Microstructure and properties of step aged rare earth alloy magnets

Alloys with compositions Co-25.5 wt % Sm-8 wt % Cu-15 wt % Fe-3 wt % Zr and Co-Sm-Cu-Fe-1.5 wt % Zr have been step aged to produce magnets with coercive force (iHc) in the range of 10–25kOe, much higher than those reported so far in the literature for the Zr alloys. The high coercive force magnets 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 two 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 step aging does not change the morphology, but only changes the chemical composition of the two phases. Best properties are obtained in materials with a coherent microstructure of optimum boundary phase thickness and optimum chemical composition. The highest values of iHc obtained so far are ∼26kOe and ∼16kOe for the 3% Zr and 1.5% Zr alloys respectively. The best hard magnetic properties of (BH) max = 33 MGOe and iHc = 13kOe are for a 25% Sm-20%Fe-4 Cu-2% Zr alloy.