Hidefumi Kishimoto

Coercivity enhancement in Ce-Fe-B based magnets by core-shell grain structuring

Ce-based R2Fe14B (R= rare-earth) nano-structured permanent magnets consisting of (Ce,Nd)2Fe14B core-shell grains separated by a non-magnetic grain boundary phase, in which the relative amount of Nd to Ce is higher in the shell of the magnetic grain than in its core, were fabricated by Nd-Cu infiltration into (Ce,Nd)2Fe14B hot-deformed magnets. The coercivity values of infiltrated core-shell structured magnets are superior to those of as-hot-deformed magnets with the same overall Nd content. This is attributed to the higher value of magnetocrystalline anisotropy of the shell phase in the core-shell structured infiltrated magnets compared to the homogeneous R2Fe14B grains of the as-hot-deformed magnets, and to magnetic isolation of R2Fe14B grains by the infiltrated grain boundary phase. First order reversal curve (FORC) diagrams suggest that the higher anisotropy shell suppresses initial magnetization reversal at the edges and corners of the R2Fe14B grains.

Microstructural and magnetic properties of Nd-Fe-B alloys processed by equal-channel angular pressing

Equal-channel angular pressing (ECAP) is a well-established thermo-mechanical processing technique. This technique allows virtually unlimited strain and manipulation of texture by processing route, while the cross-section of the sample remains unchanged during processing. In order to clarify the effectiveness of ECAP on preparing anisotropic permanent magnets, the microstructure and magnetic properties of a melt-spun Nd13.5Fe73.8Co6.7B5.6Ga0.4 alloy processed at 773 K for 300 s by ECAP were investigated. Macrotexture analysis carried out for the exit channel of ECAP shows that the basal plane of the tetragonal Nd2Fe14B crystal aligns parallel to the shear band, i.e., the c-axis texture formation normal to the shear band induced by the ECAP process. Due to this texture formation, the technical magnetization behaviour becomes anisotropic, and the remanent magnetization is clearly enhanced along the direction perpendicular to the shear band. This anisotropic microstructure is realized at a relatively low pro...

Low temperature texture development in Nd2Fe14B/α-Fe nanocomposite magnets via equal channel angular pressing

While suitable texture has been developed in Nd2Fe14B/α-Fe nanocomposites via thermomechanical processing methods such as die upsetting by incorporating low melting point eutectic Nd-Cu additives, significant grain coarsening occurs during this process due to the high temperature and long timescales involved, resulting in a loss of exchange coupling. Equal channel angular pressing (ECAP) is a severe plastic deformation technique which has been successfully used to produce a suitable texture in single-phase Nd2Fe14B at temperatures on the order of 500°C while preserving grain sizes on the order of 20-30nm. We investigate the development of texture in a commercial Nd2Fe14B/α-Fe nanocomposite alloy with added Nd90Cu10 produced via ECAP and then characterise it using texture x-ray diffraction and magnetic measurements. It is found that initial texture can be developed in this nanocomposite system at T = 520°C via ECAP. The average grain size of Nd2Fe14B as measured via X-ray diffraction after ECAP remains bel...

The effect of Cu-based core-sheath configurations on the processing of Nd-Fe-B-based permanent magnets via equal-channel angular pressing

Equal channel angular pressing (ECAP) has been used as an alternative manufacturing route for preparation of Nd2Fe14B-based anisotropic magnets, facilitating processing temperatures much lower than conventional die upsetting. While this method can produce a suitable texture and microstructure in permanent magnetic materials, it still remains novel; involving extremely high pressures which present a high risk of both process failure and die and tooling damage. Powder metallurgical processes frequently incorporate an external layer of secondary material (commonly an outer foil layer or can) for separation between the primary material and die as well as the control of surface effects such as friction through appropriate choice of secondary material. This work implements such modifications to this manufacturing route by incorporation of an outer layer of Cu foil, the addition of which negatively affected both the powder compaction and strength of texture produced via ECAP. Also investigated was the incorporation of solid Cu bar as part of the sample cross section. This modification facilitated processing without any compromise on observed magnetic properties, whilst also reducing damage to both the die and tooling. This type of methodology may aid in improving the reliability of producing bulk anisotropic permanent magnets via ECAP.

Effect of Processing Parameters on the Magnetic Properties and Macrotexture of a Nd 13.5 Fe 73.8 Co 6.7 B 5.6 Ga 0.4 Alloy Processed by Equal Channel Angular Pressing With Back Pressure

Equal channel angular pressing (ECAP) is a well-established thermo-mechanical processing technique, which could induce the c-axis texture of Nd₂Fe₁₄B in a melt-spun Nd_13.5Fe_73.8Co_6.7B_5.6Ga_0.4 alloy. However, the effects of ECAP processing parameters, such as temperature, back pressure (BP), and multiple-pass ECAP routes, remain unknown for this alloy. In this paper, we have investigated the effects of these processing parameters on the c-axis texture formation. It is found by X-ray diffraction macrotexture analysis that the maximum intensity of (001) pole figures for the tetragonal-Nd₂Fe₁₄B phase (I_max) shows an increase from 2.7 to 4.1 m.r.d. (multiples of random distribution) by increasing the ECAP temperature from 723 to 823 K, while the difference in remanent magnetization between easy and hard directions (AMr) rises from 24.0 to 41.5 Am²/kg. When the BP was increased from 0.25 to 0.5 GPa at 823 K, I_max showed an increase from 2.8 to 4.1 m.r.d. However, I_max saturated for BPs above 0.5 GPa, suggesting that BP has limited effect on the texture formation, although it is necessary for the compaction of the alloy powders. Two multiple-pass ECAP routes conventionally known as routes A and C were employed for two-pass ECAP at 823 K. It is found that route A processing is effective in enhancing the texture formation, while the texture is lost by a subsequent pressing when adopting route C. Therefore, the compaction of Nd_13.5Fe_73.8Co_6.7B_5.6Ga_0.4 alloy powder using route A ECAP passes with 0.5 GPa BP at 823 K results in pronounced texture, which is beneficial for anisotropic hard magnetic properties.