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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.

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.

Recently, we found that a Mg–Sc alloy with a bcc (β) phase exhibits superelasticity and a shape memory effect at low temperature. In this work, we examined the stress-induced and thermally induced martensitic transformation of the β-type Mg–Sc alloy and investigated the crystal structure of the thermally induced martensite phase based on in situ X-ray diffraction (XRD) measurements. The lattice constants of the martensite phase were calculated to be a = 0.3285 nm, b = 0.5544 nm, and c = 0.5223 nm when we assumed that the martensite phase has an orthorhombic structure (Cmcm). Based on the lattice correspondence between a bcc and an orthorhombic structures such as that in the case of β-Ti shape memory alloys, we estimated the transformation strain of the β Mg–Sc alloy. As a result, the transformation strains along the 001, 011, and 111 directions in the β phase were calculated to be + 5.7, + 8.8, and + 3.3%, respectively.

The effect of alloying elements on grain boundary sliding was systematically investigated using several binary magnesium alloys (X = Ag, Al, Li, Sn, Pb, Y and Zn) via both experimental and numerical methods. The alloying element clearly affected damping properties related to grain boundary sliding, as measured by nanoindentation tests. The properties, such as damping capacity and strain rate sensitivity, apparently depended on grain boundary characteristics, i.e., the grain boundary energy. By increasing and decreasing the grain boundary energy, the alloying element was found to play a role in enhancing and suppressing grain boundary sliding, respectively. First-principles calculations revealed that the lithium element had weak bonding to magnesium due to a few operations of the electric orbit. On the other hand, rare-earth elements exhibited relatively strong bonding to magnesium, because of electron interactions with the first nearest neighbor site, and tended to prevent grain boundary sliding. These results suggest that grain boundary energy is a reliable parameter for predicting grain boundary sliding and developing a magnesium alloy, which has good elongation-to-failure and/or secondary formability at room temperature.

The effect of crystal orientation on incipient plasticity during nanoindentation was investigated by experiments and molecular statics simulation. Pop-in behavior is a result of dislocation activity, and is therefore influenced by crystal orientation. Experimental results using single crystals indicated that indentations on the basal plane had higher pop-in loads and larger pop-in displacements than those on the prismatic plane, an effect also captured by molecular statics simulation. The difference can be traced to the types of activated dislocations, with not only basal but also pyramidal dislocations active for indentations on the basal plane, but only basal dislocations triggered at the first pop-in on the prismatic plane.

We show that, while interfaces formed in a Mg-3Zn-0.5Y (at%) magnesium alloy by severe plastic deformation (SPD) by high pressure torsion (HPT) at room temperature (RT) are segregated with Zn (without aid of any thermal treatment), only a small fraction of grain boundaries are segregated when extruded at 573 K (300 °C). We have examined the effect of strain rate and temperature on the diffusion behavior and segregation of zinc in magnesium alloys. At first, we have established the driving force for segregation by evaluating segregation energy and the interface energy gain by density functional theory calculations. The mechanism of segregation is established through calculation of excess vacancy concentration and critical dislocation velocity as a function of strain rate. It is estimated that Zn atoms are transported by SPD induced vacancy flux in case of HPT at RT, whereas the Zn atoms are dragged by equilibrium vacancies and dislocations on extrusion at 573 K (300 °C). The amount of segregation to the different twins and grain boundaries with different interfacial energies have been calculated by thermodynamic parameters and found to be in the range of 1–8 at% of Zn for the case of HPT process and 0.7–1.6 at% of Zn for extrusion processed specimens. These estimates correspond to the solute concentrations determined experimentally.

Deformation structures have been examined by transmission electron microscopy after 4% plastic deformation in tension and compression in a Mg-2.2 at%Y alloy extruded at 698 K to obtain a recrystallized grain size of about 20 μ m . The extruded alloy showed similar yield stresses of about 240 MPa in both tension and compression at room temperature. Dislocations formed by tensile stress showed a tendency of 〈 a 〉 type dislocations to cross slip to prismatic planes, resulting in long continuous dislocation loops. Dislocation loops with 〈 c 〉 components were also observed. Occasionally, { 10 1 ¯ 2 } type twinning was also observed in grains with large orientation away from basal texture. These grains contained predominantly basal slip. Compression deformed samples showed a limited number of { 10 1 ¯ 2 } type twins. The matrix contained loops of non-basal 〈 a 〉 type dislocations, together with basal dislocations with 〈 c 〉 component. Inside the twins occurred stacking faults and loops of 〈 c 〉 type dislocations with segments perpendicular to the basal plane. Activation of several deformation modes and complex dislocation structures explains the strain hardening behavior and low anisotropy of the alloy.

The effect of grain size and forming speed on room temperature formability was examined using Mg-0.3at%Mn binary alloy with different average grain sizes (2.5 and 18.5 µm). Erichsen tests revealed that the limited dome height (LDH) was clearly influenced by these factors. LDHs increased with finer grain sizes and/or lower forming speeds. The highest LDH was obtained to be 8.2, which is superior to that in the conventional aluminum (A5083) alloy (LDH=4.3), in the present Erichsen testing conditions. The results obtained from the tensile tests under strain rate ranges similar to those of the Erichsen tests showed high strain rate sensitivity ( m -value, where m =0.06–0.13 for the meso-grained alloy and 0.11–0.22 for the fine-grained alloy, respectively), irrespective of the extrusion direction vs. the tensile direction. Deformed microstructural observations after tensile and Erichsen tests revealed the existence of deformation twinning; however, the area/volume fraction of its microstructural feature was too small to affect the deformation mechanism. Instead of the deformation twinning-related fracture, cavitation due to grain boundary sliding was the origin of fracture during the present Erichsen tests. The further activation of grain boundary sliding was an effective method to improve the room temperature formability of magnesium alloys.

The effect of solute segregation at deformation twin boundaries on the strength (hardness) and damping capacity was investigated using an extruded Mg-Y binary alloy. The existence of deformation twin brought about an increase in both the hardness and the damping capacity. However, a subsequent annealing had a different effect, in which segregation at twin boundaries led to a decrease in the damping capacity. This is because the segregation of yttrium stabilized the twin boundaries; as a result, these boundaries inhibited alternate shrinkage and growth of deformation twin, by which the vibration energy is absorbed.

The impact of alloying elements on crack propagation and atomistic phenomenon at { 10 1 ¯ 2 }-type twin boundaries in magnesium was investigated via both experiments and calculations. The alloying elements clearly affected the crack propagation behavior. Cracks were difficult to propagate along matrix-deformation twinning interfaces in alloys that had high fracture toughness. In such magnesium alloys, the solute atoms, e.g., silver, manganese and zinc atoms, create adhesive interactions between magnesium atoms. Closed-shell and covalent-like bonding of these types of solute atoms would influence strong adhesion, which impedes the nucleation of a new surface at the twin boundary.

The change in twin boundary mobility due to twin boundary segregation was investigated in pure magnesium and four binary solid solution magnesium alloys. The damping capacity in the vicinity of {10-12} twin boundaries were measured before and after annealing by nano-dynamic mechanical analysis. The annealing led to lower damping capacity in all magnesium alloys, which was in contrast to the results in pure magnesium. This is caused by the segregation of solute atoms in incoherent twin boundaries. The alloying element that has a characteristic of low segregation energy for twin boundaries effectively prevented the damping capacity degradation.

This study represents the quantitative kink-band strengthening effect using uniaxial hot-compressed Mg-9%Y-6%Zn alloy. When room-temperature micro-Vickers indentation was performed on a specific area containing kink boundaries under a low indentation load of 10 gf, the magnitude of increase in hardness due to kink-band strengthening was closely related with the number of kink boundaries beneath the indentation, i.e., the kink-band strengthening effect can be expressed by the number of kink boundaries. It is noted that this increased value is calculated by subtraction from the hardness value of the matrix without kink boundaries. The present estimated kink-band strengthening values via microscopic empirical analysis is in a similar range to those of prior macroscopic analysis obtained from the results of compression tests using bulk specimens.

The possibility to control the deformation kink bands morphology was investigated by the caliber rolling process using the Mg-9 at.%Y-6 at.%Zn alloy with a volume fraction of ~100% in a long-periodic stacking ordered (LPSO) phase. Changing the insertion direction of the rolled billet was an effective method to control the area fraction of deformation kink bands formation. When the rolled billet was rotated 180° to the rolling direction in each rolling pass, deformation kink bands formed in not only the coarse-sized but also the fine-sized LPSO phases. These induced deformation kink bands had a higher density than those of the conventional inserted caliber rolled alloy. This is mainly due to the difference in shear direction to the rolled billet in every rolling pass. The rolled alloy with a high area fraction of deformation kink bands showed superior mechanical properties of hardness and strength at room temperature.

The effect of alloying elements on nano-ordered wear properties was investigated using fine-grained pure magnesium and several types of 0.3 at. pct X (X = Ag, Al, Ca, Li, Mn, Y, and Zn) binary alloys. They had an average grain size of 3 to 5 μm and a basal texture due to their production by the extrusion process. The specific wear rate was influenced by the alloying element; the Mg-Ca and Mg-Mn alloys showed the best and worst wear property, respectively, among the present alloying elements, which was the same trend as that for indentation hardness. Deformed microstructural observations revealed no formation of deformation twins, because of the high activation of grain boundary-induced plasticity. On the contrary, according to scratched surface observations, when grain boundary sliding partially contributed to deformation, these alloys had large specific wear rates. These results revealed that the wear property of magnesium alloys was closely related to the plastic deformation mechanism. The prevention of grain boundary sliding is important to improve the wear property, which is the same as that of a large-scale wearing configuration. One of the influential factors is the change in the lattice parameter with the chemical composition, i.e., ∂(c/a)/∂C. An alloying element that has a large value of ∂(c/a)/∂C effectively enhances the wear property.

The effect of twin boundaries on indentation behavior was investigated in Mg-0.3 at.%X (X = Al, Li, Y and Zn) binary alloys containing preexisting { 10 1 ¯ 2 } deformation twins. These alloys exhibited the highest hardness values reported among both fine- and coarse-grained alloys studied in the literature, irrespective of the alloying elements present. The activation volumes for plastic deformation were found to lie between 18b3 and 60b3, consistent with a rate-controlling mechanism of dislocation slip. These activation volumes were similar to previously reported results for alloys devoid of twins prior to testing, suggesting that although twin boundaries do not affect the dominant deformation mechanism, they lead to an increase in hardness. On the other hand, the activation volumes obtained from indentation tests were different from those obtained by uniaxial compression tests in samples without pre-existing twins, due to the occurrence of microstructural evolution, i.e., deformation twin formation, during plastic deformation.

For the first time, we report here segregation of Zn atoms to twin boundaries formed under 9% compression at room temperature (RT) in a Mg-3Zn-0.5Y (in at%) alloy. The wrought processed (textured) alloy was severely plastically deformed (SPD) through high pressure torsion (HPT) at RT. Zn segregation (2 to 4 at%) was observed on newly formed boundaries, such as twin boundaries (formed on compression only in HPT), low angle boundaries formed by deformation and grain boundaries due to recrystallization. First principles calculations performed on low angle boundaries shows that Zn atom prefers to segregate to the dislocations in the boundary.

The enhancement method of room temperature formability was investigated by controlling the rate-controlling mechanism. Wrought processed pure magnesium and its alloys (Mg-Al-Zn and Mg-Mn alloys), which have a basal texture, were used in this study. Pure magnesium had two kinds of average grain sizes (~7 µm and ~30 µm), and the other alloys were 20–30 µm in average grain size. The forming load vs. displacement curves in Erichsen tests were not influenced by the forming rate in the Mg-Al-Zn alloy; however, the fine-grained pure magnesium and Mg-Mn alloy showed a high forming rate dependence. The limited dome heights were 4.7, 2.0 and 5.2 for the fine-grained pure magnesium, Mg-Al-Zn and Mg-Mn alloys, respectively. Deformed microstructural observations revealed that, in the meso-grained pure magnesium and the Mg-Al-Zn alloy, deformation twins formed and were closely related to crack propagation. On the other hand, the formation of such deformed structures was difficult to identify in the fine-grained pure magnesium and meso-grained Mg-Mn alloy. The good stretch formability and high rate dependence of the fine-grained pure magnesium and Mg-Mn alloy resulted from the major contribution of grain boundary sliding to deformation during Erichsen testing. The addition of an alloying element, which plays a role in the enhancement for grain boundary sliding, is quite effective to improve the room temperature stretch formability.

The impact of alloying elements on the room temperature tensile behaviour was investigated for a wide range of strain rates using eight types of extruded Mg-0.3 at.% X (X = Ag, Al, Li, Mn, Pb, Sn, Y and Zn) binary alloys with an average grain size of 2–3 μm. The solid solution alloying element affected not only tensile plasticity but also rate-controlling mechanism for these fine-grained magnesium alloys. Most of the alloys exhibited an elongation-to-failure of 20–50% , while the alloys with a high m-value exhibited large tensile plasticity, such as an elongation-to-failure of 140% in a strain rate of 1 × 10−5 s−1 for the Mg–Mn alloy. This elongation-to-failure is more than two times larger than that for pure magnesium. This is due to the major contribution of grain boundary sliding (GBS) on the deformation. Microstructural observations reveal that grain boundary segregation, which is likely to affect gain boundary energy, plays a role in the prevention or enhancement of GBS. The present results a...

The effect of crystal orientation on nanoindentation behavior at both quasi-static and high strain rates was investigated using single-crystalline magnesium oriented in basal and prismatic configurations. Both the basal and prismatic planes had similar activation volumes, 55 and 73[superscript b3] for deformation at room temperature, as well as a small temperature dependence up to 423 K (150 °C). Microstructural observations beneath the indentations revealed that { 10[bar over 1]2 } type deformation twins were formed in both orientations irrespective of testing temperature. With twins forming beneath the indenter and multiple orientations of loading, it is believed that cross-slip and/or multiple slip are likely rate-controlling for global deformation, which also aligns with observations on nanoindentation of polycrystalline coarse-grained magnesium. The locations of the twins were consistent with expectations based on indentation mechanics as assessed by finite element simulations. The finite element simulations also predicted that an indenter tip with a shaper tip radius would tend to promote { 10[bar over 1]2 } twins., United States. Army Research Office. Institute for Soldier Nanotechnologies, Japan Society for the Promotion of Science (Grant-in-Aid (C) No. 25420765)

This study investigated strain-rate sensitivity (SRS) in an as-extruded AZ31 magnesium (Mg) alloy with grain size of about 10 mm. Although the alloy shows negligible SRS at strain rates of >10-5 s-1 at room temperature, the exponent increased by one order from 0.008 to 0.06 with decrease of the strain rate down to 10-8 s-1. The activation volume (V) was evaluated as approximately 100b3 at high strain rates and as about 15b3 at low strain rates (where b is the Burgers vector). In addition, deformation twin was observed only at high strain rates. Because the twin nucleates at the grain boundary, stress concentration is necessary to be accommodated by dislocation absorption into the grain boundary at low strain rates. Extrinsic grain boundary dislocations move and engender grain boundary sliding (GBS) with low thermal assistance. Therefore, GBS enhances and engenders SRS in AZ31 Mg alloy at room temperature.

Microstructural investigation of extremely strained samples, such as severely plastically deformed (SPD) materials, by using conventional transmission electron microscopy techniques is very challenging due to strong image contrast resulting from the high defect density. In this study, low angle annular dark field (LAADF) imaging mode of scanning transmission electron microscope (STEM) has been applied to study the microstructure of a Mg-3Zn-0.5Y (at%) alloy processed by high pressure torsion (HPT). LAADF imaging advantages for observation of twinning, grain fragmentation, nucleation of recrystallized grains and precipitation on second phase particles in the alloy processed by HPT are highlighted. By using STEM-LAADF imaging with a range of incident angles, various microstructural features have been imaged, such as nanoscale subgrain structure and recrystallization nucleation even from the thicker region of the highly strained matrix. It is shown that nucleation of recrystallized grains starts at a strain level of revolution N=1/4 (earlier than detected by conventional bright field imaging). Occurrence of recrystallization of grains by nucleating heterogeneously on quasicrystalline particles is also confirmed. Minimizing all strain effects by LAADF imaging facilitated grain size measurement of 150±25 nm in fully recrystallized HPT specimen after N=5., Graphical Abstract

We have focused on dispersion of quasicrystalline i-phase in a “Rare Earth-free” magnesium alloy Mg–6Zn–3Al (ZA63). The alloy was chill-cast to produce orthorhombic φ phase interdendritically and quasicrystalline i-phase particles with strong pinning effect at grain boundaries, giving the boundaries a very curved shape. The cast alloy was subjected to a two-step direct extrusion process to study phase (particle) distribution, grain structure, microtexture and related parameters using a host of techniques associated with scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and the resulting mechanical properties. The first extrusion (at 563 K) with a small extrusion ratio of 6.25:1 produced a dispersion of φ phase along the extrusion direction and i-phase decorating the boundaries of grains of size of about 30 μm with a mild basal texture. The second extrusion (at 483 K) with an extrusion ratio of 25:1 reduced the average grain size to ∼ 2 μ m, with comparable yield strength 287.0 ± 4.1 MPa in tension and 299.0 MPa in compression (elongations ∼ 21%). The i-phase was found to be stable by differential scanning calorimetry (DSC) and the φ phase appeared to transform to the i-phase.

The impact of alloying element on the anelastic behavior at temperatures from 273 to 573 K was examined using pure magnesium and eight solid-solution binary magnesium alloys. All alloys, as well as pure magnesium, had meso-grained structures with an average grain size of approximately 30 µm. Measurement of the damping capacity revealed the occurrence of grain boundary relaxation at intermediate and elevated temperatures in all materials. The onset temperature for grain boundary relaxation depended on the alloying element. Pure magnesium and four binary alloys (Mg–Al, Mg–Li, Mg–Mn and Mg–Sn alloys) had an activation energy of approximately 1 eV, which is close to that of grain boundary diffusion in magnesium; in contrast, higher activation energies were obtained for some alloys, i.e., ~ 1.5 eV for the Mg–Ag and Mg–Zn alloys, and ~ 2–3 eV for the Mg–Ca and Mg–Y alloys, respectively. This work reveals that grain boundary segregation of the solute atoms is an influential factor for grain boundary relaxation in magnesium alloys.

The effect of process temperature on microstructure and mechanical properties was investigated using Mg–4at%Y–2at%Zn alloy produced by caliber rolling at temperatures of 373 K and 748 K. Caliber rolled bars were successfully fabricated with the introduction of deformation kink bands in the long periodic stacking ordered (LPSO) structure, regardless of rolling temperature. Microstructural observations showed that the alloy rolled at 748 K had the same microstructural features as previously reported wrought-processed magnesium alloys that contained the LPSO phase. In contrast, the alloy rolled at the lower temperature (373 K) had a high density of residual strains and deformation twins, as well as deformation kink bands. These microstructural features affected the mechanical properties; the alloy rolled at 373 K had higher properties of strength and hardness compared with those of the alloy produced at 748 K. However, the lower-temperature rolled alloy showed a decrease in compressibility, due to the existence of micro-cracks. Three-dimensional microstructural observation revealed that such defects were unlikely to be present in deformation kink bands and at the interface between kink bands and LPSO/α-Mg phases.

The segregation of lithium atoms at grain boundaries was successfully confirmed by microstructural observation through electron energy-loss spectroscopy measurements. Results obtained from room-temperature tensile tests and observations of the deformed microstructure revealed that the grain boundary segregation of lithium atoms played a role in enhancing grain boundary sliding but accelerated grain boundary embrittlement.

The microstructural evolution and mechanical properties were investigated using a caliber rolled Mg–5Y-2.5Zn (in at.%) alloy. Deformation kink bands could be induced by the caliber rolling process, which is the same as that of conventional wrought processed (extruded or rolled) Mg–Y–Zn alloys. The number of rolling passes was an influential factor for the microstructure, i.e., the bending angle of the long periodic stacking ordered (LPSO) phase associated with deformation kink bands. The mechanical properties of strength and hardness also depended on the number of rolling passes. These properties were improved with increased number of rolling passes (up to 6 passes). The yield strength and hardness before the rolling process were 260 MPa and ~90 Hv. On the other hands, after caliber rolling, these properties showed over 500 MPa and 129 Hv, respectively, for the alloy with 6 rolling passes. Microstructural observations revealed that such good properties resulted from the existence of deformation kink bands and dense dislocations induced by caliber rolling. Controlling the morphology of the LPSO phase associated with deformation kink bands was found to be an effective strategy; the bending angle of this phase of more than 20° is outstanding for further improving the mechanical properties.