Grain Boundary Reconstruction Research Articles

Grain boundary reconstruction for high-performance regeneration of Ce-abundance sintered Nd-Ce-Fe-B magnet scraps

In this study, we focus on the short-process regeneration of high-Ce content sintered Nd-Ce-Fe-B magnet scraps, aiming to enhance the coercivity and overall magnetic properties through the reconstruction of grain boundaries (GBs) based on the analysis of rare earth (RE) loss and proportions at each stage of the regeneration process. CeHx, (Nd0.75Pr0.25)Hx, and their mixtures were added as intergranular additives to produce sintered magnets. When 2.31 wt% of (Nd0.75Pr0.25)Hx is added, a high-performance magnet is obtained with the remanence of 11.72 kG, coercivity of 11.56 kOe, and maximum energy product of 32.23 MGOe, reaching performance recovery rates of 98.8%, 103.5%, and 98%, respectively, of the original magnets. It is indicated that the intergranular additives were beneficial to the formation of continuous smooth RE-rich GBs. In addition, a portion of Nd/Pr diffused into the 2:14:1 phase as a partial replacement for Ce, resulting in the migration of Ce from the interior of the grains to the outer region. This led to the formation of new RE-rich triple junction phases on the exterior, thereby reducing the magnetic dilution effect of Ce in the primary phase and enhancing the magnetic crystal anisotropy. These findings offer new insights into the regeneration of high-Ce sintered Nd-Ce-Fe-B scraps.

High temperature deformation behaviour and microstructure evolution in Cu containing High Strength Low Alloy (HSLA) steel

ABSTRACT The hot deformation behaviour of a Cu containing high strength low alloy (HSLA) steel has been evaluated by carrying out hot compression tests over a temperature range of 900 °C to 1100 °C and constant true strain rates of 0.0003 to 1 s−1. Although single peak flow curve is observed predominantly, a tendency to exhibit multiple peaks at low strain rates (0.0003 and/or 0.001 s−1) at temperature greater than 950 °C. The important parameters pertaining to dynamic recrystallisation (DRX), such as critical, peak, steady state stress and strain were ascertained, and the DRX state diagram depicting the variation of critical parameters governing DRX with Zener-Hollomon parameter Z was established. The volume fraction of recrystallised grains, computed from flow stress, as expected, follows a sigmoidal trend indicating higher kinetics at low strain rates for a given temperature. Reconstruction of prior austenite grain boundaries (PAGB) based on electron back scattered diffraction (EBSD) technique for a few select specimens clearly elucidated the effect of temperature and strain rate on the DRX behaviour of the steel. The DRX state diagram established here clearly demarcates the deformation space i.e. temperature-strain rate-stain space into regimes where the material exhibits full DRX, hardening/softening + DRX and only hardening (no DRX) behaviour. This would be useful for designing suitable forging sequence to achieve the desired microstructure and mechanical properties.

Reconstruction of 3D grain boundaries from rock thin sections, using an advanced polarised-light microscopy method.

Grain boundaries play a significant role in materials by initiating reactions and collecting impurities. Here we present FAGO (Fabric Analyser Grain boundary recOnstruction), a first step towards the automatic determination of three-dimensional (3D) grain boundary geometry using polarised light. The trace of the grain boundaries from 2D rock thin sections are determined primarily from data acquired using an automatic fabric analyser microscope and the FAME software (fabric analyser-based microstructure evaluation; Peternell and colleagues and Hammes and Peternell). Based on the Fabric Analyser G50's unique arrangement of nine differently oriented light sources the retardation can be determined independently for each light direction and at each pixel in the field of view. FAGO combines these retardation datasets for each individual pixel together with retardation profiles across grain boundaries, to determine the orientations of the boundaries. The grain boundary traces are then broken up into segments of equal orientation, using the profile-obtained orientation data. Finally, a 3D grain boundary model is reconstructed. The data processing is almost fully automatic using the MATLAB® environment. Only minor manual inputs are required.

Physics-Based Image Reconstruction of SiC Grain Boundaries

Physics-Based Image Reconstruction of SiC Grain Boundaries

Ballistic-diffusive phonon heat transport across grain boundaries

The propagation of a heat pulse in a single crystal and across grain boundaries (GBs) is simulated using a concurrent atomistic-continuum method furnished with a coherent phonon pulse model. With a heat pulse constructed based on a Bose-Einstein distribution of phonons, this work has reproduced the phenomenon of phonon focusing in single and polycrystalline materials. Simulation results provide visual evidence that the propagation of a heat pulse in crystalline solids with or without GBs is partially ballistic and partially diffusive, i.e., there is a co-existence of ballistic and diffusive thermal transport, with the long-wavelength phonons traveling ballistically while the short-wavelength phonons scatter with each other and travel diffusively. To gain a quantitative understanding of GB thermal resistance, the kinetic energy transmitted across GBs is monitored on the fly and the time-dependent energy transmission for each specimen is measured; the contributions of coherent and incoherent phonon transport to the energy transmission are estimated. Simulation results reveal that the presence of GBs modifies the nature of thermal transport, with the coherent long-wavelength phonons dominating the heat conduction in materials with GBs. In addition, it is found that phonon-GB interactions can result in reconstruction of GBs.

Investigation of Parent Austenite Grains from Martensite Structure Using EBSD in a Wear Resistant Steel

Crystallographic reconstruction of parent austenite grain boundaries from the martensitic microstructure in a wear resistant steel was carried out using electron backscattered diffraction (EBSD). The present study mainly aims to investigate the parent austenite grains from the martensitic structure in an as-rolled (reference) steel sample and samples obtained by quenching at different cooling rates with corresponding dilatometry. Subsequently, this study is to correlate the nearest cooling rate by the dilatometer which yields a similar orientation relationship and substructure as the reference sample. The Kurdjumov-Sachs orientation relationship was used to reconstruct the parent austenite grain boundaries from the martensite boundaries in both reference and dilatometric samples using EBSD crystallographic data. The parent austenite grain boundaries were successfully evaluated from the EBSD data and the corresponding grain sizes were measured. The parent austenite grain boundaries of the reference sample match the sample quenched at 100 °C/s (CR100). Also the martensite substructures and crystallographic textures are similar in these two samples. The results from hardness measurements show that the reference sample exhibits higher hardness than the CR100 sample due to the presence of carbides in the reference sample.

Advanced Methods and Tools for Reconstruction and Analysis of Grain Boundaries from 3D-EBSD Data Sets

We report the recent development of a 3D orientation data post-processing software, which we refer to as QUBE. Amongst other functionalities, it offers the possibility to specify the spatial and orientational distribution of boundary normals. We describe a method to reconstruct a voxel-accurate and smooth 3D boundary triangle mesh by algorithmic means. A proof of concept is given by a benchmark on a generic dataset and we demonstrate a first result with the description of selected grain boundaries in an Fe-28%Ni sample.

Growth, annealing and thermo-electrical properties of Cd1−xZnxS thin films for microbolometers

Cd1−xZnxS (0<x<0.14) nanocrystalline thin films were grown and tested for use as sensing elements in microbolometer-type uncooled infrared detectors instead of vanadium oxides. Films of thickness 80–700 nm were deposited on SixNy/silicon as well as on glass and SiO2/silicon wafers using both thermal evaporation and MOCVD. The films were characterized by AFM, SIMS and electrical measurements. CdS thin films showed good parameters for use in microbolometer-type infrared uncooled sensors. The thermal coefficient of resistance (TCR) was in the range of 1.1–3.6% for the different film thicknesses. The TCR and electrical resistance of CdS films were optimized using annealing in different environments (vacuum, nitrogen, hydrogen and NF3) and by using a capping silicon nitride layer. The correlations between grain size, grain boundaries structure and TCR were established. Owing to the nanoscale film structure and thickness, significant grain growth and grain-boundary reconstruction occurred during minute long annealing time at temperatures as low as 200 °C.

Grain Boundary Engineering: Progress and Challenges

The progress of grain boundary engineering (GBE) is overviewed and the challenges for further investigations emphasized. It points out that, the electron backscatter diffraction (EBSD) reconstruction of grain boundaries, which gives the information of connectivity interruption of general high angle boundaries (HABs), is more significant than purely pursuing high frequency of so-called special boundaries. The criterion for the optimization of grain boundary character distribution (GBCD) needs to be established. The energy spectrum and the degradation susceptibility of grain boundaries of various characters including HABs and low Σ（Σ≤29） coincidence site lattice (CSL) needs to be studied and ascertained. And finally, the newly proposed model of non-coherent Σ3 interactions for GBCD optimization are discussed.

3-Dimensional Characterization of Polycrystalline Bulk Materials Using High-Energy Synchrotron Radiation

The implementation of 3-Dimensional X-Ray Diffraction (3DXRD) Microscopy at the Advanced Photon Source is described. The technique enables the non-destructive structural characterization of polycrystalline bulk materials and is therefore suitable for in situ studies during thermo-mechanical processing. High energy synchrotron radiation and area detectors are employed. First, a forward modeling approach for the reconstruction of grain boundaries from high resolution diffraction images is described. Second, a high resolution reciprocal space mapping technique of individual grains is presented.

Reconstruction of grain boundaries during austenite–ferrite transformation

The early stages of the γ–α transformation in steel were studied in situ by hot-stage optical microscopy and analyzed theoretically by using large-scale molecular dynamic simulations. We show that modification of the grain boundary structure, often seen as a precursor of new phase formation, is in fact an essential step of the transformation.

Reconstruction of grain boundaries in polycrystals by filtered back-projection of diffraction spots

A reconstruction method is presented for the generation of three-dimensional maps of the grain boundaries within powders or polycrystals. The grains are assumed to have a mosaic spread below 1°. They are mapped non-destructively by diffraction with hard X-rays, using a uniform mm2-sized beam. First the diffraction spots are sorted with respect to grain of origin by the indexing programGRAINDEX. Next, for each grain the reconstruction is performed by a variant of the filtered back-projection algorithm. The reconstruction method is verified by a simulation over ten grains. Using 64 reflections for each grain, sub-pixel accuracy is obtained. The potential of the method is outlined.

Reconstruction of grain boundaries in copper and gold by simulation

The reconstructions of high-angle twist grain boundaries on the four densest atomic planes in fcc copper, as described by a Lennard-Jones potential, and gold, as described by an embedded-atom-method potential, are investigated using the recently developed method of grand-canonical simulated quenching. It is found that the grain boundaries on the widely spaced (111) and (100) planes do not reconstruct, while those on the less widely spaced (110) and (113) planes do reconstruct. The effect that reconstruction can have on the physical properties of an interfacial system is illustrated by comparing the elastic properties and ideal cleavage energies of reconstructed grain boundaries with those of corresponding unreconstructed grain boundaries.

Reconstruction of Twist Grain Boundaries in Gold

ABSTRACTThe reconstruction of high-angle twist grain boundaries on the four densest atomic planes in gold are investigated using the recently developed method of grand-canonical simulated quenching. It is found that the grain boundaries on the two densest planes, (111) and (100), do not reconstruct, while those on the (110) and (113) planes do.