Size-dependent formation and thermal stability of high-order twins in hierarchical nanotwinned metals

Introducing hierarchical twins into nanotwinned (NT) materials is regarded as an effective way to further improve their mechanical properties. It can be imagined that, with the increase of the order of hierarchical twins, it is insufficient to solely take single twin spacing into consideration. For example, the effect of the spacings of primary and secondary twins should be considered together for tertiary twinning. By virtue of theoretical modelling and atomistic simulations, we investigate the influence of low-order twin spacings on high-order twinning. The optimization strategy of high-order twin density and spacings with respect to low-order twin spacings are proposed. It is demonstrated that there exists a trade-off between high-order twin density and twin spacing which can be tuned by the low-order twin spacings. In addition, the atomistic deformation mechanisms related to low-order twin spacings are discussed. Different size-dependent propagation behaviors of partial dislocations are unveiled, relying on the combination of low-order twin spacings. At last, the great thermal stability of high-order twins is also verified, which is attributed to a strong pinning effect of partial dislocations onto low-order twins, leading to a special stress partitioning phenomenon. Our findings may provide a theoretical benchmark for the fabrication of high-order hierarchical nanotwinned (HNT) structures and thus, assisting the design of high-performance mechanical materials.