Study of SmTM_7.3 microstructure by a simplified method and its correlation to properties

By using a newly developed potentiostatic etching technique, the microstructural evolution of an SmTM_7.3 (TM=Co/Fe/Cu/Zr) alloy during different stages of heat treatment has been easily and successfully observed under a conventional scanning electron microscope (SEM). Specimens up to several centimeters were examined, with their features being statistically measured. It is found that the cell structure has been well developed with a median size of 110 nm after aging 2 h at 1123 K, while the intrinsic coercivity ( _iH_c) was as low as 2 kOe. The cell size grows slowly during subsequent heat treatment to a median size of 220 nm after the whole treatment. For aging at 1123 K for 4 h, _iH_c value is around 4 kOe, and the platelet phase appears. After aging at 1123 K for 8 h, a large zonal phase and grain boundary phase are visible. It is thus believed that the zonal phase appears after aging for a time period between 4 and 8 h, corresponding to the _iH_c of 600 kA/m (7.5 kOe). No new microconstituent appears after the isothermal aging for more than 8 h, however growth proceeds. After the whole heat treatment cycle, _iH_c is much greater than 2400 kA/m (30 kOe), with the zonal phase ranging in size from 300 to 900 nm (in width, averaged 400 nm), 1 to 5 μm in length (averaged 3.6 μm), and 1 to 6 μm in separation (averaged 3.5 μm). The zonal and the grain boundary phases are thus concluded to be the dominating structure for the extremely high coercivity of this alloy. A model is proposed to explain the etching sequence of the microconstituents during the potentiostatic etching, and is successful to explain the resultant microstructure.