Development of high-coercivity state in high-energy and high-temperature Sm-Co-Fe-Cu-Zr magnets upon step cooling

The work compares the peculiarities of the high-coercivity state formation in the Sm-Co-Fe-Cu-Zr high-temperature and high-energy permanent magnets (HTPM and HEPM) in the course of the heat treatment with the stepwise decreasing temperature from 830 to 400 °C. Two types of magnets with varying Fe concentration, i.e., Sm(Co0.88-xFexCu0.09Zr0.03)7 with x = 0–0.12 (the HTPMs) and Sm(Co0.91-xFexCu0.06 Zr0.03)7.5 with x = 0.24–0.33 (the HEPMs) were studied at different temperatures of heat treatment for phase formation by x-ray diffraction followed by magnetic property measurements. Microstructure characterization was performed using transmission electron microscopy, whereas the three-dimensional elemental distribution at near-atomic scale was obtained using atom probe tomography. In HEPMs, the main increase in coercivity and relaxation of stresses accompanied by intensive enrichment of the 1:5 phase in Cu are observed at high temperatures (Т ≈ 700 °C). In HTPMs, the coercivity monotonously increases in the entire temperature range of the slow cooling from 700 to 400 °C at a rate of 0.5 °C/s. At the temperature close to the Curie temperature (∼550 °C) of the Sm(Co,Cu)5-type phase, the anomaly of the coercivity increment has been observed. The interphase stresses grow and the elemental redistribution appears to be accelerated simultaneously. The non-uniform Cu distribution in the 1:5 phase can be described by the formation of Cu-rich interlayers at the interface of the Sm(Co,Cu)5 and Sm2(Co,Fe)17-type phases