that these peaks do not correspond to Fe₃Co superlattice but to the extension of the 550°C anomaly.
The effect of ordering on the lattice constant makes an increase in cell size in the 40∼60 at%Co alloys and a decrease in the 23∼40 at%Co alloys.
Neutron diffraction and Mössbauer measurement made on a slow cooled 25 at%Co alloy has shown no evidence for Fe₃Co superlattice.