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To quote from a good textbook, which I don't remember:

"Twins which are produced by mechanical deformation are called mechanical twins. Mechanical twins are produced in bcc or hcp metals under conditions of rapid rate of loading (shock loading) or decreased temperature."

I well understand this has to do with slip systems and dislocation veloctiy. But somehow I cannot connect them. What is the connection?

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It is important to distinguish the origin of twinning, because it produces different features (and different kinds of what is commonly referred to as "twinning").

For example twinning can develop during the crystal growth as a special feature of the crystal morphology (contact or penetration twins). On the other hand, a single crystal undergoing a structural transition on cooling can develop what is called a 'domain structure' determined by the change of point-group symmetry; that is, in low temperature phase the original single crystal is actually composed of different domains (it is no longer a single crystal, whichever its external shape) oriented according to the crystallographic relationships holding for the point-group desymmetrization.

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  • $\begingroup$ What about shock loading then? $\endgroup$
    – user586228
    Commented Jan 7, 2021 at 15:51
  • $\begingroup$ in this case you can describe the situation in terms of stress and strain tensors; in this case both tensors do not conform to the crystal symmetry because the stress can be arbitrarely applied (for this point you can take a look in some textbook about crystallophysics). This is different from what happens in ferroelastic crystals (cited above) $\endgroup$
    – gryphys
    Commented Jan 7, 2021 at 16:21

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