Macroscopic Twinning Strain in Nanocrystalline Cu

Most deformation twins in nanocrystalline face-centered cubic (NC fcc) metals are reported to produce zero-macrostrain, which is attributed to either random activation of partials (RAP) or cooperative slip of three partials (CSTP). Here, we report that when the RAP mechanism is suppressed, ∼44% twins in NC Cu produced zero-macrostrain via the CSTP mechanism. This indicates that both RAP and CSTP are major mechanisms to generate zero-macrostrain twins. In addition, our results also indicate that stress state affects the twinning mechanism in NC fcc metals, and monotonic activation of partials with the same Burgers vector dominates twin formation under monotonic stress.