Alloys with Compositions Co-25.5wt/o Sm-Sw/o Cu-15 w/o Fe-3w/o Zr and Co-Sra-Cu-Fe-1.5 w/o Zr have ibeen step aged to produce magnets with coercive force (iHc) in the range of 10-25k0e, much higher than those reported so far in the litera ture for the Zr alloys. The high coercive force magnet s are typically aged at 800-850°C for 10-30 hours following the solution treatment at 1150°C. Subsequently, these are step aged to produce materials with high coercivity. The microstructure in all these alloys has a 2 phase cellular morphology with 2:17 phase surrounded by a 1:5 boundary phase. The long aging treatments at 800-850°C lead to coarsening of the two phase structure. The subsequent stepaging does not change the morphology, but only changes the chemical composition of th:* two phases.Best properties are obtained in materials with a cohe rent microstructure of optimum boundary phase thickness and optimum chemical composition. The highest values of iHc ob tained so far are % 26k0e and a. 16 kOe for the 3% Zr and 1.52 Zr alloys respectively. The (best hard magnetic properties of (B?0 max = 33 MGOe and iHc = 13k0e are for a 25% Sm-20%Fe-4 Cu-2%Zr alloy.

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