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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.

Age-hardenability and corresponding improvement of the mechanical properties of Mg–1Al–0.7Ca and Mg–1Al–0.7Ca–0.7Y alloy sheets are addressed with respect to the microstructure and texture evolution during thermomechanical treatments. A fine grain structure and weak texture with the basal pole split into the sheet transverse direction are retained in the Mg–1Al–0.7Ca–0.7Y sheet even after the homogenization at 500 °C, due to the grain boundary pinning by Y-containing precipitates possessing a high thermal stability. Contrarily, the Mg–1Al–0.7Ca sheet shows a coarse microstructure and basal-type texture after the homogenization. The peak-aged condition is attained after the aging at 250 °C for 1800 s of both homogenized sheets, while the Y-containing sheet shows a higher hardness than the Mg–1Al–0.7Ca sheet. TEM analysis and thermodynamic calculation show the formation of metastable precipitates composed of Al, Ca, Y and Mg in the Mg–1Al–0.7Ca–0.7Y sheet at the homogenized and peak-aged conditions. A significant increase in the yield strength is obtained in the peak-aged condition from 162 MPa after the homogenization to 244 MPa, which arises from the increased size and number density of the precipitates. The high age-hardenability of the Mg–1Al–0.7Ca–0.7Y sheet attributes to the superior mechanical properties with an improved ductility promoted by the weak texture.

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

The microstructure and texture evolution during static recrystallization of Mg-Nd alloys were investigated. Depending on the third alloying element added into Mg-Nd alloy, either Al or Zn, diverse textures are formed during recrystallization annealing. The rolled Mg-2Nd-1Zn sheet shows a high activity of prismatic slip and the formation of a weak texture with basal pole spread in the sheet transverse direction. In contrast, a strong texture with a basal pole spread in the rolling direction was formed in the Mg-2Nd-1Al sheet. An equivalent growth of the recrystallized grains at the early annealing stage retains their orientations during grain growth.

Microstructure, crystallographic texture and age hardenability of the non-flammable Mg-Al-Zn-Ca-Y alloy sheets were investigated as a systematic compositional change. By reducing the Al content to 1 wt.%, fine and homogeneous microstructure was preserved even after the solution-treatment at 500°C, while a weaken texture was obtained. The high Zn/Y ratio and high Ca solute amount in the matrix complementarily instigate the texture weakening with the basal poles tilted away from the sheet normal direction. Age hardenability was significantly improved by decreasing the Al content to 0.6 wt.% and the corresponding increase in the dissolved Ca solute. The increase in the solute elements concomitant with the reduced Al content leads to the retarded recrystallization and enhanced age hardenability. A balanced alloying of Al, Ca and Y enables the Mg sheets with improved mechanical properties as a result of texture and microstructure controls.

The effect of Al content on the texture evolution and recrystallization behavior of the non-flammable Mg sheet alloys containing Ca and Y was investigated in this study. With a decrease in the Al content from 3 wt.% to 1 wt.%, the amounts of the other alloying elements dissolved in the matrix, especially Ca, are increased. The increase of the alloying elements in a solid solution brought out the retarded recrystallization and weakened texture with the basal poles tilted toward the sheet transverse direction. Extension twinning activity increased when Al content with decreasing, resulting in the texture broadening towards the sheet transverse direction in the as-rolled sheets. The textures of the AZXW1000 and AZXW2000 sheets weaken uniformly in all sample directions during annealing, while the AZXW3000 sheet shows less weakening of the rolling direction split component. The texture weakening of the alloys with lower Al contents is attributed to the retarded recrystallization caused by the larger amount of the dissolved Ca solutes. Based on the non-basal texture and relatively stable grain structure, the Mg alloy sheet containing a relatively small amount of Al is advantageous to improve the formability.

Materials characterization 175, 111044 (1-12) (2021). doi:10.1016/j.matchar.2021.111044, Texture modification during sheet rolling appears differently in binary Mg-RE (rare-earth) or Mg-Ca alloy, compared to their ternary counterparts containing Zn. The differences in texture development and the active deformation mechanisms under tensile loading were investigated in the binary alloys. These analyses are based on in-situ synchrotron experiments and electron backscatter diffraction measurements to reveal direct experimental evidence of the texture development and the active deformation mechanisms. Higher activations of nonbasal < a > and pyramidal < c+a > dislocations were found in the Nd or Ca containing Mg alloys, compared to the Mg-Zn alloy. The texture development shows an obvious feature, which is a broadening of the basal pole intensity distribution perpendicular to the loading direction and a strengthening of the pole at the loading direction in all examined sheets. This texture development relates to the higher activation of prismatic < a > slip. The addition of Zn in the Mg-RE or Mg-Ca alloys further promotes the activation of nonbasal < a > and pyramidal < c+a > dislocations. This enhanced prismatic < a > slip in the Zn containing ternary alloys is confirmed by EBSD misorientation analysis., Published by Science Direct, New York, NY

Journal of magnesium and alloys 9(6), 2207-2222 (2021). doi:10.1016/j.jma.2021.07.029, Biodegradable implants are taking an increasingly important role in the area of orthopedic implants with the aim to replace permanent implants for temporary bone healing applications. During the implant preparation process, the material's surface and microstructure are being changed by stresses induced by machining. Hence degradable metal implants need to be fully characterized in terms of the influence of machining on the resulting microstructure and corrosion performance.In this study, micro-computed tomography (��CT) is used for the quantification of the degradation rate of biodegradable implants. To our best knowledge, for the first time quantitative measures are introduced to describe the degradation homogeneity in 3D. This information enables a prediction in terms of implant stability during the degradation in the body.Two magnesium gadolinium alloys, Mg-5Gd and Mg-10Gd (all alloy compositions are given in weight% unless otherwise stated), in the shape of M2 headless screws have been investigated for their microstructure and their degradation performance up to 56 days. During the microstructure investigations particular attention was paid to the localized deformation of the alloys, due to the machining process. In vitro immersion testing was performed to assess the degradation performance quantified by subsequent weight loss and volume loss (using ��CT) measurements.Although differences were observed in the degree of screw's near surface microstructure being influenced from machining, the degradation rates of both materials appeared to be suitable for application in orthopedic implants. From the degradation homogeneity point of view no obvious contrast was detected between both alloys. However, the higher degradation depth ratios between the crests and roots of Mg-5Gd ratios may indicated a less homogeneous degradation of the screws of these alloys on contract to the ones made of Mg-10Gd alloys. Due to its lower degradation rates, its more homogeneous microstructure, its weaker texture and better degradation performance extruded Mg-10Gd emerged more suitable as implant material than Mg-5Gd., Published by Elsevier, Amsterdam [u.a.]

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

In this work, the influence of twin-roll casting (TRC) speed on the microstructure of the through-thickness uniformity, centerline segregation, and surface quality of three wrought Mg alloys was investigated. The microstructural features of the AZ31, ZX11, and ZWK200 alloys produced at TRC speeds ranging from 1.8 m/min to 2.2 m/min (for the AZ31 and ZWK200), and 1.5–2.5 m/min (for the ZX11 alloy) were analyzed. There were clear differences in the microstructure uniformity depending on the alloy composition. Columnar grains coexisting with globular grains were found in the AZ31 and ZX11 alloys, whereas the ZWK200 alloy showed a homogeneous fine-grained microstructure characterized by a weaker texture even at the highest TRC speed used. While there is a tendency to reduce the centerline segregation as the TRC speed is decreased during casting of the AZ31 alloy, the formation of this defect cannot be prevented in the ZX11 and ZWK200 alloys by only varying the TRC speed.

The deformation behavior of extruded Mg alloys with a Ca or Nd addition (up to 0.5 wt.%) is addressed with respect to a specified thermo-mechanical treatment, realized by pre-compression and subsequent heat treatment at intermediate temperature. The twinning&ndash, detwinning process is discussed with respect to the initial texture and applied heat treatment. Isothermal aging leads to precipitation and segregation along twin boundaries and dislocations in the pre-compressed Mg alloys, and, thus, variation in the mobility of twin boundaries (TB) is observed in the investigated alloys. Despite individual scenarios of TB mobility in particular grains, in general, the same TB mobility modes are observed in the alloys independently on Ca or Nd alloying. The microstructure development, particularly the twin volume fraction and the mobility of tensile {10-12} twin boundaries, is tracked using scanning electron microscopy, including backscattered electron (BSE) imaging and electron backscatter diffraction (EBSD) mapping.

Wires of magnesium alloys possess a high potential, e.g., as filler materials, for joining applications but also for biodegradable applications, such as suture materials. While the typical process of producing wires is based on a wire drawing process, direct extrusion by using adjusted dies to deal with high degrees of deformation allows a one-step manufacturing of wires to some extent. In this work, the extrusion of wires with a thickness of 1 mm and even lower is shown feasible for pure magnesium and three Al-containing magnesium alloys (AZ31, AZ80, AZ91). The surface quality and the mechanical properties are improved with increasing Al content. It is shown that, despite the large difference in the degrees of deformation, the properties and their development are similar to those of extruded round bars. Wrapping tests were carried out as an exemplary more complex forming procedure, and the behavior is correlated to the microstructure and texture of the extruded wires.

This work investigates the deformation mechanisms active during high temperature plane strain compression and their impact on the microstructural and texture evolution of an Mg-Zn-Ca-Zr (ZXK) alloy. The microstructures of samples deformed at different strains and different loading directions were examined using electron backscatter diffraction measurements. Likely activated slip systems in grains and twins were determined by in-grain misorientation axis (IGMA), and their activity was compared to the simulation results using the Viscoplastic Self-Consistent (VPSC) model. The importance of the active deformation modes on texture development, specifically the formation of particular texture components, has been linked with a profuse activity of non-basal dislocations and the preferential activation of pyramidal I slip over pyramidal II slip. Along with prismatic slip, the activation of { 10 1 ¯ 2 } twins and pyramidal slip seems to be pivotal for the stabilization of the so-called TD-texture component in a specific loading configuration.

Slip activity from various slip modes largely determines the yield strength and ductility of Mg alloys. Solid solution elements in Mg can change the slip activity dramatically. In this paper, far-field high energy X-ray diffraction microscopy (FF-HEDM) is employed to study slip activity in a Mg-3wt%Y alloy during an in situ tensile experiment. The specimen was incrementally loaded up to 3% engineering strain along the rolling direction. At each load step, FF-HEDM data were collected to track the crystallographic orientation, center of mass, and stress tensor changes of nearly 1000 grains in the probed volume. By analyzing the change in orientation and stress tensor of individual grains at different load steps, it is possible to identify the activated slip systems and measure their critical resolved shear stress (CRSS) values. Prismatic slip and pyramidal I slip are found to be very active in this alloy. The estimated CRSS values for basal slip, prismatic slip and pyramidal I slip are 12 MPa, 38 MPa, and 36 MPa, respectively. These CRSS values were applied in a dislocation-based elastic viscoplastic self-consistent (EVPSC) model that successfully simulated the tensile stress-strain curve from the FF-HEDM experiment. The model also qualitatively predicted the crystal rotation in most of the selected grains, though it underestimated the internal stress and the magnitude of crystal rotation in these grains. Influence of solute Y on the strength and ductility of Mg alloys is discussed.

Due to their excellent strength-to-weight ratio, Mg alloys are attractive for applications where weight savings are critical. However, the limited cold formability of wrought Mg alloys severely restricts their widespread usage. In order to study the role that deformation twinning might play in limiting the elongation-to-failure ( $${\varepsilon} _{\text {f}}$$ ), in-situ tensile tests along the extrusion axis of Mg-1Mn (wt pct) were performed at 323 K, 423 K, and 523 K. The alloy exhibited a strong basal texture such that most of the grains experienced compression along their $$\langle {c}\rangle $$ -axis during deformation. At 323 K, fracture occurred at about 10 pct strain. Although basal, prismatic, and pyramidal $$\langle {c+a}\rangle $$ slip activity was observed along with extension twinning, contraction twinning significantly influenced the deformation, and such twins evolved into {10 $${\bar{1}}$$ 1}–{10 $${\bar{1}}$$ 2} double twins. Crystal plasticity simulation showed localized shear deformation within the contraction twins and double twins due to the enhanced activity of basal slip in the reoriented twin volume. Due to this, the twin–matrix interface was identified to be a potential crack initiation site. Thus, contraction twins were considered to have led to the failure of the material at a relatively low strain, suggesting that this deformation mode is detrimental to the cold formability of Mg and its alloys. With increasing temperature, there was a significant decrease in the activity of contraction twinning as well as extension twinning, along with a decrease in the tensile strength and an increase in the $${\varepsilon} _{\text {f}}$$ value. A combination of basal, prismatic, and pyramidal $$\langle {c+a}\rangle $$ slips accounted for a large percentage of the observed deformation activity at 423 K and 523 K. The lack of contraction twinning was explained by the expected decrease in the critical resolved shear stress values for pyramidal $$\langle {c+a}\rangle $$ slip, and the improved $${\varepsilon} _{\text {f}}$$ values at elevated temperatures were attributed to the vanishing activity of contraction twinning.

Mg-5wt%Y extruded alloy showed excellent tensile ductility along the extrusion direction. An in situ tensile test with three-dimensional X-ray diffraction (3DXRD) identified prismatic, basal, and pyramidal slip in different grains during deformation based on the analysis of grain rotation. Ex situ slip trace analysis using electron backscatter diffraction confirms the extensive activation of non-basal slip systems, which can explain the high ductility of this material. Critical resolved shear stress (CRSS) ratios between non-basal slip and basal slip are estimated from Schmid factor analysis.

Während in einer Vielzahl von Anwendungen in Form von Gussteilen das hohe Leichtbaupotenzial von Magnesium im Automobil hinreichend bewiesen ist, beschränken sich Blechbauteile auf sehr wenige Nischen. Zwei wesentliche Gründe für die zögerliche Nutzung des Leichtmetalls in Blechbauteilen sind zum einen der Preis für das Halbzeug, zum anderen aber auch limitierte werkstoffliche Eigenschaften bekannter Legierungen, die einen Herstellprozess automobiler Bauteile deutlich verkomplizieren. Während durch das Gießwalzverfahren wesentliche Fortschritte in der wirtschaftlichen Herstellung von Magnesium- Blechen in den vergangenen Jahren erreicht wurden, bleibt die Auswahl an verfügbaren Legierungen dünn. An letzterem Umstand setzt ein Konsortium zwischen der Volkswagen AG, dem Helmholtz-Zentrum Geesthacht und der Firma Posco an. Mithilfe von gezielter Legierungsentwicklung kann gezeigt werden, wie verbesserte Werkstoffeigenschaften zu einer vereinfachten Produktion automobiler Bauteile führen.