A theoretical model is suggested which describes two plastic deformation mechanisms in deformed columnar-grained nanotwinned metals loaded parallel to coherent twin boundaries. The first mechanism represents the formation and expansion of split dislocation loops within individual twins, while the second one assumes the emission of split jogged dislocations from grain boundaries into grain interiors. Within the model, both split dislocation loops and jogged dislocations are generated at pre-existent full dislocations at grain boundaries. Under these assumptions, the critical stresses for the nucleation and expansion/motion of split dislocation loops and jogged dislocations are calculated. It is demonstrated that at a specified grain size, the critical stress for the formation and motion of jogged dislocations is always smaller than that for the expansion of split dislocation loops across twin lamellae. The results explain the inconsistencies of molecular dynamics simulation results [H. Zhou et al., Nano Lett. 14 (2014) 5075; Y. Zhu et al., Int. J. Plasticity 72 (2015) 168] concerning the possibility for the coexistence of split dislocation loops and jogged dislocations in nanotwinned metals and multilayers.
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