Effect of Nb and C additives on the microstructures and magnetic properties of rapidly solidified Sm-Co alloys

Highly coercive Sm-Co-based permanent magnets have been achieved through simple modification of binary Sm12Co88 alloys with Nb, C or combined Nb and C at concentrations ranging from 1 to 10 atomic percent processed via rapid solidification. Melt spinning at 40 m/s resulted in the formation of the metastable TbCu7-type structure in all alloys. While the unalloyed, as-solidified Sm12Co88 alloy displayed a coercivity of 0.5 kOe, alloying additions resulted in a systematic and profound increase in coercivity. Nb additions resulted in as-solidified coercivities up to 9 kOe, C additions up to 37 kOe, and combined NbC additons 8 kOe. The Nb and NbC additions led to a reduction in grain size, while C additions altered the morphology, producing a grain-boundary phase that effectively isolated the magnetic grains from one another. The magnetization processes for Nb- and NbC-modified Sm-Co were determined to be nucleation-controlled, while a transition to pinning-controlled magnetization was observed for the C-modified alloy.