Grain-size-dependent zero-strain mechanism for twinning in copper

It is generally accepted that deformation twinning in coarse-grained metals contributes the macroscopic strain, while most deformation twins in nanocrystalline (NC)metals, contrary to popular belief, yield zero net macroscopic strain via either the cooperative or random activation of all three Shockley partials. In the former, the three partials with a particular (b ₂:b ₁:b ₃) triplet unit are successively emitted, while in the latter the three partials are randomly activated in equal numbers. Here we report that there exists a transition between the two zero-strain deformation twinning mechanisms, i.e., from cooperative activation of partials to random activation of partials in NC Cu with medium stacking-fault energy, that occurs with decreasing grain size at room temperature and different strain rates. This experimental finding provides insight into the understanding of deformation twinning.