A. Kirchner

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.

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.

A comparison of the magnetic properties and deformation behaviour of Nd-Fe-B magnets made from melt-spun, mechanically alloyed and HDDR powders

Commercial melt-spun and HDDR Nd-Fe-B powders as well as mechanically alloyed Nd-Fe-B powders have been used for hot pressing and subsequent die upsetting. A comparison of all three types of magnets with respect to their magnetic properties and deformation behaviour is discussed in this paper. The highest values of remanence, , and energy density, , found for the die-upset melt-spun material can be explained in terms of its small Nd content of 14 at% and its high degree of texture connected with the strong shape anisotropy of the deformed nanocrystalline grains. Fracture surfaces of this material have coarse grains at the former flake boundaries which results in a coercivity, , of only 1 T. An energy density of has been measured for a die-upset mechanically alloyed material. In spite of a Nd content of 16 at% in this alloy, a remanence of 1.2 T and a coercivity of 1.6 T have been attained. The large coercivity is due to (i) a very homogeneous distribution of the Nd-rich intergranular phase, (ii) a grain size of only observed after die upsetting and (iii) the fact that there is no formation of faceted grains during hot deformation. The microstructure of HDDR magnets (14 at% Nd) has larger average grain sizes and some faceted grains. Consequently these magnets have the smallest values of the degree of texture, remanence, , and energy density, . By addition of further elements the coercivity of this material could be held at . After thermal demagnetization the three types of hot-deformed Nd-Fe-B magnets have a relatively large initial susceptibility, which is due to the presence of classical magnetic domains as well as interaction domains. For the investigated fine-grained Nd-Fe-B materials the deformation mechanism can be described by using a solution-precipitation creep model, governed by interface-reaction-controlled creep.

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.

Investigations of the corrosion behaviour of nanocrystalline Nd-Fe-B hot pressed magnets

Abstract The corrosion behaviour of Nd–Fe–B nanocrystalline magnets made from differently processed powders such as melt spun, intensively milled and HDDR (Hydrogenation–Disproportionation–Desorption–Recombination) powders has been investigated in sulphuric acid solution and in air at 25°C. It is observed that the corrosion resistance of the investigated magnets can be correlated with the grain size of the hardmagnetic phase and the distribution and composition of the Nd-rich intergranular phases. The magnet made from HDDR powder exhibits a comparable, if not better, corrosion resistance in acid solution than magnets made from melt spun and intensively milled powders. Small additions of Co, Al and Ga improved the corrosion resistance of the investigated magnets by replacing the high corrosion sensitive intergranular Nd-rich phases with more noble phases. The surface layer formed during anodic polarization of the HDDR magnet was analysed by Auger electron spectroscopy indicating the formation of (Nd,Fe)-oxide with small amounts of Co and Al. The measured electrostatic surface potential differences of magnets containing Co, Al and Ga were lower than those of magnets without alloying additives.

Textured NdFeB HDDR magnets produced by die-upsetting and backward extrusion

The hydrogenation disproportionation desorption recombination (HDDR) process was applied to produce isotropic, submicron powder in 500 g batches in a specially designed HDDR reactor. The hot pressing characteristics of the material were determined and it was shown that the material has an excellent stability against grain growth. The coercivity of 1000-1080 is almost constant over a hot pressing temperature range of 700-, making the material highly suitable for subsequent hot deformation. Die-upset HDDR magnets were prepared in order to study the basic deformation behaviour. A remanence of 1.13 T in the axial direction and a coercivity of were achieved. Similar properties were obtained for the backward extruded magnets produced at and only a small decrease in alignment along the axial direction of the ring was found. Grain sizes were very uniform and on the submicron scale. Platelet-shaped grains were observed in the die-upset magnets. The formation of interaction domains, along the axial and radial directions for the die-upset and backward extruded magnets respectively, were established by high-resolution Kerr microscopy. The high degree of texture in the hot deformed HDDR magnets was also confirmed by Lorentz microscopy revealing continuous equispaced domains extending over the entire thinned sample with only small directional variations.

Hot Deformation Of Nanocrystalline Nd-Fe-B Alloys

Die-upset experiments on hot compacted commercial melt-spun powders MQP-A and MQP-B were carried out under an argon atmosphere at 500 to 800/spl deg/C with strain rates of 10/sup -4/ to 10/sup -1/ s/sup -1/. The kinetic analysis of the strain-rate stress relations yields the following dependences for the strain rate: (i) on flow stress, a power law with a stress exponent of 3, (ii) on temperature, a thermal activated process with activation energies of 280 and 400 kJ/mol for the MQP-A and -B samples, and (iii) on mean grain size, a reciprocal proportionality for MQP-A. These results indicate an interface-controlled solution precipitation process as the microstructural mechanism of hot deformation and texturing. Hereby the grain-boundary phase seems to be not required to be in the liquid state for deformation. Its presence is necessary for a crack-free deformation at high strain rates. In the case of radially textured ring magnets made by backward extrusion an area reduction of about 65% is necessary for optimization of the needed press stresses as well as the magnetic properties. Typical values are B/sub r/=1.25 T, /spl mu//sub 0/H/sub cj/=1.1 T and (BH)/sub max/=280 kJ/m/sup 3/.

Corrosion behavior of polymer-bonded NdFeB-based nanocrystalline magnets

The corrosion resistance of polymer bonded nanocrystalline NdFeB magnets with Nd-rich, stoichiometric, and Fe-rich compositions and that of their main constituent single phases is assessed in aqueous environments. For the evaluation of corrosion properties, gravimetric tests as well as potentiodynamic polarization measurements were performed in different nitrogen-purged electrolytes: 0.1 M sulfuric acid (pH=1), 0.5 M phthalate buffer (pH=5), and 0.1 M sodium hydroxide (pH=13). The single phases show anodic dissolution in strong acidic environment and stable passivation in alkaline solution. In weakly acidic phthalate buffer Nd-rich and Fe phases exhibit limited passive regions, whereas Fe/sub 3/B and Nd/sub 2/Fe/sub 14/B do not show stable passivation. Polymer-bonded magnets of different compositions have a significantly improved resistance toward strongly acidic environments as compared to hot pressed magnets. Enrichment in Fe content and additions like Co and Nb improve their corrosion behavior.

Highly coercive milled and melt-spun (Pr,Nd)FeB-type magnets and their hot workability

Highly coercive isotropic (Pr/sub 1-x/Nd/sub x/)/sub 2/Fe/sub 14/B-type magnets (0/spl les//spl times//spl les/1) have been processed using high energy ball milling and melt-spinning techniques, and an assessment of their hot deformation behavior has been carried out. A very high coercivity of /spl mu//sub 0/H/sub c/=2.7 T and a remanence of J/sub r/=0.72 T were found after annealing partly amorphous Pr/sub 15/DyFe/sub 75.9/B/sub 8/Zr/sub 0.1/ ribbons. Samples containing Dy (1at.%) and Zr (0.1 at.%) exhibit an improved stability against grain growth with increasing the annealing temperature. It has been found that the deformation forces necessary for texturing by hot deformation are the lowest for the as-milled Pr-containing alloys. As-milled Pr/sub 15/Fe/sub 77/B has been textured by die-upsetting at 750/spl deg/C obtaining magnetic properties as high as J/sub r/=1.27 T, /spl mu//sub 0/H/sub c/=1.20 T and (BH)/sub max/=307 kJ/m/sup 3/.