A study of PrFeBCu permanent magnetic alloys

Abstract

The microstructures and magnetic properties of alloys Pr 20.5Fe 74.0B 3.5Cu 2.0 (A) and Pr 17.0Fe 76.5B 5.0Cu 1.5 (B) have been investigated. In the cast condition the microstructure of alloy A consisted of three phases: a Pr 2Fe 14B matrix phase, a praseodymium-rich grain boundary phase and free iron inside the matrix phase. Little of the PrFe 4B 4 (approximate composition) boride phase which is frequently observed in RE-Fe-B-type alloys (RE, rare earth) was observed in alloy A. The microstructure of alloy B in the cast state consisted of four phases: the same three phases as those observed in alloy A together with the PrFe 4B 4 phase. A greater amount of the grain boundary phase and a smaller grain size were observed in alloy A compared with alloy B. In annealed (1000 °C for 5 h) and slowly cooled alloy A an extra phase with needle-like or irregular form was observed in the grain boundary region. The ratio Pr:Fe in the extra phase was analysed to be about 1:2 and this phase was suggested to be a PrFe 2-type phase (Pr(Fe 0.94Cu 0.06) 2). In alloy B the microstructure was not altered radically by annealing except for the disappearance of the free iron. Alloy A exhibited much better permanent magnetic properties than alloy B and this was attributed to the smaller grain size and improved magnetic isolation of the individual grains. The effects of low temperature annealing on the magnetic properties at 500 °C have also been investigated.