METALLOGRAPHIC STUDY ON ND-FE-CO-B SINTERED MAGNETS

Metallographic study has been carried out on the Nd-Fe-Co-B alloys to clarify the change in coercivity with the Co composition substituted for Fe in the Nd//2Fe//1//4B type sintered magnets. The decrease in coercivity caused by the Co substitution is attributed to the formation of soft-magnetic phases such as Nd(Fe, Co)//2 and Nd(Fe, Co)//3. The mole ratio of Nd to (Fe plus Co) of these soft-magnetic phases decreases with increasing the Co substitution. The Co concentration is much higher in these soft-magnetic phases than in Nd-Fe-Co-B magnet makes the Nd(Fe, Co)//2 compound unstable to form the non-magnetic Nd//3(Fe, Co, Al) compound, resulting in the increase in the coercivity of Nd-Fe-Co-B magnet.