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.
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.
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 are clearly expected to open doors to the development of magnesium alloys with good secondary formability at room temperature through the control of alloying elements.
We measured the impact toughness of three alloys (Mg, Mg-0.3 at%Ca-0.6 at%Zn, and Mg-0.3 at%Ca-0.6 at%Al) by the impact three-point bending test. The plastic deformability and impact toughness were ...higher in the ternary alloys than in pure Mg. The generalized stacking fault energy and grain boundary cohesive energy were estimated by first-principles calculations for Mg, binary Mg–Ca, ternary Mg–Ca–Zn, and ternary Mg–Ca–Al alloys. The calculation results agreed with the trend in the experimental results. We suggest that addition of Ca along with Zn or Al reduced plastic anisotropy and strengthened the grain boundaries, leading to higher in impact toughness of Mg alloys.
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Dislocation structures in a near-isotropic Mg-Y extruded alloy Singh, Alok; Somekawa, Hidetoshi; Mukai, Toshiji
Materials science & engineering. A, Structural materials : properties, microstructure and processing,
06/2017, Letnik:
698
Journal Article
Recenzirano
Deformation structures have been examined by transmission electron microscopy after 4% plastic deformation in tension and compression in a Mg-2.2at%Y alloy extruded at 698K to obtain a recrystallized ...grain size of about 20μm. The extruded alloy showed similar yield stresses of about 240MPa 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, {101¯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 {101¯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.
Recently, magnesium alloys with a dispersion of quasicrystalline icosahedral i-phase have been shown to exhibit very high strengths combined with ductility. However, effect of the i-phase and its ...amount has not been investigated comprehensively. To make a systematic investigation, two alloys of composition Mg–6x Zn–x Y, where x=0.5 and 1at%, were chill cast and extruded at three different temperatures to produce various grain sizes with a dispersion of i-phase. The extruded alloys were tested in tension and compression. Very fine grains of micron and submicron size have been obtained, resulting in very high yield strengths of up to about 400MPa accompanied by total elongations of over 12%. Microstructural features such as precipitation and texture have been studied, and mechanical properties such as strength and ductility in tension and compression have been determined. As the grain size is refined, depending on the alloy composition and extrusion temperature, the texture is also modified, such that a higher strength resulting from finer grain size is accompanied by a reasonable elongation, of over 12%. In aged condition the Hall–Petch plots for tensile and compressive yield strengths are nearly parallel, with slopes in the range of 237–307MPaμm−1/2. The Hall–Petch slopes of critically resolved shear stress of slip and twinning are nearly the same at about 63MPaμm−1/2.
The mechanical properties were investigated by Mg–0.3
at.% Ca–1.8
at.% Zn alloy, which was produced by hot extrusion, having an average grain size of about 1
μm and spherical precipitates in the ...matrix. The extruded alloy showed a good balance of the yield strength (
σ
ys
=
291
MPa) and plane-strain fracture toughness (
K
IC
=
28.3
MPa
m
1/2), obtained by the stretched zone analysis, which were higher than those of conventional wrought magnesium alloys. The microstructure control of both the grain refinement and the dispersion of precipitates in the matrix was a possible method for improvement of the mechanical properties in magnesium alloys.
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