Hybrid nanograin rare earth magnets with improved thermal stability

Attempts to fill a wide gap in thermal stability between Nd–Fe–B and Sm–Co based magnets by making a conventional hybrid Nd15Fe79B6∕Sm(Co,Fe,Cu,Zr)7.4 type of magnets encountered two technical difficulties: incompatible processes and interdiffusion between two materials. Because a fundamental change in coercivity mechanism occurs when reducing the grain size in these magnets to nanometer range, high coercivity can be obtained in both nanograin Sm2(Co,Fe)17 and Nd2Fe14B after a short anneal at ∼700°C. Thus, the processes of making two different nanograin magnets are completely compatible. Further, very short time of elevated temperature processing can minimize the interdiffusion. Applying this approach, hybrid nanograin Nd2Fe14B∕Sm2(Co,Fe)17 magnets with (BH)max=27.4MGOe and HcM=14kOe were made using rapid inductive hot compaction and hot deformation. In addition, better magnetic performance of (BH)max=34.5MGOe and HcM=16.9kOe was obtained in a hybrid nanograin Pr2(Fe,Co)14B∕Pr(Co,Fe)5 (80wt%∕20wt%) magnet.