Strong strain hardening in ultrafast melt-quenched nanocrystalline Cu: The role of fivefold twins

Abstract

Nanocrystalline and nanotwinned metals exhibit ultrahigh strength but suffer from low ductility due to the absence of the strain hardening effect. Here, we report sustained strong strain hardening up to 20% compressive strain in a melt-quenched nanocrystalline Cu structure, which contains numerous fivefold twins, stacking faults, and twin boundaries. Our molecular dynamics simulations reveal that the strong strain hardening results from the synergistic effect of constant nucleation and impedance of dislocations, restricted twin boundary migration, and abundant dislocation reactions in fivefold twin networks. Specifically, we find that fivefold twins both nucleate and impede dislocations, and the migration of fivefold twin boundary is restricted by the core of fivefold twins. Moreover, we observe a new migration mechanism, in which fivefold twin boundary migrates by two atomic planes directly, enabled by the gliding of two different Shockley partial dislocations in the opposite directions. Finally, dislocation transmission, which is adverse to strain hardening, occurs very scarcely in fivefold twins. This is caused by the large misfit strains in fivefold twins and abundant immobile dislocations generated by frequent dislocation reactions in fivefold twin networks. This work reveals the advantage of fivefold twins over nanotwins to overcome the strength-ductility trade-off.