J.C. Baird

Limitation of current hardening models in predicting anisotropy by twinning in hcp metals: Application to a rod-textured AM30 magnesium alloy

When a strongly textured hexagonal close packed (HCP) metal is loaded under an orientation causing profuse twinning or detwinning, the stress-strain curve is sigmoidal in shape and inflects at some threshold. Authors have largely attributed the dramatic stress increase in the lower-bound vicinity of the inflection point to a combined effect of a Hall-Petch mechanism correlated to grain refinement by twinning, and twinning-induced reorientation requiring activation of hard slip modes. We experimentally and numerically demonstrate that these two mechanisms alone are unable to reproduce the stress-strain behaviors obtained under intermediate loading orientations correlated to in-between profuse twinning and nominal twinning. We argue based on adopting various mechanistic approaches in hardening model correlations from the literature. We used both a physics dislocation based model and a phenomenological Voce hardening model. The HCP material is exemplified by an extruded AM30 magnesium alloy with a � 10¯0� -fiber parallel to the extrusion direction.

Anomalous Twin Bands in AZ31 Magnesium Sheet Bending

Three-point bending was performed on an AZ31 magnesium sheet with a grain size ∼8.0 µm at room temperature. In-situ electron backscatter diffraction and metallography examinations revealed an anomalous twinning pattern during bending. In the compression zone, the {1012} twins appear in an extremely localized fashion, consisting of alternating twin bands. Each band comprises a high density of twins. In between the twin bands, twins are absent. As the bending angle increases, the twin bands grow into the tension zone although the stress disfavors the extension twinning.

Twinning Multiplicity in an AM30 Magnesium Alloy under Uniaxial Compression

Twinning under compression along two perpendicular directions of an AM30 alloy with a {1010} fiber texture was investigated. The primary extension {10 1 2} twinning occurred for both compression normal to the fiber and compression parallel to the fiber. These primary extension twins took forms of fully residual twins parallel to the fiber and the “stopped elastic” twins normal to the fiber. Both the {1011}-{1012} and {1013}-{1012} double twinning occurred in the matrix grains at the last stage of compression normal or parallel to the fiber, but as the “combined two shears” mode of the double twinning. Another time-sequence type of {1011}-{1012} double twin also activated early normal to the fiber. The primary extension twinning and contraction twinning seem to obey the Schmid law according to the texture evolution.