Twinning is considered as an important deformation mechanism in Mg alloys . The most common types are extension and contraction twinning. Which type ,contraction or extension is beneficial for weakening of basal texture and enhancement in ductility of Mg alloys?
{10-12} tension twins usually grow readily, if the strain path and texture allow extensive activation. They tend to overtake the initial microstructure and contribute to strain accomodation. During compression parallel to the extrusion direction of extruded Mg alloys {10-12} twinning contributes to strain accomodation up to 8-12% strain.
{10-11} compression twins feature a high CRSS and a low growth rate, hence their contribution to strain accomodation is usually very limited. In addition {10-11} compression twins can form internal {10-12} tension twins. The resulting {10-11}-{10-12} double twins have been correlated to flow localization, void formation and fracture.
In terms of texture modification there have been some studies, where bending of sheet material was used to introduce {10-12} twins and modifiy the texture through subsequent annealings.
In rare earth Mg alloys shear banding nucleation introduces modified recrystallization textures. The shear bands have been correlated to compression and double twins in some studies.
I hope this helps.
Dear Martin Lentz
Thanks for providing useful information. I need your further assistance.
Despite a high CRSS of contraction {10-11} twins. Please comment on contribution of contraction twinning, if under a given rolling conditions, Schmid factor of contraction twinning exceeds the value of extension {10-12} twins. Under these conditions can the enhanced contribution of contraction be related to more beneficial to accommodate strain and hence weakening the texture of Magnesium alloys?
If the external load or local stress fields enable the formation of compression twins, they might form. If you are observing any twins in micrographs you should check the corresponding misorientation angle/axis pairs to verify that these are actually compression twins.
When I've observed {10-11}compression twins they transformed quickly to {10-11}-{10-12} double twins, which are likely to reduce the ductility.
You'll find a lot of information in the articles
http://www.sciencedirect.com/science/article/pii/S0921509306026876
http://www.sciencedirect.com/science/article/pii/S0921509307004297
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