It was noted that ultra-fine polycrystalline metals which contained nano-twins simultaneously exhibited ultra-high strength and ductility. The plastic deformation of such materials was studied using molecular dynamics simulations. Based upon the latter, the sequence of dislocation events which was associated with initiation of plastic deformation, dislocation interaction with twin boundaries, dislocation multiplication, and deformation-debris formation, was traced. Two new dislocation mechanisms were reported that explained both the ultra-high strength and ductility which were found in this class of microstructure. Firstly, the interaction of a 60° dislocation with a twin boundary that led to the formation of a {001}<110> Lomer dislocation which, in turn, dissociated into Shockley, stair-rod and Frank partial dislocations, was observed. Secondly, the interaction of a 30° Shockley partial dislocation with a twin boundary generated three new Shockley partials during twin-mediated slip transfer. The generation of a high-density of Shockley partial dislocations on several different slip systems contributed to the observed ultra-high ductility, while the formation of sessile stair-rod and Frank partial dislocations (together with the twin boundaries themselves) explained the ultra-high strength. These simulations highlighted the importance of an interplay between the carriers of, and barriers to, plastic deformation in achieving simultaneous ultra-high strength and ductility.

Dislocation–Twin Interaction Mechanisms for Ultrahigh Strength and Ductility in Nanotwinned Metals. Z.X.Wu, Y.W.Zhang, D.J.Srolovitz: Acta Materialia, 2009, 57[15], 4508-18