Microstructures of precipitation-hardened SmCo permanent magnets

The microstructural features of isothermally aged and step aged and isothermally aged Sm(Co, Cu ,Fe, Zr)7.4 alloys are examined using electron microscopy and x-ray microanalysis. The microstructure is seen to consist of a cellular morphology with twinned rhombohedral 2:17 phase cell interiors, 1:5 phase cell boundaries, and elongated thin platelets with a 1:3 structure. Upon prolonged aging, Zr preferentially goes into the 1:3 phase, Cu to both the 1:5 and the 1:3 phase, and Fe to the 2:17 phase. Isothermal aging at 850 °C facilitates the growth of the cells and the low-temperature step aging facilitates the chemical partitioning of the transition metals. The role of Zr in developing the morphology and the chemical partitioning is discussed.