Phase evolution and its effects on the magnetic performance of nanocrystalline SmCo7 alloy

The evolution of the phase constitution and the microstructure, as well as their effects on magnetic performance, were investigated systematically using a prepared nanocrystalline single-phase SmCo7 alloy as the starting material for a series of annealing processes. The SmCo7 (1:7 H) phase was discovered to have a good single-phase stability from room temperature up to 600 °C. The destabilization of the SmCo7 phase results in the formation of the Sm2Co17 (2:17 R) and SmCo5 (1:5 H) phases, which exist as phase-transformation twins and particulate precipitates, respectively, with a completely coherent relationship with the 1:7 H parent phase. For the first time the formation mechanism of the 2:17 R phase-transformation twins has been proposed, in which the ordered substitution of 1/3 of the Sm atoms by Co-Co dumbbell pairs along two particular crystal directions was demonstrated. The characteristic width values of the 2:17 R phase-transformation twins, as deduced from this model of the mechanism, were unambiguously verified by the experimental results. Among the SmCo7 alloys with various phase constitutions and microstructures, the best magnetic properties were obtained in the nanocrystalline 1:7 H single-phase alloys. The present work may promote a new understanding of nanoscale-stabilized single-phase SmCo7 and its potential applications as unique high-temperature permanent magnets.