Plastic anisotropy and the role of non-basal slip in magnesium alloy AZ31B

Magnesium and its alloys do not in general undergo the same extended range of plasticity as their competitor structural metals. The present work is part I of a study that examines some of the roles deformation twinning might play in the phenomenon. A series of tensile test results are reported for the common wrought alloy AZ31. These data are employed in conjunction with a simple constitutive model to argue that View the MathML source twinning (which gives extension along the c-axis) can increase the uniform elongation in tensile tests. This effect appears to be similar to that seen in Ti, Zr and Cu–Si and in the so called TWIP phenomenon in steel.

Twinning and the ductility of magnesium alloys Part I: “Tension” twins

http://li.mit.edu/Stuff/RHW/Upload/07-01_Lou_XY_Hardening_evolution_paper.pdf

Hardening evolution of AZ31B Mg sheet

On the role of non-basal deformation mechanisms for the ductility of Mg and Mg–Y alloys

This article presents room-temperature deformation mechanisms in polycrystalline Mg alloys. Dislocation slip of basal 〈a〉 and prismatic 〈a〉 types are shown to occur nearly at the same ease when the basal planes are tilted in such a way that the Schmid-factor ratio (equivalent to the critically resolved shear stress (CRSS) ratio) of prismatic 〈a〉 to basal 〈a〉 slip is larger than a value ranging from 1.5 to 2.0, depending on the initial texture distribution and grain size. Grain-boundary sliding (GBS) also occurs at room temperature up to 8 pct of total strain, enhanced by plastic anisotropy as well as by the increasing number of grain-boundary dislocations. Twinning plays an important role in both flow and fracture behaviors. Twins are induced mostly by stress concentrations caused by the anisotropic nature of dislocation slip. Twins can be classified into two types based on their shape: a wide lenticular type and a narrow banded type. The wide twins are 
$$\{ 10\bar 12\} $$

 twins appearing in the early stage of deformation and accompany little change of surface height. The narrow twins are 
$$\{ 10\bar 11\} $$

 or 
$$\{ 30\bar 32\} $$

 appearing in the late stage of deformation and accompany a substantial change in surface height. The formation of the narrow twins seems to give rise to highly localized shear deformation within the twin, leading to strain incompatibility and to final failure.

Enhanced deformation mechanisms by anisotropic plasticity in polycrystalline Mg alloys at room temperature

The activity of non-basal slip systems and dynamic recovery at room temperature in fine-grained AZ31B magnesium alloys

Deformation mechanism in a coarse-grained Mg–Al–Zn alloy at elevated temperatures

Effect of temperature and grain size on the dominant diffusion process for superplastic flow in an AZ61 magnesium alloy

Superplastic deformation mechanism in powder metallurgy magnesium alloys and composites

An investigation of the superplastic characteristics of magnesium alloys with several grain sizes revealed that grain boundary sliding took place more easily with grain refinement. The required grain size for high strain rate superplastic forming was estimated to be ∼2 μm. The required grain structure could be obtained by several procedures, hot extrusion with a high extrusion ratio, severe plastic deformation via equal channel angular extrusion, consolidation of machined chip, and/or powder metallurgy processing of rapidly solidified powders, on a laboratory scale. The processing route of hot extrusion was selected in this study. An experimental study of superplastic press forming was conducted for a commercially extruded ZK60 alloy. The fabricated product did not essentially contain macroscopic defects, i.e. cracks or cavities. From an examination of tensile characteristics, it was found that the post-formed alloy exhibited higher strength and higher ductility compared with some conventional cast magnesium alloys, aluminium alloys, and steels. The experimental results support the possibility of using superplastically formed magnesium to produce structural components.

H. Watanabe

Papers

The activity of non-basal slip systems and dynamic recovery at room temperature in fine-grained AZ31B magnesium alloys

Deformation mechanism in a coarse-grained Mg–Al–Zn alloy at elevated temperatures

Effect of temperature and grain size on the dominant diffusion process for superplastic flow in an AZ61 magnesium alloy

Superplastic deformation mechanism in powder metallurgy magnesium alloys and composites

Application of superplasticity in commercial magnesium alloy for fabrication of structural components