This disclosure is directed to sintered bodies comprising grains and a grain boundary composition, wherein: (a) the grains comprise a composition substantially represented by a formula G2M14B, where G is Nd, Dy, Pr, Tb, or a combination thereof, and M is Co, Fe, Ni, or a combination thereof, wherein the grains are optionally doped with one or more rare earth elements; and (b) the grain boundary composition is an alloy composition substantially represented by the formula: Nd8.5-12.5Dy35-45Co32-41Cu3-6.5Fe1.5-5, wherein the subscript values are atom percent relative to the total composition of the alloy composition. Corresponding populations of particles are also disclosed.