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-0.3at.%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.
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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 Mg
2+
/Mg
0
cycling activity.
Appropriate control of the crystal orientation and the grain size combined with Ca element alloying results in a highly active magnesium metallic anode.
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.
The effect of grain size and alloying elements on the yield strength in compression was investigated using pure magnesium and its diluted solid solution alloys (Mg–Zn and Mg–Y alloys). The yield ...strength in compression was closely related to the {10–12} twinning, and followed the Hall–Petch relation in all the alloys. The value of σ0 increased in the following order, Mg–Y>Mg–Zn>pure magnesium, indicating solid solution strengthening; however, the slope in the Hall–Petch relation was not influenced much by the alloying elements. The twinning formed frequently at the grain boundaries with a low misorientation angle, and even occurred in the parent grains with a low Schmid factor in all the alloys. These results indicated that one of the most influential parameters for the Hall–Petch slope was not the Schmid factor but the grain boundary characteristics. All the stress and strain curves in the compression showed a plateau region referring to the expansion/propagation of {10–12} twinning after the yielding behavior. The length of the plateau region increased with a decrease in the grain size and the Schmid factor of the basal slip in the parent grains.
•Tube-shaped Mg-Y-Zn alloy, inducing kink boundaries, is formed without cracks.•This formed alloy exhibits an increased hardness magnitude of approximately 30 Hv.•Induced kink density is lower than ...the estimated value owing to de-kinking behavior.•The present formed alloy shows similar kink density vs. hardness to previous studies.
The secondary formability and the effect of shape application on induced kink boundaries are examined using Mg-9at.%Y-6at.%Zn alloy through experimental and numerical studies. A tube-shape is successfully formed without any cracks. This tube-shaped alloy includes kink boundaries associated with the applied shear strain. In the numerical study, a substantial shear strain is created at the contact region, but the shear direction becomes opposite after tube formation. This suggests that de-kinking behavior, such as shrinkage of kink boundaries, has a possibility to occur. In contrast, this formed alloy exhibits higher hardness by a magnitude of 30 Hv than the un-processed alloy. The correlation between kink density and hardness in this tube-shaped alloy is consistent with literature results obtained from several wrought-processed Mg-Y-Zn alloys.
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 effects of grain size and strain rate on tension behavior at ambient temperature were investigated for several extruded magnesium with an average grain size in the range between 1 and 20
μ
m. In ...quasi-static strain rate regimes (1 × 10
−2
to 10
−4
s
−1
), the activation volume was ~ 20
b
3
(
b
is the Burgers vector), which suggested that the major contribution of deformation was cross-slip and/or multiple slips. In contrast, in low strain rate regimes (<1 × 10
−4
s
−1
), the ductility increased with grain refinement, and the maximum elongation-to-failure was 230 pct for a grain size of 1.2
μ
m and a strain rate of 1 × 10
−5
s
−1
; such ductility was never observed in magnesium at room temperature. In addition, the activation volume was also reduced to ~5
b
3
. Observations of the deformed surface revealed plentiful traces of grain boundary sliding. This mechanism played an important role in deformation. As a result, while the yield strength was aligned on the Hall–Petch relation in quasi-static strain rate regimes, an inverse Hall–Petch effect was observed in low strain rate regimes.
Mechanical properties of a coarse-grained hexagonal close packed (hcp) single phase Mg-Sc alloy with two different textures, randomized (through body centered cubic (bcc) to hcp transformation) or ...weakened (through only cold rolling) texture, was investigated. Alloys of both the textures showed high elongation of over 30% with a moderately high ultimate tensile strength of about 240 MPa. Transmission electron microscopy (TEM) observations showed that both a- and c- component dislocations were activated in the grain interiors, over 20 µm away from grain boundaries. Thus, pyramidal <c+a> slip can be activated in grain interiors of a coarse-grained Mg-Sc alloy.
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