Masaaki Tokunaga

Some heat treatment experiments for Nd-Fe-B alloys

Since the Curie temperature (Tc) of R-Fe-B permanent magnets is lower than conventional 1/5 and 2/17 magnets, the irreversible loss due to the change of coercivity is critical when considering their application. The simple way to guarantee the thermal properties is to raise the coercivity at room temperature. The effect of heat treatment on coercivity was studied. The typical heat treatment is as follows: (1) The first heating at T 1 , 900°C × 2 hrs is followed by the continuous cooling at the rate of 1.3°C/min to room temperature. (2) The second heating at T 2 , near the eutectic temperature for 1 hr is followed by quenching. When employing this heat treatment, the following magnetic properties were obtained for Nd(Fe 0.92 B 0.08 )6. Br=13800 G, bH c =9150 Oe, iHc=9200 Oe and (BH)max=44.0 MGOe.

Improvement of thermal stability of Nd-Dy-Fe-Co-B sintered magnets by additions of Al, Nd and Ga

To improve the thermal stability of (Nd 0.8 Dy 0.2 ) (Fe 0.86 Co 0.06 B 0.08 )5.5 sintered magnets for heavy duty applications, such as starter motors, effects of additional elements(M=Al, Nb and Ga) On the thermal stability were investigated in the (Nd 0.8 Dy 0.2 ) (Fe 0.86-u -Co 0.06 B 0.08 M u )5.5 sintered magnets. The addition of Al increased i H c , but did not decreased irreversible losses at 220°C(Pc= 2). The sintered magnet with M=Nb and u=0.006 had lower i H c and higher irreversible losses than u=0, while the sintered magnet with M=Nb and u=0.015 had higher i H c and lower irreversible losses. Additions of Ga were resulted in higher i H c and lower irreversible losses. A combination of both Nb and Ga additions was also studied and a very low irreversible loss of under 5 % after exposure at 260°C(Pc= 2) was obtained for (Nd 0.8 Dy 0.2 ) (Fe 0.835 Co 0.06 B 0.08 Nb 0.015 Ga 0.01 )5.5 sintered magnets. The obtained magnetic properties for the magnet are B r =10450 G, b H c =10000 Oe, i H c =27100 Oe and (BH)_{max}=26.2 MGOe.

Microstructure of R-Fe-B sintered magnet

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 2 Fe 14 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 2 Fe 14 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 2 . 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 2 =600°c was 330°C and the higher Tc of the BCC phase is required to have higher thermal stabilities.

Effect of Nb additions on the irreversible losses of Nd-Fe-B type magnets

While Nd-Fe-B magnets have outstanding magnetic properties at room temperature (1) and below, their usefulness at higher temperatures is limited by three factors: irreversible loss, reversible loss of B r and reversible loss of H ci . This work examines the beneficial effects of Nb additions to Nd-Fe-B magnets, in a particular to reduce the irreversible loss.

Evidence for domain wall pinning by a magnetic grain‐boundary phase in sintered Nd‐Fe‐B based permanent magnets

Low‐field thermomagnetic analysis of sintered Nd(Dy)‐Fe(Co)‐B magnets provides new evidence that the high coercivity near room temperature is due to the pinning of residual domain walls in a ferromagnetic grain‐boundary phase. The temperature where MHc drops to near zero appears to be the Curie point of that phase. It is 50–80u2009°C below the Tc of the 2‐14‐1 matrix.

Thermal stability and corrosion resistance of HDDR processed Sm-Fe-Ti-B-N bonded magnets

Thermal stability of HDDR processed Sm-Fe-Ti-B-N bonded magnets is investigated in comparison with HDDR processed Sm-Fe-N bonded magnets. Temperature coefficients of Br and iHc of Sm-Fe-Ti-B-N bonded magnet are -0.06%//spl deg/C and -0.43%//spl deg/C, respectively, in the temperature range of 25 to 100/spl deg/C range. Irreversible loss of Sm-Fe-Ti-B-N bonded magnet at 100/spl deg/C is -2.1% and -2.5% respectively after exposure for 2 hrs and 300 hrs. This irreversible loss of Sm-Fe-Ti-B-N is lower than that of Sm-Fe-N which has coercivity 16 kOe. The corrosion resistance of Sm-Fe-Ti-B-N powder is evaluated by measuring increase in oxygen content after exposure at 120/spl deg/C. It is compared with Sm-Fe-N and Nd-Fe-Co-B powders. Sm-Fe-Ti-B-N powder shows slight increase in oxygen content after exposure at 120/spl deg/C in air for 300 hrs. It is also found to have good thermal stability and corrosion resistance.

Uniaxially anisotropic Nd-Fe-B magnets produced from the boronization of Nd-Fe powders

Abstract Uniaxially anisotropic Nd-Fe-B magnets have been obtained from the boronization of magnetically aligned Nd-Fe powders. The powders of B, Fe 3 B 7 and Fe 5 B 5 have been used as a boron source. The X-ray diffraction spectra indicate that the c -axis of Nd 2 Fe 17 lies perpendicular to the direction of the alignment field, and after the boronization treatment the c -axis of Nd 2 Fe 14 B 1 lies parallel to the direction of the alignment field. The boronization by Fe 3 B 7 has shown good orientation of the easy axis compared to those by B and Fe 5 B 5 . The addition of a small amount of B into the Nd-Fe alloys strikingly improves the orientation of the easy axis, and the resulting magnetic properties are improved. Typical magnetic properties obtained from Nd 1.65 Fe 76.5 +B 7 and Nd 16.5 Fe 76.5 B 1 +B 6 reactions are B r of 0.94 and 1.24 T, H ci of 0.74 and 0.67 MA/m, and ( BH ) max of 146 and 279 kJ/m 3 , respectively.

Magnetizability of Nd-Fe-B-type magnets with Dy additions

Many alloying additions have been made into Nd‐Fe‐B‐type magnets to alter their permanent magnet properties, in particular for applications above 100u2009°C. To this end, a common practice has been to add Dy, increasing Hci [M. Sagawa, S. Fujimura, H. Yamamoto, H. Matsuura, and K. Hiraga, IEEE Trans. Magn. 20, 1584 (1984); M. Tokunaga, M. Meguro, M. Endoh, S. Tanigawa, and H. Harada, ibid. 20, 1964 (1985)]. It is not unusual to find Hci >20 kOe in these substituted alloys. This approach has caused a dilemma. In some cases, increasing Hci above 20 kOe makes the alloy more difficult to magnetize and therefore less useful when the field available for magnetizing is 25 kOe or less. We have examined the effect of various alloying additions and heat treatment on the magnetizability of substituted Nd‐Fe‐B alloys. We show that high Hci at room temperature is not a necessary requirement to have Hci >6 kOe at 150u2009°C. We discuss the factors affecting the magnetizability of Nd‐Fe‐B‐type magnets.

Rapidly Quenched Nd-Fe-B Magnets

Over-quenched Nd-Fe-B flakes for isotropic bonded magnets show high coercivity (iHc) by post-heat treatment. This flake has the lowest temperature coefficient of iHc around -0.45/°C and the smallest grain size in the Nd-Fe-B materials. Enough thermal stability and magnetizability are obtained for isotropic bonded magnets with these characteristics of the flake. The Nd14Fe80B6 ternary composition has the best hot workability (die upsetting) from stress-strain curves for the initial stage of die upsetting at hot working temperatures. Alignment of c-axis along die upsetting direction is obtained, but decrease in iHc due to grain coarsening is resulted. Ga is the best for enhancing iHc without deterioration in hot workability. Typical magnetic properties of Nd14Fe79.25B6Ga0.75 die-upset magnets are (BH)max=36 MGOe and iHc=19 kOe. The iHc of powders made by pulverizing the die-upset magnet is nearly independent of particle size. Anisotropic bonded magnets are developed using this powder. Typical magnetic properties of Nd-Fe-B-Ga compression moulded magnets are (BH)max=15 MGOe and iHc=19 kOe.