Strain hardening due to {10 12} twinning in pure magnesium

Abstract

The effect of grain segmentation by mechanical twinning on the strain-hardening behaviour of textured and random polycrystals is assessed using the Kocks-Mecking method of analysis. Profuse twinning in the first 6–8% strain in textured polycrystals has relatively small strengthening effects despite the large volume fraction of grains undergoing twinning. This is due to the small value of the Hall–Petch constant in textured polycrystals. For random polycrystals, the Hall–Petch constant is much larger but the overall hardening effect is reduced due to the small volume fraction of grains undergoing twinning. Additional hardening effects due to the twinning crystallographic transformation on dislocation mobility are deemed small in cast polycrystals due to their low dislocation density, but may be more important in textured polycrystals with higher dislocation densities. Grain size-independent storage of dislocations accounts for the strain hardening at large strains.

Keywords:

• magnesium
• dislocations
• forest hardening
• Taylor factor
• Hall–Petch law

Acknowledgements

PL thanks the Ministry of Education of the Czech Republic for financial support under the research plan MSM 1M2560471601 ‘Eco-Centre for Applied Research of Non-Ferrous Metals’. The authors are indebted to Pat Kelly and Kevin Spencer of the University of Queensland for useful discussions and for critical reading of the manuscript, Sean Agnew from the University of Virginia for encouragement and fruitful discussions and Dirk Steglich from the GKSS Research Centre for making Kelley and Hosford's original data files available for this work.

Notes

Notes

1. The σΘ curves in Figures 5, 7 and 8 are parametric plots derived from the polynomials of best fit to the respective flow curves.

2. The average twin area in Klimanek and Pötzsch's specimens at the end of the profuse twinning stage was A = 20 µm², so a final twin size of D = A^1/2 = 4.5 µm has been assumed for all materials.

3. The total dislocation density in Cu–8%Al at the onset of twinning was about 10¹⁵ m^–2 19.