H. Harada

Nd‐Fe‐B die‐upset and anisotropic bonded magnets (invited)

The hot working conditions were examined for Nd‐Fe‐B die‐upset magnets. The Nd14Fe80B6 ternary composition has the best workability (die upsetting) from stress‐strain curves for the initial stage of die upsetting at hot working temperature. The compression ratio defined as the inverse of reduction in height should be larger than 4 to have materials aligned. Alignment of c axis along the compressive stress direction is obtained, but grain coarsening occurs during die upsetting. Of several additional elements investigated, Ga is the best for enhancing the coercivity with a little sacrifice of remanence and no deterioration in hot workability. Typical magnetic properties for Nd14Fe79.25B6Ga0.75 die‐upset magnets are Br =12.4 kG, iHc =19 kOe, and (BH)max =36 MGOe. The irreversible loss after exposure up to 160u2009°C is under 10% (Pc =2). The iHc of powder made by crushing the die‐upset magnets is nearly independent of particle size. Decrease in iHc by pulverizing to several hundred micron is about 2 kOe. Anisotropic bonded magnets were developed using this powder. Best energy products of injection and compression molded magnets with Ga addition are 10 and 15 MGOe, respectively. The irreversible losses after exposure up to 120u2009°C is lower than 5% (Pc =2), which is enough thermal stability for polymer‐bonded magnets.The hot working conditions were examined for Nd‐Fe‐B die‐upset magnets. The Nd14Fe80B6 ternary composition has the best workability (die upsetting) from stress‐strain curves for the initial stage of die upsetting at hot working temperature. The compression ratio defined as the inverse of reduction in height should be larger than 4 to have materials aligned. Alignment of c axis along the compressive stress direction is obtained, but grain coarsening occurs during die upsetting. Of several additional elements investigated, Ga is the best for enhancing the coercivity with a little sacrifice of remanence and no deterioration in hot workability. Typical magnetic properties for Nd14Fe79.25B6Ga0.75 die‐upset magnets are Br =12.4 kG, iHc =19 kOe, and (BH)max =36 MGOe. The irreversible loss after exposure up to 160u2009°C is under 10% (Pc =2). The iHc of powder made by crushing the die‐upset magnets is nearly independent of particle size. Decrease in iHc by pulverizing to several hundred micron is about 2 kOe. Anisotr...

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.

Magnetic properties and thermal stabilities of Ga substituted Nd-Fe-Co-B magnets

The substitution effect of 20 kinds of elements has been studied in order to increase the intrinsic coercive force for the Nd-Fe-Co-B alloys. The substitution of Ga is found to increase the coercivity more than any other elements. The compositions which are suitable for both magnetic properties and thermal stabilities have been examined. The Ga substitution is compared with the Al and Dy substitutions whose elements are well-known for increasing coercivity. The Ga substitution shows better magnetic properties and lower irreversible losses than the Al and Dy substitutions for the Nd(Fe 0.72 Co 0.2 B 0.08 )5.6 alloy. The magnetic properties and composition which achieve less than 5 % irreversible loss after exposure at 140°C and the permeance coefficient of -2 are as follows; 4πIr = 12.2 kG, i H c = 16.5 kOe, (BH)_{max} = 35.2M MGOe and Nd(Fe 0.7 Co 0.2 B 0.08 Ga 0.02 )5.6.

Ga added Nd-Fe-B sintered and die-upset magnets

The addition of Ga to Nd-Fe-B sintered and die-upset magnets in order to improve thermal stability was examined. The Ga addition increases the coercivity of Nd(Fe/sub 0.72/Co/sub 0.2/B/sub 0.08/)/sub 5.6/ sintered magnets markedly. Ga addition does not enhance coercivity of sintered magnets with lower Co substitution. Ga addition increases coercivity for Nd-Fe-B die-upset magnets. No decrease in coercivity due to Co substitution of up to 20 at.% is observed for Nd/sub 14/Fe/sub 79.25/B/sub 6/Ga/sub 0.75/ die-upset magnets. This is an advantage of the die-upset magnets compared with the sintered magnets. Ga has a tendency to occur in the Nd-rich phase at higher concentrations than 2 wt.%, and Ga content in the 2/14/1 matrix is around 0.2 wt.% for Nd/sub 14/Fe/sub 79.25/BGa/sub 0.75/ die-upset magnets. For sintered magnets, Ga is distributed between the matrix and the Nd-rich phase. H/sub A/ of the 2/14/1 matrix is increased by Ga addition up to 2.5 at.%. The correlation between H/sub A/ and iH/sub c/ is not good. The Ga-containing Nd-rich phase is observed in the sintered magnets, and the role of Ga in this phase needs to be evaluated to understand coercivity enhancement by Ga addition. >

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.

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.

Structure and magnetic properties of sputtered Co-Si alloy films

An interesting behavior of Si as an additive to Co alloys is expected, since Si is supposed to change the structure and the magnetic properties of Co alloys as a metalloid element. From the above reason, the relationship between the microstructures and the magnetic properties of the sputtered Co‐Si alloy films was investigated in detail. As the Si content was increased to 35 at.u2009% Si, such changes of magnetic properties were found that the coercive force of the sputtered Co‐Si alloy films reached the peak of about 550 Oe at 6–10 at.u2009% Si, below or above which the coercive force is relatively small. On the other hand, according to the transmission electron microscopic observation at relatively low magnification, the grain size of α‐Co crystallites forming the films was found to decrease as the Si content increases. The results of Lorentz electron microscopy showed large magnetic domains in the films with a Si content below 6 at.u2009%, very fine magnetic domains in the films with 6–10 at.u2009% Si content, and the...