Constitution, structure and magnetic properties of some rare-earth-cobaltaluminium alloys

Abstract

The constitution and structure of the alloys represented by the formulae Ce1−x AlxCo5 and Pr1−xAlxCo5 (where 0⩽x⩽0.6) have been studied by X-ray diffraction and metallographic techniques. The substitution of between 1 and 3 at % Al (0.06⩽x⩽0.18) for Ce in CeCo5 produces a mixture of the 1∶5 and 2∶17 phases based on CeCo5 and Ce2Co17; there are two variations of the 2∶17 phase which are isostructural with the hexagonal Th2Ni17-type and rhombohedral Th2Zn17-type phases. At the composition Ce0.76Al0.24Co5 (4 at% Al) the alloy consists only of the 2∶17-type phases and metallographically the alloy is one phase in appearance. Further substitution of Al results in the precipitation of an fcc phase, based on the Co-Al solid solution, in the 2∶17 matrix. The crystal structures of the Pr1−xAlxCo5 alloys are very similar to those of the equivalent cerium alloys, although the ternary 2∶17-type phase has only the rhombohedral structure, as does the binary Pr2Co17 phase. The metallographic structures of the Pr alloys in the composition range 1 to 3 at% Al show significant differences from the corresponding Ce alloys. Determination of the Curie temperatures of theR1}-xAlxCo5 alloys (R= Ce and Pr) in the composition range 0⩽x⩽0.24 showed good agreement between the measured and published values for both the CeCo5 and PrCo5 phases. The Curie temperatures of the ternary 2∶17-type phases were approximately 50 K and 70 K lower than those of the binary, Ce2Co17 and Pr2Co17 alloys, respectively.