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A better dispersion of quasicrystalline icosahedral i-phase is achieved in a Mg-3 at. pct Zn-0.5 at. pct Y (ZW82) alloy by a process involving two direct extrusions here. It is confirmed by particle size and distribution analysis that the finer distribution of the particles results from breakage and refinement of the larger particles within the matrix. Better particle distribution resulted in full recrystallization, reported in this alloy composition for the first time. Consequently, a tension-compression yield symmetry was achieved even with a relatively large grain size of about 5 μm. A weaker basal texture (maximum intensity 5.283) played an important role in the strengthening. Yield strengths of about 259 MPa in tension and 269 MPa in compression, with large tensile elongation of 28 pct and compressive elongation of about 20 pct, were obtained. Thus, full recrystallization assisted by finer dispersion of icosahedral phase strengthens the alloy against twinning to remove yield asymmetry, even in a relatively larger grain size alloy.

Persistent magnesium (Mg) dissolution/deposition during cycling is crucial for the practical use of Mg rechargeable batteries, and the alloying-enhanced performance has recently attracted much attention. Nevertheless, the microscopic relationship among the alloys, the defects, and the performance remains under debate. Here, via comprehensive Density Functional Theory calculations, we revealed the effect of alloying-induced grain boundaries (GBs) and demonstrated a microscopic mechanism of how the GBs and alloys affect the performance. Mg atoms at the [0001](100) tilt GB and (110) surface are preferentially stripped during discharge, resulting in a “pit-type” morphology. Surprisingly, alloying does not change Mg's dissolution tendency at GBs. Instead, it can tune the number of tilt GBs, as alloying with Ca or Na can create more GBs than alloying with Li, Al, and Zn, resulting in improved discharge performance. Considering the experimental observation, we also propose a new picture of a GB-dependent electrochemical energy diagram extending from the conventional electrochemical theory.

Segregation of rare earth alloying elements are known to segregate to grain boundaries in Mg and suppress grain boundary sliding via strong chemical bonds. Segregation of Mn, however, has recently been found to enhance grain boundary sliding in Mg, thereby boosting its ductility. Taking the Mg (2¯114) twin boundary as an example, we performed a first-principles comparative study on the segregation and chemical bonding of Y, Zn, and Mn at this boundary. We found that both Y-4d and Mn-3d states hybridized with the Mg-3sp states, while Zn–Mg bonding was characterized by charge transfer only. Strong spin-polarization of Mn pushed the up-spin 3d states down, leading to less anisotropic Mn–Mg bonds with more delocalized charge distribution at the twin boundary, and thus promotes grain boundary plasticity, e.g., grain boundary sliding.

Magnesium (Mg) and its alloys usually show relatively low strength and poor ductility at room temperature due to their anisotropic hexagonal close-packed (HCP) crystal structure that provides a limited number of independent slip systems. Here we report that unique combinations of strength and ductility can be realized in bulk polycrystalline pure Mg by tuning the predominant deformation mode. We succeeded in obtaining the fully recrystallized specimens of pure Mg having a wide range of average grain sizes, of which minimum grain size was 650 nm, and clarified mechanical properties and deformation mechanisms at room temperature systematically as a function of the grain size. Deformation twinning and basal slip governed plastic deformation in the conventional coarse-grained region, but twinning was suppressed when the grain size was refined down to several micro-meters. Eventually, grain boundary mediated plasticity, i.e., grain boundary sliding became dominant in the ultrafine-grained (UFG) specimen having a mean grain size smaller than 1 μm. The transition of the deformation modes led to a significant increase of tensile elongation and breakdown of Hall-Petch relationship. It was quantitatively confirmed by detailed microstructural observation and theoretical calculation that the change in strength and ductility arose from the distinct grain size dependence of the critical shear stress for activating different deformation modes.

The change in twin-boundary mobility that is caused by twin-boundary segregation was investigated in pure magnesium and four binary solid-solution magnesium alloys (Mg-Ag, Mg-Sn, Mg-Y and Mg-Zn all...

A novel metallurgical approach was adopted to enhance the electrochemical activity of Mg anodes for magnesium rechargeable batteries. The primary electrochemical processes were considered to be grain boundary-mediated. Therefore, appropriate control of the grain size combined with Ca alloying of the Mg anode resulted in a remarkable electrochemical Mg2+/Mg0 cycling activity.

In a Mg-Sc system, a bcc (β) phase exists in a high-temperature region, and the quenched β shows drastic age-hardening and shape-memory properties. In addition to the disordered β phase, the ordered β (B2) phase is reported to exist in a low-temperature region. We report that the quenched β phase can be ordered through aging treatment at 573 K. This finding indicates that the mechanical characteristics and shape-memory functionality of β-type Mg-Sc alloys can be controlled by ordering the β phase.

The impact of grain boundary structures on mechanical properties and its deformation behavior was investigated on a wrought processed Mg-0.3 at%Mn binary alloy. The Mg-Mn alloy, which had an average grain size of 1.2 µm and a high density of low-angle grain boundaries, was fabricated via caliber rolling. This alloy exhibited yield strength of more than 300 MPa and had low strain rate dependence in both tension and compression tests at room temperature. The major deformation mechanism was dislocation slip for tension and deformation twinning for compression, respectively, as is well established for conventional wrought processed magnesium alloys. However, an extruded Mg-Mn alloy having an average grain size of 1.8 µm is reported to exhibit a good elongation-to-failure and large strain rate dependence. While the same cast billet was used, the result obtained from caliber rolled alloy is inconsistent with that from the extruded alloy. This is due to the difference in grain boundary structures, i.e., the existence of low-angle grain boundaries in the caliber rolled alloy. Low-angle grain boundaries, which do not have free volume, are difficult for segregation of alloying elements. The mechanical properties and deformation mechanism can be clearly controlled by grain boundary structures through wrought process.

Precipitation of a stable quasicrystalline i-phase has been discovered in a Mg-6Zn-3Al (wt%, ZA63) magnesium alloy, possibly a first involving a stable i-phase. Dissolution of i-phase solidified at grain boundaries leads to precipitation in the Mg-matrix during cooling due to a solvus boundary observed by phase diagram calculation. Phase transformation, nucleation characteristics, morphology and interfaces with the matrix have been studied. Differential scanning calorimetry (DSC) and in-situ X-ray diffraction (XRD) showed dissolution and precipitation of i-phase during heating and cooling cycles, forming a hysteresis between about 250 and 325°C. The quasilattice parameter a R of i-phase was calculated to be 5.1704 A at room temperature, close to the c-parameter 5.1831A of the Mg-matrix. At dissolution/nucleation, the diffraction peaks of i-phase were sharper, (quasi)lattice constants a R and c Mg become equal to each other (5.225A) and the lattice strain in the matrix was minimized. Precipitation occurred with a fivefold plane parallel to a matrix pyramidal {0111} plane, which gave rise to two possible orientation relationships (OR): 5f||[0001] (OR3’) and 2f||[0001] (OR1). Presence of both ORs in the same precipitates generated icosahedral twinning. The precipitates were faceted sharply on fivefold, twofold and threefold planes which were on basal, prismatic and pyramidal planes of the Mg matrix. Results suggest the OR selection to be nucleation temperature dependent. Based on the crystallographic relationships, 3-dimensional tiling model of icosahedral lattice has been used to model lattice match with the matrix.

Segregation of rare earth alloying elements are known to segregate to grain boundaries in Mg and suppress grain boundary sliding via strong chemical bonds. Segregation of Mn, however, has recently been found to enhance grain boundary sliding in Mg and thereby boosting its ductility. Taking the Mg (-2114) twin boundary as an example, we have performed a first-principles comparative study on the segregation and chemical bonding of Y, Zn, and Mn at this boundary. We find that both Y-4d and Mn-3d states hybridize with the Mg-3sp states, while Zn-Mg bonding is characterized by charge transfer only. Strong spin-polarization of Mn pushes the up-spin 3d states down, leading to less anisotropic Mn-Mg bonds with more delocalized charge distribution at the twin boundary, and thus promotes grain boundary plasticity, e.g., grain boundary sliding.

In this study, a Mg-0.3at.%Y alloy was provided for a severe plastic deformation by high pressure torsion (HPT) and subsequent annealing. After the HPT by 5 rotations, nanocrystalline structures with a mean grain size of 0.23 μm having deformed characteristics were obtained. Fully recrystallized microstructures with mean grain sizes ranging from 0.66 μm to 32.7 μm were obtained by subsequent annealing at various temperatures. Room temperature tensile tests revealed that ultrafine grained (UFG; grain sizes smaller than 1 μm) specimen exhibited very high yield strength over 250 MPa but limited ductility. In contrast, good balance of strength and ductility was realized in fine grained specimens with grain sizes around 2~5 μm. Particularly, the yield strength and total tensile elongation of a specimen with a mean grain size of 2.13 μm were 184 MPa and 37.1%, respectively, which were much higher than those of pure Mg having a similar grain size. The significant effects of grain size and Y addition on the mechanical properties were discussed.

The effect of alloying elements on room temperature stretch formability and its deformation mechanism was investigated using pure magnesium and its binary alloys containing an element of Ag, Al, Ca, Li, Mn, Pb, Sn, Y and Zn, respectively. All of the alloys as well as pure magnesium were produced by extrusion and had a similar average grain size. The alloying elements clearly affected stretch formability as measured by Erichsen testing. The addition of manganese or yttrium element played a role in improving formability; on the other hand, most of the alloying elements did not improve formability, as exhibited by their limited dome height values, which were similar to or lower than that of pure magnesium. This resulted from their differences in deformation mode. An intragranular misorientation axis analysis using electron back-scatter diffraction results revealed that the Mg-Y alloy had a high fraction of grains amenable to non-basal dislocation slips, which is a helpful deformation mode for improving formability. In contrast, pure magnesium and the Mg-Mn alloy, which had a high limited dome height, did not have a high fraction of such grains. Grain boundary sliding compensated for the lack of -component, such as activating non-basal dislocation slips.

The impact of chemical composition, i.e., the manganese element, on room-temperature mechanical properties and its deformation behavior was examined using a series of Mg-Xat% (X = 0.05, 0.1, 0.3, 0.45, 0.6 and 0.8) Mn binary alloys. The average grain size of these alloys was controlled to be 2–3 µm by extrusion temperature. The manganese segregates at grain boundaries, and the fraction of dispersed particles increases with increasing manganese addition. The results obtained from tensile tests reveal that the chemical composition is unlikely to affect the strain rate dependence, which is the major deformation mechanism. The strain rate sensitivity (m-value) is calculated to be ~ 0.1 and ~ 0.2–0.3 in the quasi-static and in the low strain rate regimes, respectively, for all the alloys. This indicates that the contribution of grain boundary sliding on deformation increases at lower strain rates. On the other hand, the elongation-to-failure in tension is clearly influenced by manganese concentration. The chemical composition of 0.3 at% manganese brings about a good ductility of nearly 95% and 145% in the strain rates of 1 × 10−3 and 1 × 10−5 /s, respectively; however, a further manganese addition leads to a decrease in the elongation-to-failure. Deformed microstructural observations show that cavities nucleate at the interface between matrix and particles without any deformation twin formation, and they are the origin of fracture. A high amount of manganese causes formation of the dispersed particles; as a result, it is ineffective to improve the ductility.

We investigated the effect of solute elements on interfacial segregation and fracture in Mg alloys by first-principles density functional theory calculations in conjunction with both experiment and interfacial fracture mechanics. Based on the assumption of brittle fracture in Mg alloys, the interfacial separation caused by segregated solutes in Mg can be efficiently described by the energy-based criterion of fracture, which is in good agreement with the fracture toughness obtained by experimental tests of Mg–M binary alloys. The electronic interaction—that is, the change in the electronic state between the interface and surface—mainly influences the ideal work of separation regardless of the type of interface. We found that IIIB (d1) and IVB (d2) solutes, such as Zr, show distinctive hybridization between the p band of Mg and the d band of the solute, which characterizes the strong fracture toughness of Zr-doped Mg alloys in both the calculations and experiments.

Fine-grained Mg-Xat.%Bi dilute binary alloys having different concentrations of bismuth element were produced by conventional extrusion process. The ductility decreased with increasing bismuth addition; however, the Mg-0.3 at.%Bi alloy containing binary phase particles exhibited an elongation-to-failure in tension of 170% even at a strain rate of 1 × 10−3/s at room-temperature. This superior mechanical property results from grain boundary sliding, which is closely related to equilibrium grain boundary structures. This ductility has never been observed in any other magnesium alloys and the temperature of 0.32Tm for obtaining the present superplastic-like behavior is the lowest among the conventional metallic materials.

In-situ TEM imaging has been carried out on thin foils of pure Mg and Mg-0.3at.%Y alloy under tension, to study crack nucleation and propagation. During propagation along grain boundaries, twin nucleation occurred from the crack tip. In case of pure Mg, the crack did not propagate along the twin boundaries but continued along the grain boundaries. However, in case of Mg-0.3at.%Y alloy, in which the grain boundaries were segregated with yttrium, the crack propagated along newly formed twin boundaries nucleated at the crack tip. This directly demonstrates the effect of alloying on the fracture behavior.

Due to the excellent balance of strength and ductility exhibited by some Mg-Zn-RE (Y subgroup rare earth element) alloys, which contain icosahedral quasicrystalline precipitates, it is of interest to examine their deformation mechanisms. The internal strain evolution Mg-3at%Zn-0.5 at%Y with 4 vol% i-phase was measured using in-situ neutron diffraction. The extruded samples exhibit an initially weak || extrusion direction “rod texture,” distinct from the normally strong texture of extruded Mg alloys, but the grain size is unexceptional (16.7 ± 2.1 μm). The initially weak texture contributes to a nearly symmetric yielding response between tension and compression. The hardening responses are asymmetric, however, since {10.2} extension twinning is significantly more active during compressive straining, despite the initially weak texture. In-situ neutron diffraction tension and compression experiments parallel to the extrusion direction, together with elasto-plastic self-consistent (EPSC) crystal plasticity modeling, reveal the strength and hardening behavior of individual slip and twinning modes. The previously published twinning-detwinning (TDT) model is implemented within the EPSC framework, and it is proven effective for describing the observed, mild tension-compression asymmetry. This is not possible with previous EPSC-based models of twinning. Finally, the description of hardening within the TDT model is modified, in order to accurately describe the evolution of internal strains within the twins.

Magnesium and its alloys have the lowest density among structural metallic materials; thus, this light-weight metal has great potential for reducing the weight, for example, of vehicles and trains. However, due to its crystal structure, deformability is poor; in particular, under compressive stress. In this study, we modified magnesium with bismuth as an alloying element, which has the characteristics of being likely to form precipitates instead of grain boundary segregation. The Mg-Bi binary alloy showed excellent deformability and high absorption of energy in high-strain rate regimes at room temperature via contribution of grain boundary sliding. These properties, which are closely comparable to those of conventional middle-strength aluminum alloys (Al-Mg and Al-Mg-Si series alloys), have never been observed before in magnesium alloys. The development of such properties opens the door for not only academic but also industrial research in magnesium.

Microstructures and mechanical properties were investigated using Mg-9at.%Y-6at.%Zn alloy produced by several types of wrought-processing, i.e., forging, rolling, extrusion, equal-channel-angular extrusion, caliber rolling and high-pressure-torsion. Regardless of the wrought-processing method, deformation kink bands were induced to the long-periodic stacking ordered phase. Hardness increased with higher magnitude of applied strains. However, the inducing direction of shear strain affected the hardness values, and multiple shear strains were effective in improving the mechanical properties. For instance, the alloy produced by extrusion-torsion process exhibited yield strength in compression exceeding 600 MPa, which was over 1.7 times higher than that of the direct extruded alloy.

The kink bands strengthening effect was investigated using Mg-9at.%Y-6at.%Zn alloys produced by caliber rolling or extrusion at intermediate and elevated temperatures (373 K and more than 673 K). The results of microstructural observations showed that deformation kink bands were induced in all the alloys, regardless of the wrought-processing method and the processing temperatures. Micro-Vickers hardness tests showed that the regions including kink bands exhibited a higher hardness than those without kink bands, which reveal intrinsic kink bands strengthening. Interestingly, the trend of this kink bands strengthening effect could be expressed by an empirical relationship, which was divided into two lines associated with the wrought-processing temperature. This tendency did not appear to depend on the wrought-processing method. The alloys produced at the intermediate temperature showed markedly superior hardness, since, in addition to the kink bands strengthening, the residual strains/dislocations around kink boundaries contributed to obstruct dislocation slips.

The microstructures of pure magnesium produced by high-pressure-torsion (HPT) are influenced by the grade of its purity. Conventional purity magnesium (99.9 mass% purity) readily obtains finer grained structures than those of ultra-high purity magnesium of 99.9999 mass%. This is due to the impurity elements playing a significant role in obstruction of dislocations induced by HPT process; as a result of promoting dynamic recrystallization as compared to that in the ultra-high purity magnesium. Nano-indentation tests reveal that these HPT-ed microstructures affect the mechanical properties (hardness and damping capacity) and the deformation mechanism. Higher damping capacity and larger strain rate dependence on indentation hardness are obtained in the HPT-ed commercial grade magnesium, since grain boundary plasticity, such as grain boundary sliding, contributes to deformation, associated with a high volume fraction of grain boundaries.