Yasuto Nozawa

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 160 °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 120 °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 160 °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...

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. >

Magnetic properties of isotropic and anisotropic Nd-Fe-B bonded magnets

Abstract A trial to make Nd-Fe-B rapidly quenched flakes anisotropic by grain growth during heat treatment in the magnetic field has been carried out. No magnetic anisotropy is observed, but the optimum heat treatment temperature is lowered by about 100°C by applying 14 kOe of magnetic field. There is, as yet, no explanation of this phenomenon. Another way of making anisotropic bonded magnets is to use pulverized powders of anisotropic dense die-upset magnets as a starting material. By examination of the process parameters for Nd-Fe-B die-upset magnets it was found from stress-strain curves that the Nd 14 Fe 80 B 6 ternary composition has the best workability and to achieve aligned material the compression ratio (defined as the inverse of reduction in height) should be larger than 4. Grain coarsening resulted in a decrease in coercivity ( i H c ) during die upsetting. Ga is the best additional element for enhancing i H c with a little sacrifice of remanence ( B r ) and no deterioration in hot workability. Further improvement of thermal stability by raising the Curie temperature ( T c ) of the Nd-Fe-B-Ga alloy can be achieved by Co substitution. The i H c of the powder made by pulverizing the die-upset magnets is nearly independent of particle size. Anisotropic bonded magnets were developed using this powder. The ( BH ) max of compression-moulded magnet with Ga addition and Co substitution, Nd 14 Fe 71.75 Co 7.5 B 6 Ga 0.075 , is 15 MGOe. The irreversible losses after exposure up to 140°C is lower than 5% ( P c =2), which is sufficient thermal stability for polymer-bonded magnets.

Effect of coarse grains on magnetic properties of Nd-Fe-Co-B-Ga die-upset magnets

Coarse grains contained in Nd-FeCo-Ga-B hot-pressed and die-upset magnets are examined. Coarse grains contained in hot-pressed magnets are found to maintain their shapes during die upsetting and to increase deformation resistance B/sub r/, H/sub k/, and (BH)/sub max/ of die-upset magnets decrease with increase in the volume fraction of coarse grains. Equiaxed coarse grains nucleate and grow during melt spinning. Columnar coarse grains grow during hot pressing. It is shown that the volume fraction of coarse grains before die upsetting should be minimized to increase (BH)/sub max/. >

Rare earth magnets for applications over a wide temperature range

Abstract Sm2Co17-type sintered magnets and Nd-Fe-B die-upset magnets have been investigated over the temperature range 4.2–423 K. Their magnetic properties were compared with those of SmCo5 and Nd-Fe-B sintered magnets. This study indicates that the Sm2Co17-type magnets appear to be the best for applications at high temperatures up to 423 K. The magnetic properties of a Sm2Co17 magnet at 423 K are: Br = 10.8 kG, iHc = 15.7 kOe and (BH)max = 27.1 MGOe. The irreversible loss of the Sm2Co17 magnet at a permeance coefficient of 2 is 1.1% after exposure at 573 K for half an hour. Sintered Nd-Fe-B magnets exhibit better thermal stability compared to their die-upset conterparts. The second quadrant loop of the Nd-Fe-B magnets exhibit an anomalies behavior at 4.2 K due to the spin reorientation phenomenon which occurs at 140 K.

Nd-Fe-B anisotropic bonded magnets

To understand the role of the Ga addition which enhances coercivity in Nd14Fe79.25B6Ga0.75 die upset magnets, metallurgical investigations using Auger electron spectroscopy (AES) and energy-dispersive x-ray spectroscopy-transmission electron microscopy (EDX-TEM) were done. Ga has a tendency to be condensed in the Nd rich phase (higher than 2 wt%) and Ga content in the matrix is around 0.2 wt%. The Ga solution in these phases seems to strengthen the pinning force defined at the interface of the matrix and the Nd rich phase.To improve thermal stability further, the Co substitution for Fe in Nd14Fe79.25-uCouB6Ga0.75 (u= 0 to 20) magnets was investigated. The Co substitution up tou = 20 does not decrease coercivity, while for sintered magnets the Co substitution decreases coercivity very markedly. The irreversible losses foru = 0 and 7.5 after exposure at 180° C are over 10% and under 5%, respectively.Using pulverized Nd14Fe71.75Co7.5B6Ga0.75 die-upset magnets, anisotropic bonded magnets (compression molded) with 15 MGOe of (BH)m2 and 19 kOe ofHc were obtained. The irreversible loss is under 5% after exposure up to 140° C.