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- ItemRare-earth-free MnAl-C-Ni permanent magnets produced by extrusion of powder milled from bulk(Amsterdam : Elsevier, 2020) Feng, Le; Freudenberger, Jens; Mix, Torsten; Nielsch, Kornelius; Woodcock, Thomas GeorgeRare-earth-free MnAl-C-Ni permanent magnets have been produced for the first time by extruding powders milled from bulk. The resulting materials, fabricated using different conditions, contained a large volume fraction (> 0.92) of the desired τ-phase. In terms of the maximum energy product, the best performance obtained for a whole, transverse section of the extruded material was (BH)max = 46 kJm−3, and was (BH)max = 49 kJm−3 for a sample taken from near the edge of this section. Analysis showed that this material was comparable to the long-established benchmark, comprising MnAl-C-based magnets extruded in industry from bulk or from gas-atomised powder. Such materials are no longer available. The microstructure of the materials produced here consisted of fine, recrystallised grains, which exhibited an <001> fibre texture with intermediate texture quality and of larger, non-recrystallised regions, which contained hierarchical twinning and a high density of defects. The volume fraction and size of the non-recrystallised regions was greatly reduced by decreasing the size of the initial powder particles. This led to a large increase in the squareness factor of the demagnetisation curve and consequently to the high (BH)max values observed.
- ItemInvestigating the Static Recrystallization Behavior of 22MnB5 Manganese–Boron Steel through Stress Relaxation Analysis(Basel : MDPI, 2023) Birnbaum, Peter; Pilz, Stefan; Neufeld, Kai; Kunke, AndreasA constitutive model was developed to characterize the static recrystallization (SRX) and evolution of the grain size of the industrially relevant press-hardening steel, 22MnB5, subsequent to the hot forming of sheet metal. Isothermal stress relaxation tests were conducted using the BAEHR 805 A/D thermomechanical simulator, encompassing a temperature range of 950 to 1050 °C, prestrain levels ranging from 0.01 to 0.1, and strain rates spanning from 0.01 to 0.8 s−1. The results obtained from the isothermal stress relaxation tests facilitated the formulation of an Avrami equation-based model, which aptly describes the kinetics of SRX in relation to the temperature, prestrain, and strain rate. Notably, an increase in temperature led to accelerated recrystallization kinetics, signifying temperature-dependent behavior. When the temperature increased from 950 to 1050 °C, the recrystallization time was reduced to approximately one-third. Additionally, the prestrain exhibited a positive influence on the acceleration of SRX kinetics. A quintupling of the prestrain from 0.01 to 0.05 resulted in a reduction of the static recrystallization duration to approximately one-fifth. Among the parameters studied, the strain rate had the least impact on the SRX kinetics, as doubling the strain rate from 0.01 to 0.8 only resulted in a halving of the recrystallization duration. Moreover, an analysis of the microstructural evolution in response to the forming parameters was undertaken. While the grain-size investigation post-isothermal stress relaxation tests provided results in line with the SRX kinetics calculations, the observed effects were comparatively subdued. Furthermore, a comprehensive examination was conducted using electron backscatter diffraction (EBSD) analysis, aiming to explore the effects of specific stress relaxation states on the morphology of martensite. The findings reveal fully recrystallized globulitic microstructures, characterized by relatively minor differences among them.
- ItemAutomated meshing of electron backscatter diffraction data and application to finite element micromagnetics(Amsterdam [u.a.] : Elsevier, 2019) Gusenbauer, Markus; Fischbacher, Johann; Kovacs, Alexander; Oezelt, Harald; Bance, Simon; Zhao, Panpan; Woodcock, Thomas George; Schrefl, ThomasThis paper gives a procedure for automatically generating finite element meshes with an adaptive mesh size from Electron Backscatter Diffraction (EBSD) data. After describing the procedure in detail, including preliminary and image processing steps, an example application is given. The method was used to carry out finite element (FE) micromagnetic simulations based on real microstructures in the hard magnetic material, MnAl. A fast micromagnetic solver was used to compute hysteresis properties from the finite element mesh generated automatically from EBSD data. The visualization of the magnetization evolution showed that the reversal is governed by domain wall pinning at twin boundaries. The calculated coercive fields are very sensitive to changes of the Gilbert damping constant, even for low field rates. © 2019 The Authors
- ItemStructural and luminescence imaging and characterisation of semiconductors in the scanning electron microscope(Bristol : IOP Publ., 2020) Trager-Cowan, C.; Alasmari, A.; Avis, W.; Bruckbauer, J.; Edwards, P.R.; Ferenczi, G.; Hourahine, B.; Kotzai, A.; Kraeusel, S.; Kusch, G.; Martin, R.W.; McDermott, R.; Naresh-Kumar, G.; Nouf-Allehiani, M.; Pascal, E.; Thomson, D.; Vespucci, S.; Smith, M.D.; Parbrook, P.J.; Enslin, J.; Mehnke, F.; Kuhn, C.; Wernicke, T.; Kneissl, M.; Hagedorn, S.; Knauer, A.; Walde, S.; Weyers, M.; Coulon, P.-M.; Shields, P.A.; Bai, J.; Gong, Y.; Jiu, L.; Zhang, Y.; Smith, R.M.; Wang, T.; Winkelmann, A.The scanning electron microscopy techniques of electron backscatter diffraction (EBSD), electron channelling contrast imaging (ECCI) and cathodoluminescence (CL) hyperspectral imaging provide complementary information on the structural and luminescence properties of materials rapidly and non-destructively, with a spatial resolution of tens of nanometres. EBSD provides crystal orientation, crystal phase and strain analysis, whilst ECCI is used to determine the planar distribution of extended defects over a large area of a given sample. CL reveals the influence of crystal structure, composition and strain on intrinsic luminescence and/or reveals defect-related luminescence. Dark features are also observed in CL images where carrier recombination at defects is non-radiative. The combination of these techniques is a powerful approach to clarifying the role of crystallography and extended defects on a material's light emission properties. Here we describe the EBSD, ECCI and CL techniques and illustrate their use for investigating the structural and light emitting properties of UV-emitting nitride semiconductor structures. We discuss our investigations of the type, density and distribution of defects in GaN, AlN and AlGaN thin films and also discuss the determination of the polarity of GaN nanowires. © 2020 The Author(s). Published by IOP Publishing Ltd.