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Journal of magnesium and alloys 10(1), 146-159 (2022). doi:10.1016/j.jma.2021.07.003, Published by Elsevier, Amsterdam [u.a.]

In magnesium alloys with multiple substitutional elements, solute segregation at grain boundaries (GBs) has a strong impact on many important material characteristics, such as GB energy and mobility, and therefore, texture. Although it is well established that GB segregation is inhomogeneous, the variation of GB solute composition for random boundaries is still not understood. In the current study, atomic-scale experimental and simulation techniques were used to investigate the compositional inhomogeneity of six different GBs. Three-dimensional atom probe tomography results revealed that GB solute concentration of Nd in Mg varies between 2 to 5 at.%. This variation was not only seen for different GB orientations but also within the GB plane. Correlated atomistic simulations suggest that the inhomogeneous segregation behavior observed experimentally stems from local atomic rearrangements within the GBs and introduce the notion of potential excess free volume in the context of improving the prediction of per-site segregation energies.

Mg-1Ce alloys were pre-rolled at room temperature to various reductions to achieve different as-rolled microstructures. The deformed materials were then treated with a pulsed electric current (electropulse treatment, EPT) to study the effect of the as-rolled microstructure on the recrystallization texture formation. The experimental results show that an optimum window exists for pre-rolling reductions, within which the as-rolled microstructures and sharp basal textures obtained will evolve into fine recrystallized grains and weak textures during the subsequent EPT. Microstructure analysis indicates a competition between two processes, the nucleation and the nucleus growth, in contributing to the texture weakening of the alloy. While both the application of pulsed electric current and the increase in the pre-rolling reductions could strengthen the advantage of the nucleation rate of the non-basal nuclei over those with basal orientations, the contribution from nucleus growth depends mainly on the available deformed basal-oriented matrix, which decreases with increasing pre-rolling reductions. The results of the present study might provide useful guidance for the optimization of process parameters in the future application of the ‘pre-rolling + EPT’ technique on the Mg alloy sheet.

Commercial purity as-cast magnesium was hot rolled and subsequently annealed at different temperatures in order to investigate its grain growth behavior and link it to the texture evolution. Annealing at an intermediate temperature of 220 °C gave rise to abnormal grain growth with a few grains reaching a grain diameter 10 times larger than the mean. Increasing the annealing temperature to 350 °C yielded normal grain growth. Both types of grain growth revealed a strengthening of the (0001) 11 2 - 0 texture component. It is hypothesized that a dislocation density gradient after recrystallization grants (0001) 11 2 - 0 grains a size advantage during early stages of growth. The type of growth will be, however, determined by the mobility of the present grain boundaries and triple junction drag, which are strongly dependent on the annealing temperature. The above hypothesis of the interplay between these parameters was explored through curvature- and residual dislocation-density-gradient-driven grain growth simulations using a formerly developed level-set approach. The simulation outcome suggests that application of such a modeling approach in microstructure studies of magnesium can provide valuable new insights into the problem of grain growth and associated texture evolution.

The deformation behavior of two specifically oriented grains in a magnesium tricrystal, and the corresponding changes in the character of their 90° 〈 10 1 ¯ 0 〉 tilt boundary were investigated. Identical samples with grain boundaries aligned parallel to the direction of loading were deformed by plane strain compression at room temperature up to select strains. Orientation measurements showed that character/geometry of the grain boundary changed drastically during plastic straining, which was accompanied by lattice rotation of the adjoining crystallites. The boundary misorientation angle decreased from 90° to 71° and all the way down to 10° at strains of ɛt = – 0.15 and – 0.4, respectively. This was interpreted in terms of grain boundary - dislocation interaction, where lattice dislocations due to favored basal slip collided with the boundary and caused a gradual annihilation of grain boundary structural units. When deformation in the two grains became clearly non-uniform at larger strains of – 0.3 and – 0.4 due to the presence of the boundary, the adjacent regions of the latter underwent a pronounced lattice curvature of about 0.005 deg/µm persisting over a 4 mm distance from the boundary.

Scientific understanding of the behavior of wrought magnesium alloys is quite mature, with literally thousands of papers published on the topic, along with several reviews. Most of this research is relatively recent, being published after the year 2000. With such a large body of work available to the reader, it could easily be missed that the field of magnesium metallurgy is poised for significant advances. Access to synchrotron and neutron scattering has revealed new knowledge about deformation and fracture behavior, and the complex interaction between solutes, dislocations, interfaces, and disconnections is only just becoming clear. Supercomputing now makes it possible to use atomic-scale simulation techniques to make material calculations such as stacking fault energy predictions, and the simulation of atom-by-atom movement at boundaries and twins in three dimensions is now feasible. Advanced microscopy techniques such as atom probe tomography are allowing us to examine the complex chemical nature of grain boundaries on the nanoscale. These are just some of the areas ripe with potential for new discovery. With so much information available to the reader, it can be difficult to identify those areas of study in which the greatest knowledge gaps exist or those which are most ripe for exploration. For this reason, several authors have teamed up to write this short discussion piece which highlights research areas which the authors agree have the most potential for impact in the field. Refereed/Peer-reviewed

Processes : open access journal 10(11), 2222 (2022). doi:10.3390/pr10112222 special issue: "Special Issue "Advances in Novel Biomaterials for Bone Regeneration" / Special Issue Editors: Prof. Dr. Annelie-Martina Weinberg, Guest Editor; Dr. Nicole Sommer, Guest Editor; Dr. Valentin Herber, Guest Editor", Published by MDPI, Basel

The deformation behavior of specially grown magnesium tricrystals was studied, where two of the grains were chosen to primarily exhibit prismatic slip, while the third grain was oriented for tension twinning. In all three grains basal slip was initially suppressed. Identical tricrystalline samples with grain boundaries and the triple line aligned parallel to the direction of loading were deformed by plane strain compression at 370 °C up to select strains. The mechanical response and microstructure evolution in the deformed tricrystals were comprehensively characterized and analyzed with a particular focus on the role of grain boundaries for deformation twinning, dislocation slip activity and dynamic recrystallization.

Metals : open access journal 11(12), 1958 (2021). doi:10.3390/met11121958, Published by MDPI, Basel

Binary and ternary Mg-1%Er/Mg-1%Er-1%Zn alloys were rolled and subsequently subjected to various heat treatments to study texture selection during recrystallization and following grain growth. The results revealed favorable texture alterations in both alloys and the formation of a unique &plusmn, 40&deg, transvers direction (TD) recrystallization texture in the ternary alloy. While the binary alloy underwent a continuous alteration of its texture and grain size throughout recrystallization and grain growth, the ternary alloy showed a rapid rolling (RD) to transvers direction (TD) texture transition occurring during early stages of recrystallization. Targeted electron back scatter diffraction (EBSD) analysis of the recrystallized fraction unraveled a selective growth behavior of recrystallization nuclei with TD tilted orientations that is likely attributed to solute drag effect on the mobility of specific grain boundaries. Mg-1%Er-1%Zn additionally exhibited a stunning microstructural stability during grain growth annealing. This was attributed to a fine dispersion of dense nanosized particles in the matrix that impeded grain growth by Zener drag. The mechanical properties of both alloys were determined by uniaxial tensile tests combined with EBSD assisted slip trace analysis at 5% tensile strain to investigate non-basal slip behavior. Owing to synergic alloying effects on solid solution strengthening and slip activation, as well as precipitation hardening, the ternary Mg-1%Er-1%Zn alloy demonstrated a remarkable enhancement in the yield strength, strain hardening capability, and failure ductility, compared with the Mg-1%Er alloy.

Journal of magnesium and alloys 8(1), 252-268 (2020). doi:10.1016/j.jma.2019.07.010, Published by Elsevier, Amsterdam [u.a.]

Based on transmission electron microscopy results from pure Mg single crystal examined in the current work, and Mg alloys and other hexagonal metals in literature, a characteristic dislocation substructure inside 10 1 ¯ 2 twins is identified. Abundant non-basal [c] and ⟨c + a⟩ perfect dislocations, as well as basal I1 stacking faults with widths on the order of 100 nm distributed preferentially in the vicinity of a twin boundary, with a low density zone in the middle of the twin. Considering the ubiquity of 10 1 ¯ 2 twins, this characteristic dislocation substructure should be considered in modeling of hexagonal metal alloy deformation.

Mg single crystals most favourably oriented for prismatic slip were subjected to plane strain compression at room and elevated (200 and 370 °C) temperatures. At room temperature deformation was accommodated by cooperative { 10 1 ¯ 2 } extension and { 10 1 ¯ 1 } contraction twinning, occurring in a combined fashion, instead of prismatic slip. At higher strains deformation was found to localize within contraction twin bands that underwent dynamic recrystallization. Such bands were also revealed to be crack initiation sites due to extensive localized shear. Prismatic slip was activated at elevated temperatures (above 200 °C) only, although being geometrically favored.

In-situ SEM simple shear tests were performed on magnesium bicrystals of random misorientations at room temperature and 150 °C. { 10 1 ¯ 2 } extension twins nucleated at the grain boundaries and grew with increasing strain. Rectilinear slip traces appeared in the grain interior of the bicrystal which corresponded to basal slip. By deflection at twin boundaries, these slip lines readily continued through the twins. Besides slip traces due to basal slip, additional slip lines were observed inside twins at 150 °C that evidenced the activation of non-basal slip. Supplemental tests were performed on single crystals, in which twinning/detwinning via migration of twin boundaries was observed. The obtained results are discussed with respect to direct and indirect slip transfer across { 10 1 ¯ 2 } twin boundaries, the transmutation of 〈a〉 into 〈c+a〉 dislocations and the activation of first-order pyramidal slip as a result of slip-twin interaction.

Sample geometries for micro-mechanical testing, e.g. micro-pillars and micro-cantilevers are primarily produced using gallium focused ion beam technology. However, the effects of the gallium ions on the mechanical properties of metals which are embrittled by liquid metal gallium are still unknown. In this work, micro-compression samples from single crystalline and ultrafine-grained aluminum are fabricated using both xenon and/or gallium ions. The different ions have little effect on the yield strength of single crystalline aluminum. However, for the ultrafine-grained aluminum, the strength is reduced with increasing Ga dose, and considerable differences in the deformation morphology and resulting microstructures are observed.

In a variety of modern, multi-phase steels, damage evolves during plastic deformation in the form of the nucleation, growth and coalescence of voids in the microstructure. These microscopic sites play a vital role in the evolution of the materials’ mechanical properties, and therefore the later performance of bent products, even without having yet led to macroscopic cracking. However, the characterization and quantification of these diminutive sites is complex and time-consuming, especially when areas large enough to be statistically relevant for a complete bent product are considered. Here, we propose two possible solutions to this problem: an advanced, SEM-based method for high-resolution, large-area imaging, and an integral approach for calculating the overall void volume fraction by means of density measurement. These are applied for two bending processes, conventional air bending and radial stress superposed bending (RSS bending), to investigate and compare the strain- and stress-state dependent void evolution. RSS bending reduces the stress triaxiality during forming, which is found to diminish the overall formation of damage sites and their growth by the complimentary characterization approaches of high-resolution SEM and global density measurements., Metals;9(3)

Alloying additions in magnesium can modify common basal textures during recrystallization based on their solid solubility and precipitation behavior. With a variety of tenable theories in the literature, a far greater understanding is required of how restricted growth due to drag effects and annealing texture formation are linked. In the current work, a complex magnesium alloy Mg-3Al-1Zn-0.3Ca (wt.%) with multiple substitutional elements was subjected to a combination of deformation and annealing treatments in order to examine how a variation of bulk solute concentrations would influence its segregation and precipitation behavior. The work focuses in particular on solute segregation to grain boundaries and demonstrates that the type and level of segregation play a key role in controlling the growth behavior in such a way that restricts preferential growth of grains with a basal texture. Deeper knowledge in this area can be expected to advance current alloy design strategies by tweaking the solute concentration in the solid solution through targeted heat treatments to result in the required amounts of second phase precipitation and grain boundary segregation.

In order to improve the understanding of damage evolution in mechanically heterogeneous microstructures, like the ones of dual-phase steels, the influence of the applied stress state is a key element. In this work, we studied the influence of the globally applied stress state on the evolution of damage in such a microstructure. Classical damage models allow predictions of damage during deformation based on considerations of the material as an isotropic continuum. Here, we investigate their validity in a dual phase microstructure that is locally dominated by its microstructural morphological complexity based on a statistical ensemble of thousands of individual voids formed under different stress states. For this purpose, we combined a calibrated material model incorporating damage formation to assess the local stress state in samples with different notch geometries and high-resolution electron microscopy of large areas using a deep learning-based automated micrograph analysis to detect and classify microstructural voids according to their source of origin. This allowed us to obtain both the continuum stress state during deformation and statistically relevant data of individual void formation. We found that the applied plastic strain is the major influence on the overall number, and therefore the nucleation of new voids, while triaxiality correlates with the median void size, supporting its proposed influence on void growth. In contrast, coalescence of voids leading to failure appears related to local instabilities in the form of shear band formation and is therefore only indirectly determined by the global stress state in that it determines the global distribution, density and size of voids.

Magnesium, the lightest structural metal, is approximately four times lighter than steel—the most widely used metal in industrial applications. Currently available Mg alloys, however, are impractically expensive for use in automotive structural components, as severe ductility problems require forming operations at elevated temperatures and an exclusion from critical safety components. With a strong impetus in research having sprung up during the last two decades, addition of rare-earth elements in small quantities emerged as a potential solution for simultaneously delivering the ductility and weight requirements for automotive applications. These improvements are arguably achieved by virtue of texture weakening and enhancement of non-basal slip. However, ways by which rare-earth elements modify texture remain very elusive, and no consensus on the driving mechanisms has been reached in the literature as of yet. We take a look back at different paradigms held for the action of rare-earth additions, and examine key facts that may reconcile controversies. We attempt to identify critical gaps and suggest venues to overcome them. These gaps, once filled, may promote Mg alloys to become a stronghold for lightweighting, which will exceptionally benefit our environment and wellbeing.

Two solid solution binary magnesium-rare earth (RE) alloys, Mg-1wt.% Gd and Mg-1wt.% Dy were subjected to large strain hot rolling followed by recrystallization annealing at different temperatures for 60 min. Recrystallization and grain growth in Mg-1Gd led to stepwise change in the deformation texture, highlighted by a complete disappearance of the basal texture component developed during rolling. In case of Mg-1Dy, texture changes upon annealing were less accentuated, characterized by gradual softening of the deformation basal texture with increasing annealing temperatures. The development of favorable RE-textures during annealing is believed to be a result of solute segregation to planar defects, such as grain boundaries. It was observed using atom probe tomography that Gd atoms show a much higher grain boundary segregation tendency than Dy atoms. The enhanced segregation behavior of Gd manifests during annealing into a strong growth advantage of recrystallized off-basal orientations over basal orientations originating from the deformation microstructure, continuing as discontinuous grain growth at higher annealing temperatures and longer durations. No distinctive growth advantage was witnessed during recrystallization and grain growth of Mg-1Dy. Different segregation effects between the two RE elements and the resulting annealing texture evolution strongly impacted the room temperature tensile ductilities of both alloys. Mg-1Gd seemed to exhibit concomitant precipitation hardening effects, thereby displaying higher tensile strength along with enhanced ductilities. This work demonstrates the exciting possibility of being able to selectively exploit grain boundary characteristics using RE elements as an effective mechanism to tailor the microstructure during thermomechanical processing, thereby improve the mechanical performance of Mg alloys.

One of the key aspects influencing microstructural design pathways in metallic systems is grain boundary motion. The present work introduces a method by means of which direct measurement of grain boundary mobility vs. misorientation dependence is made possible. The technique utilizes datasets acquired by means of serial electron backscatter diffraction (EBSD) measurements. The experimental EBSD measurements are collectively analyzed, whereby datasets were used to obtain grain boundary mobility and grain aspect ratio with respect to grain boundary misorientation. The proposed method is further validated using cellular automata (CA) simulations. Single crystal aluminium was cold rolled and scratched in order to nucleate random orientations. Subsequent annealing at 300 °C resulted in grains growing, in the direction normal to the scratch, into a single deformed orientation. Growth selection was observed, wherein the boundaries with misorientations close to Σ7 CSL orientation relationship (38° 〈111〉) migrated considerably faster. The obtained boundary mobility distribution exhibited a non-monotonic behavior with a maximum corresponding to misorientation of 38° ± 2° about 〈111〉 axes ± 4°, which was 10–100 times higher than the mobility values of random high angle boundaries. Correlation with the grain aspect ratio values indicated a strong growth anisotropy displayed by the fast growing grains. The observations have been discussed in terms of the influence of grain boundary character on grain boundary motion during recrystallization.

Magnesium alloys have come to the fore as attractive lightweight structural materials owing to their high specific strength, whilst some of them are considered for bioresorbable implant applications. A new Mg4Li-1Ca (LX41) alloy, processed by two-step rolling followed by annealing, showed extraordinary properties outperforming the existing Mg alloys in terms of specific strength, strain hardening ability, ductility, and cytocompatibility. The record -breaking superiority in mechanical properties combined with cytocompatibility qualifies the alloy for applications in lightweight engineering structures and bone implants. (C) 2016 Elsevier B.V. All rights reserved.

Specially oriented Mg single crystals were deformed at ambient and elevated temperatures in channel-die compression with the c -axis inclined at 45° to the compression direction and 〈 10 1 ¯ 0 〉 parallel to the lateral constraint direction. The specimens deformed by basal slip, entailing a rotation of the c -axis towards the compression direction. At room temperature, starting already during early stages of deformation, macroscopic bands comprising a mesh of anomalous { 10 1 ¯ 2 } extension twins formed parallel to the constraint direction. These twins were characterized by a negative Schmid factor and produced a strain opposite to the imposed deformation. At the final true strain of −1, a significant volume fraction of the matrix was consumed by twins, causing a reorientation of the c -axis farther away from the compression direction, and hence delaying the formation of a hard basal texture component. An analysis of the displacement gradient tensors for the six possible extension twin variants showed that the selected twin variants, of which the bands were composed, involved a lattice rotation opposite to the one caused by basal slip. Less twinning was observed at elevated temperatures with the macroscopic bands disappearing at 370 °C. The obtained results are discussed with respect to deformation heterogeneity and the role of anomalous twinning for the deformation behavior of magnesium.

Mg single crystals of selected ‘hard’ and ‘soft’ orientations were deformed at ambient temperature in channel-die compression. Specimens with a ‘hard’ basal starting orientation displayed a limited ductility and fractured along { 11 2 4 } planes. Compression perpendicular to the c -axis of orientations well aligned for { 10 1 2 } extension twinning revealed a very contrasting deformation behavior. Specimens compressed along the crystallographic 〈 11 2 0 〉 direction exhibited an exceptionally high room temperature ductility, whereas compression along 〈 10 1 0 〉 resulted in failure after an initial sigmoidal stress–strain response. In both cases { 10 1 2 } extension twins were found to fully consume the matrix at low strains and convert the starting orientations into orientations corresponding to the respective high Schmid factor twin variants. High strains have also been attained for a prismatic starting orientation where the c -axis was constrained. In specimens, favorably oriented for basal slip, anomalous { 10 1 2 } extension twins formed, clustered in bands parallel to the constraint direction. These twins produced a strain opposite to the imposed plane strain deformation and provided a pronounced dynamic grain boundary strengthening effect.

Unconventional rolling schemes characterized by high reductions and a low number of passes were employed to deform a new magnesium sheet alloy to large strains in order to obtain a heterogeneous deformation microstructure with large local variations in defect density. Microstructure heterogeneity was present in the form of large elongated grains, twinning, and shear banding. The rolled material was further deformed in tension at room temperature along different sheet directions to study the effect of such microstructure on plastic flow and planar anisotropy. Other rolled samples were subjected to annealing treatments before tensile testing to investigate the mechanisms of recrystallization in conjunction with deformation heterogeneity. Recrystallization chains were seen to form by stages of progressive subgrain rotation and subgrain growth giving rise to new grain boundaries capable of long-range migration during annealing. Unlike the as-rolled condition where planar anisotropy of the yield strength and maximum elongation was evident due to several reasons discussed in the study, the annealed condition with its homogeneous equiaxed grain structure and weak textures demonstrated an almost isotropic plastic flow behavior.

The origin of texture components often associated with rare-earth element (REE) additions in wrought magnesium alloys is a long-standing problem in magnesium technology. While their influence on the texture is unquestionable, it is not yet clear why certain texture components, such as $$ \langle 11\bar{2}1 \rangle ||{\text{extrusion}}\;{\text{direction}}, $$ are favored over other components typically observed in traditional magnesium alloys. The objective of this research is to identify the mechanisms accountable for these RE textures during early stages of recrystallization. Electron backscattered diffraction and transmission electron microscopy analyses reveal that REEs in zinc-containing magnesium alloys corroborate discontinuous dynamic recrystallization. REEs promote isotropic growth for all nuclei generated through the bulging mechanism. During nucleation, the effect of REEs on orientation selection was explained by the diversified activity of both $$ \langle 10\bar{1}0 \rangle $$ and [0001] Taylor axes in the same grain with a marked preference for [0001] rotations to occur first. During nuclei growth, no growth preference was observed when sufficient REEs are added in the zinc-containing magnesium alloys, instead isotropic nuclei growth across all grain orientations occurs. This phenomenon is attributed to REEs segregating to grain boundaries (GBs), in agreement with prior computational and theoretical results (Barrett et al., Scripta Mater 146:46–50, 2018) that show a more isotropic GB energy and mobility after segregation.