Kazuhiro Kumasaka1 and Kenichi Ono1
1Ibaraki Electrical Communication Laboratory, Nippon Telegraph and Telephone Public Corporation, Ibaraki
Magnetic properties have been investigated in Fe-Co-W alloys containing less than 15%W and less than 40%Co. Specimens were aged at 823∼1273 K after cold swaging by 0∼96% reduction in area. The age-hardening characteristics for these alloys have been observed by measuring electrical resistivity, Vickers hardness, X-ray diffraction, thermal dilatation and electron microscopy. The results obtained are summarized as follows.
(1) Residual induction Br, squareness ratio Br/B22.3 and coercive force Hc are raised to maximums by aging near 973 K. These maximum magnetic values become larger with increasing cold swaging reduction in area.
(2) With increasing W content, Br/B22.3 and Hc are increased and Br has a maximum at about 13%W. These magnetic properties raised to the maximums at about 20%Co. Magnetic properties with Br: 1.80∼1.92 T and Hc: 5∼8 kA/m are obtained in more than 15%Co and more than 10%W. For instance, Fe-20%Co-13%W alloy shows Br: 1.92 T, Br/B22.3: 0.93 and Hc: 5.5 kA/m.
(3) Increasing Vickers hardness and Hc by aging is accompanied with decreasing lattice constant and electrical resistivity of the matrix. Vickers hardness raised to maximum at the aging stage when lattice constant and electrical resistivity close near constant values. Hc reaches a maximum at a more advanced aging stage. It can be considered that the maximum Hc is obtained at the precipitation growth stage. The activation energy in the Hc increasing stage is in the range 290∼340 kJ/mol. This indicates that Hc is raised by the precipitation which is controlled by the W atoms diffusion rate.
(4) Precipitates of the aged alloy after cold rolling are very fine, compared with precipitates of the aged alloy after quenching. This indicates that the fine precipitate structure increases Hc.
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