Dislocation-twin interaction mechanisms for ultrahigh strength and ductility in nanotwinned metals

Ultrafine polycrystalline metals containing nanotwins exhibit simultaneous ultrahigh strength and ductility. We study the plastic deformation of such materials through molecular dynamics simulations. Based upon these simulations, we trace the sequence of dislocation events associated with the initiation of plastic deformation, dislocation interaction with twin boundaries, dislocation multiplication and deformation debris formation. We report two new dislocation mechanisms that explain the observation of both ultrahigh strength and ductility found in this class of microstructures. First, we observe the interaction of a 60 degrees dislocation with a twin boundary that leads to the formation of a {001}< 110 > Lomer dislocation which, in turn, dissociates into Shockley, stair-rod and Frank partial dislocations. Second, the interaction of a 30 degrees Shockley partial dislocation with a twin boundary generates three new Shockley partials during twin-mediated slip transfer. The generation of a high-density of Shockley partial dislocations on several different slip systems contributes to the observed ultrahigh ductility, while the formation of sessile stair-rod and Frank partial dislocations (together with the presence of the twin boundaries themselves) explain observations of ultrahigh strength. Our simulation highlights the importance of interplay between the carriers of and barriers to plastic deformation in achieving simultaneous ultrahigh strength and ductility. (C) 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.