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.