D. Hinz

Hot deformation of nanocrystalline Nd-Fe-B alloys

Abstract Commercial melt-spun powders MQP-A and MQP-B were hot compacted in vacuum. Thereafter (i) die-upsetting experiments were performed under an argon atmosphere at 500 to 800 °C with strain rates of 10 −4 to 10 −1 s −1 or (ii) radially textured ring magnets were made by backward extrusion. The remanence achieved for die-upset MPQ-A samples is B r = 1.3 T (compared to 0.8 T obtained after hot-compaction). In the case of ring magnets typical values of B r , μ 0 H cj and ( BH ) max are 1.25 T, 1.1 T and 280 kJ m −3 , respectively. The deformation and formation-of-texture processes can be explained by the model of solution-precipitation creep. The main driving forces of the deformation processes are the chemical potentials of the atomic species in the liquid phase or at the surfaces of the crystallites. The activation energies of these processes are 280 and 400 kJ mol −1 for MQP-A and MQP-B samples, respectively. The strain rate was found to be proportional to σ 3 and d −1 where σ is the flow stress and d is the mean grain size of the sample. These values indicate an interface-controlled solution-precipitation process. The liquid grain-boundary phase does not seem to be required for deformation and texturing. However its presence is necessary for a crack-free deformation at high strain rates.

Absence of magnetic domain wall motion during magnetic field induced twin boundary motion in bulk magnetic shape memory alloys

A detailed study of twin boundary motion in NiMnGa single crystals together with in situ magnetic domain observation is presented. Optical polarization microscopy in connection with a magneto-optical indicator film technique was used to investigate the reorganization of the magnetic domains during twin boundary motion over a wide magnetic field range. Images at different field strengths demonstrate that no magnetic domain wall motion within the twins takes place, even during the structural reorientation by twin boundary movement. This absence of interaction of magnetic and structural domains is different from currently proposed models, which assume domain wall movement under an external field.

Evolution of interaction domains in textured fine-grained Nd2Fe14B magnets

The magnetic microstructure of thermomechanically processed (die-upset) melt-spun NdFeB magnets with different degrees of texture in thermally demagnetized and in dc-demagnetized states has been investigated systematically using magnetic force microscopy. An isotropic magnetic microstructure with a fine contrast on a scale between 200 and 400 nm has been clearly observed in the hot-pressed precursor in the thermally demagnetized state. Broad and well-pronounced interaction domains with lateral expansion over many individual grains have been found in the magnet with a maximum degree of texture. The crystallographic texture of this magnet has been analyzed by electron backscatter diffraction using a high-resolution scanning electron microscope. On a mesoscopic scale, most of the crystallites have a misorientation smaller than 10 deg, yielding macroscopically a high value of remanent magnetization (1.34 T). The presence of a threshold value in dependence of the size of the interaction domains on the degree o...

Nanocrystalline high performance permanent magnets

Abstract Recent developments in nanocrystalline rare earth–transition metal magnets are reviewed and emphasis is placed on research work at IFW Dresden. Principal synthesis methods include high energy ball milling, melt spinning and hydrogen assisted methods such as reactive milling and hydrogenation-disproportionation-desorption-recombination. These techniques are applied to NdFeB-, PrFeB- and SmCo-type systems with the aim to produce high remanence magnets with high coercivity. Concepts of maximizing the energy density in nanostructured magnets by either inducing a texture via anisotropic HDDR or hot deformation or enhancing the remanence via magnetic exchange coupling are evaluated.

Magnetostructural transformation in Ni–Mn–In–Co ribbons

A series of Ni50−xMn37In13Cox (x=0–9) ribbons was synthesized using the melt-spinning technique. Different ordered structures (5M, 7M, B2, and L21) were formed depending on composition. Both martensitic transformation temperature and entropy change increase with increasing Co for 0⩽x⩽3, while they decrease with further increasing to 3<x⩽9. Increasing Co increases the Curie temperature of the austenite but decreases that of the martensite. An optimized composition range of 4.5⩽x⩽5.5 is obtained where a magnetic-field-induced reverse transformation from nonmagnetic martensite to ferromagnetic austenite is realized.

Local texture in Nd–Fe–B sintered magnets with maximised energy density

Abstract Sintered Nd–Fe–B magnets, one of which showed a maximised energy density as high as (BH) max =451 kJ/m 3 (56.7 MGOe) at room temperature, have been produced by optimisation of alloy composition, minimisation of impurities, improved alignment of the powder particles and a controlled sintering process. For the first time, large scale microtexture analysis using electron backscatter diffraction (EBSD) has been carried out on sintered Nd–Fe–B magnets. Large orientation maps in both directions, parallel and perpendicular to the texture axis, were acquired in a thermal field emission gun scanning electron microscope (FEGSEM). Domain patterns of the same areas have been obtained using magneto-optical Kerr-microscopy. Thereby, a quantitative description of local texture could be correlated effectively with domain patterns of individual grains and with backscattered electron images. Distinction between individual grains in the Kerr-images was more difficult due to the excellent alignment of the grains in the magnet with the maximised energy density. Pole figures were used to evaluate EBSD patterns.

Near net shape production of radially oriented NdFeB ring magnets by backward extrusion

Abstract Anisotropic NdFeB magnets find application in electric motors where high flux densities combined with high coercivities are required. Such magnets can be produced by hot deformation of nanocrystalline NdFeB alloys. In particular, the backward extrusion process results in radially oriented ring magnets. The hot working process can be carried out in such a way that the ring magnets obtained after cutting off the top and the bottom are suited for direct use on the rotor in an electric motor. This is possible for a wide range of dimensions, which means that the ring magnets can be produced in a near net shape. Especially, smaller wall depths of the rings are realizable compared with sintered magnets. Major benefits are the simpler rotor construction, a greatly reduced rotor assembly time and simpler magnet retention on the rotor due to greater mechanical integrity of the magnetic structure. After a brief explanation of the backward extrusion process, the used dies and the corresponding rings are presented. Finally, the homogeneity of the magnetic properties and of the deformation process are discussed.

Improved hot workability and magnetic properties in NdFeCoGaB hot deformed magnets

The hot workability and the magnetic properties of die-upset and backward extruded melt-spun magnets with the composition of Nd/sub 14.2/Fe/sub 80.8/B/sub 5/ (MQP-A), and Nd/sub 13.6/Fe/sub 73.6/Co/sub 6.6/Ga/sub 0.6/B/sub 5.6/ (MQU-F) have been investigated. Magnets of MQU-F powder show an improved hot workability and better magnetic properties compared to the ternary alloy. Remanence, coercivity and energy product of die-upset MQU-F magnets were found to be B/sub /spl tau//=1.3 T, /spl mu//sub 0j/H/sub c/=1.7 T, and (BH)/sub max/=326 kJm/sup -3/, respectively. SEM and TEM investigations revealed a smaller average grain size in MQU-F magnets, which has a positive effect on the hot workability and magnetic properties. Radially oriented ring magnets were produced from both alloys and characterized magnetically. The temperature coefficients of ring magnets were determined to be /spl beta/=(dH/sub c//dT)=-0.6%//spl deg/C and /spl alpha/=(dB/sub /spl tau///dT)=-0.09%//spl deg/C. A higher deformation speed can be used for the production of MQU-F magnets, which should make the process more economic.

Fully dense MgB2 superconductor textured by hot deformation

Abstract Bulk textured MgB 2 material of nearly full density showing a weak c -axis alignment of the hexagonal MgB 2 grains parallel to the pressure direction was obtained by hot deformation of a stoichiometric MgB 2 pellet prepared by a gas–solid reaction. The texture of the material was verified by comparing the X-ray diffraction patterns of the hot deformed material with isotropic MgB 2 powder. A small, but distinct anisotropy of the upper critical field up to H c 2 a , b / H c 2 c ∼1.2 depending on degree of texture was found by resistance and susceptibility measurements. No anisotropy of the critical current density determined from magnetization measurements was found for the textured material.

Compression-induced texture change in NiMnGa-polymer composites observed by synchrotron radiation

Composites consisting of magnetic shape memory (MSM) particles embedded in a polyester matrix were prepared. Single-crystalline MSM particles were obtained by mortar grinding of melt-extracted and subsequently annealed Ni50.9Mn27.1Ga22.0 (at. %) fibers. The crystal structure of the martensite is tetragonal (5M) with c<a=b. Magnetic characterization of these composites shows indirect evidence for stress induced twin boundary motion in the MSM particles, as the compressed composite is easy to magnetize in the direction of compression and more difficult to magnetize in the perpendicular directions. The texture of all the embedded MSM particles is investigated before and after compression by means of synchrotron radiation. In the initial state, the MSM particles in the composite have a random texture, i.e., there is no preferred orientation of the c axis. After a 30% compression (height reduction), the MSM particles have a (004)-fiber texture in the direction of compression. This is unambiguous evidence for s...