M. Tokunaga

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...

Sm2(Fe,M)17Nx compounds and magnets

The effect of additives has been studied for the Sm2Fe17Nx compounds. Only Co addition increases the Curie temperature. The Ti, V, or Co additions increase the anisotropy field. The additions of these elements are favorable for the magnet fabrication. The magnet fabrication has been carried out by the Zn metal bonding and mechanical alloying. The (BH)max is 8.7 MGOe and the iHc is 14 kOe for the Zn‐bonded magnet. The (BH)max is 10.8 MGOe and the iHc is around 25 kOe for the mechanical alloy magnet.

Micromagnetic studies of inhomogeneous nucleation in hard magnets

Abstract Nucleation fields of reversed domains in hard magnets with magnetic planar defect are calculated based upon micromagnetism. We show that the reversal processes are classified into three types depending on the parameters characterizing the defect. In a certain region of the parameters, there exists the case where the magnetization in the defect is forced to rotate reversely by an applied field which does not yet bring about the reversal rotation in the host matrices. We suggest that this situation gives a crucial role for the angular dependence of the nucleation fields in Ne-Fe-B sintered magnets.

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.

The influence of Ga-substitution on the coercivity of Nd-(Fe,Co)-B sintered permanent magnets

In Co-substituted Nd-Fe-B sintered magnets, several additional phases such as Nd(Fe,Co)/sub y/B/sub ?/ with y=2, 3, and 4 have been found besides the hard magnetic phase and the Nd-rich intergranular phase. The low coercivity of the magnet is attributed to the occurrence of these soft ferromagnetic phases. In Ga-doped Nd-(Fe,Co)-B sintered magnets, new intergranular phases such as Nd(Ga,Fe,Co) and Nd(Ga,Fe,Co)/sub 2/ have been found instead of the soft ferromagnetic Co-rich phases. From X-ray microanalysis data it is concluded that Ga substitutes the transition-metal atoms in the 2:14:1-phase, and Ga is not detected in the Nd-rich phase at all. The increase of the coercivity of Ga-doped magnets is correlated to a higher volume fraction of intergranular phases and an increased wetting during sintering due to these phases. >

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.

The effect of Co addition on the fracture strength of NdFeB sintered magnets

Abstract The bending strength and fracture toughness of the ternary and Co-containing NdFeB sintered magnets have been studied. The microstructure and the fracture behavior were examined utilizing micro-analysis. It is shown that the bend fracture of the NdFeB magnets is mainly intergranular. The Co addition with proper content increases the bending strength of NdFeB sintered magnets by about 80% and this is mainly due to the strengthening of Nd-rich grain boundary phase.

Studies on Nd(Fe/sub 0.92-x/B/sub 0.08/Ga/sub x/)/sub 5.5/ sintered permanent magnets

The temperature dependence of the hysteresis loop and the anisotropy field H/sub A/ of sintered magnets of the composition Nd(Fe/sub 0.92-x/B/sub 0.08/Ga/sub x/)/sub 5.5/ was investigated using pulsed-field equipment. Analysis based on the nucleation model was performed. The spin reorientation temperatures were determined by measurements of the temperature dependence of the initial AC susceptibility chi /sub ac/(T). It is shown that chi /sub ac/(T) curves can be used to determine the temperature where the FOMP (first-order magnetization process) transition starts. >

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/. >