The kinetics and energetics of dislocation mediated de-twinning in nano-twinned face-centered cubic metals

Polycrystalline Cu with hierarchy microstructures – ultrafined grains about 500 nm and included twins several to tens of nanometers thick – show maximum strength at an averaged twin thickness of 15 nm. Li et al. Nature 464 (2010) 877–880 (Ref. [1]), reported that the primary plastic deformation transits from inclined dislocation gliding to de-twinning in nano-twinned (nt) Cu when twin thicknesses decrease from 25 nm to 4 nm. Their investigation showed that enhanced de-twinning accounts for the strength softening in nt-Cu. From the kinetic and energetic aspects of de-twining process, we present respectively two models. In the kinetic model, we assume that thermally activated dislocation nucleation is the controlling mechanism for plastic deformation, while the transformation energy associated with de-twinning is considered to be governing in the energetic model. Applications of the models on strain-rate sensitivity and temperature dependence of strengths in nt-Cu suggest that the energetic mechanism also plays an important role for de-twinning in nt-Cu.