Contribution of Twinning to Low Strain Deformation in a Mg Alloy
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WHEN a single crystal of magnesium is compressed along its prismatic pole, the first 0.06 or so of strain is entirely attributable to twinning on the f10 12g planes.[1] In such a case, the yield stress is controlled entirely by the f10 12g twinning stress. Sharply textured polycrystalline magnesium alloys follow the general trends seen in single crystals[2] but the question that motivates the present work is: how much of the strain that immediately follows macroscopic yielding is due to f1012g twinning and what is the effect of grain orientation? The answer to this will impact on efforts to control the yield strength of wrought magnesium alloys. The most obvious way to isolate the contribution of f10 12g twinning to the macroscopic strain is to measure the twin-volume fraction, f. The twinning shear of f10 12g twins in magnesium is low (ct = 0.13), so the resolved axial strain component, et, of a twinning system, s, can be given in terms of its scalar Schmid factor, ms (expressed in the sample reference frame) as:[3] et ¼ ms fs ct :
½1
The f10 12g twin mode is seen to dominate in the conditions considered here so in what follows the term twinning will be used uniquely to indicate f1012g twinning. The fraction of this twin in low strain compression of extruded and rolled magnesium alloys
MATTHEW R. BARNETT, ARC Future Fellow and Professor of Metallurgy, and ALIREZA GHADERI, Postdoctoral Research Fellow, are with the ARC Centre for Excellence for Design in Light Metals, Institute for Frontier Materials, Deakin University, Geelong, VIC, Australia. Contact e-mail: [email protected] JOSEPH D. ROBSON, Reader, is with the Department of Materials Science, University of Manchester, Manchester, U.K. Manuscript submitted July 12, 2013. Article published online October 8, 2013 METALLURGICAL AND MATERIALS TRANSACTIONS A
has been measured using various techniques. Optical microscopy was employed by Jiang et al.[4] in their study of AZ31 extruded tubes and the twin-fraction was seen to attain to ~35 pct by a compressive strain, eT, of 0.04. However, negligible twin fractions were seen at a strain of 0.02, but, judging from a few of the published flow curves, these strain values contain a significant proportion of machine compliance. Two of us have reported twin fractions of 25 pct based on optical analysis for a plastic compressive strain of 0.015, also in extruded AZ31.[5] In this case, no significant effect of grain size was seen over the range 5.1 to 55 lm and the axial strain was attributed entirely to twinning. However, in a more recent follow-up study,[6] lower twinning fractions were seen (17 pct), which is probably a reflection of the sensitivity of optical techniques to etching effects. The dramatic change in orientation (~86 degh1 210i) that accompanies f1012g twinning has been exploited by a number of workers to measure twin fractions. Clausen et al.[7] employed neutron diffraction in their study of extruded AZ31 and obtained, after correcting for the compliance evident in their stress–strain curve, a twinfracti
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