Microstructure of R-Fe-B sintered magnet

Abstract

The microstructure including grain boundaries of (Nd0.86Dy0.14)(Fe0.92B0.08)z sintered magnets was studied using FE-SEM, AES and TEM. It is revealed that there is a composition fluctuation in the R rich phase, and that there is a BCC phase with a = 0.29nm which is coherent with the R <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> Fe <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">14</inf> B matrix between the R rich phase and the matrix. The change of iHc and the microstructure with composition and heat treatment for(Nd0.86Dy0.14) (Fe0.92B0.08)z sintered magnets(z=4.0, 5.0, 5.4 and 6.2) was investigated, iHc increased as the z-value decreased from 6.2 to 4.0. The width of the BCC phase which surrounds R <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> Fe <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">14</inf> B grains has a tendency to become greater with decreasing z-value from z=5.4 to 4.0. No BCC phase was observed for z=6.2, where iHc was only 200 Oe. There were many BCC platelets in specimens with z=4.0, 5.0 and 5.4 which had been heat treated at 900°C for two hours and cooled gradually to room temperature. After a final heat treatment near 600°c, the BCC platelets disappeared, except for z=5. The substitution of Co for Fe in the alloy (Nd0.8Dy0.2)(Fe0.92-xCoxB0.08)5.5 was studied in the range x=0 to 0.10. An irreversible loss of under 5% after exposure at 200°C (Pc=2) was obtained with x=0.06 and Tc=380°C. For the alloy with x=0.06, the irreversible loss was found to depend on the final heat treating temperature, T <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> . A specimen heat treated for one hour at 600°C and water quenched showed the lowest irreversible loss. The Tc of the BCC phase for T <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> =600°c was 330°C and the higher Tc of the BCC phase is required to have higher thermal stabilities.