Ageing effects in sintered (Nd,Dy)/sub 15/(Fe,Co,Mo,Al)/sub 77/B/sub 8/ permanent magnets

The effect of annealing on the coercivity /sub j/H/sub c/ of sintered Nd-Fe-B is contradictorily discussed in literature, /sub j/H/sub c/ depends on annealing temperature T/sub A/, annealing time, cooling rate, and details of the chemical composition. In the case of (Nd,Dy)/sub 15/(Fe,Co,Mo,Al)/sub 77/B/sub 8/ a decrease of /sub j/H/sub c/ is observed for T/sub A/>or=450 degrees C. Passing a strong minimum at T/sub A/=575 degrees C and a maximum at T/sub A/ approximately=800 degrees C, /sub j/H/sub c/ approaches its post-sintering value at higher annealing temperatures. Above the minimum, /sub j/H/sub c/ varies practically reversibly if T/sub A/ is increased or decreased. The ratio of coercivities measured after annealing at 750 degrees C and 575 degrees C increases with increasing measuring temperature and is more than 5 above 150 degrees C. Analyzing the behavior of /sub j/H/sub c/ by /sub j/H/sub c/=cH/sub A/-N/sub e/ mu /sub 0//sup -1/J, it is found that c=0.43 for T/sub A/=750 degrees C and c=0.29 for T/sub A/=575 degrees C, but N/sub e/=1.4 is independent of T/sub A/.<>