Microstructural studies on Pr–Fe–B–Cu magnets produced by upset forging of cast ingot

Recent work has shown that Pr–Fe–B–Cu-type permanent magnets have been produced from cast ingot materials using upset forging and that intrinsic coercivity can be enhanced substantially by a post forging heat treatment at 1000°C for 5 h plus 500°C for 3 h. In the present work, the microstructures of both as-upset forged and heat treated magnets have been investigated by optical and by scanning electron microscopy (SEM) in an attempt to explain the origin of this increase in the intrinsic coercivity. Backscattered electron images on the SEM, energy dispersive X-ray analysis (EDX) and thermomagnetic analysis confirm the presence of Pr2Fe17 (Fe/Pr≈8.2) after the upset forging process and heat treatment at l000°C. The quantity of this phase is substantially reduced after the second heat treatment at 500°C. In addition a Pr34Fe62Cu4 (Pr6Fe13Cu) phase has also been observed by SEM and EDX analysis after the anneal at 500°C. The increase in the coercivity on annealing has been attributed to the grain boundary improvement isolation and to the reduction in the amount of the Pr2Fe17 phase after heat treatment at 500°C.