Dislocation–twin interaction mechanisms for ultrahigh strength and ductility in nanotwinned metals
Introduction
Refining the scale of microstructure in metallic systems commonly leads to increasing strength, but at the expense of decreased toughness. Recently, Lu et al. [1] synthesized ultrafine pure crystalline copper (Cu) containing a high-density of growth twins via a pulsed electrodeposition technique. The resulting material is unusual in that it simultaneously exhibits a high yield strength, high ductility, high strain-rate sensitivity and high electric conductivity [2], [3], [4]. Transmission electron microscopy (TEM) studies of these nanotwinned metals revealed dislocation pile-ups at the twin boundaries [2], suggesting that the enhanced mechanical strength is associated with the effectiveness of twin boundaries as barriers to dislocation motion. Jin et al. [5], [6] studied the mechanisms of interaction between dislocations and twins in different face-centered cubic (fcc) metals. While they found that these interactions can generate dislocation locks, the detailed interaction mechanisms are both material- and loading condition-dependent. Zhu et al. [7] showed that twin boundaries are deep traps for screw dislocations, suggesting that twin boundary-mediated slip transfer is the rate-controlling mechanism for the observed increased strain-rate sensitivity with increasing twin density. All of these studies indicate that the ultrahigh strength of nanotwinned crystalline metals is related to nanotwin-induced strain hardening. However, several important issues remain open. What are the mechanisms of twin formation in pulsed electrodeposition synthesis? What is the origin of the observed ultrahigh ductility? What are the atomistic mechanisms by which twin boundaries lead to strain hardening? In this paper, we employ molecular dynamics (MD) simulations to study the mechanisms that contribute to the observed ultrahigh strength and ductility of ultrafine, nanotwinned Cu.
In the MD simulations presented herein, we employ embedded atom method (EAM)-type interatomic potentials that are suitable for describing Cu in order to examine a microstructure containing an ultrafine array of growth twins under uniaxial, fixed true strain-rate conditions. We examine the deformation mechanism by monitoring the evolution of the dislocation–twin microstructure during the initiation and propagation of plastic deformation with atomic resolution. Slip transfer across twin boundaries was frequently observed, which indicates twin boundaries serve as barriers to dislocation motion and those barriers are not impenetrable. We observe the generation of a Lomer dislocation from the interaction of a 60° full dislocation and a twin boundary. The subsequent dissociation of the Lomer dislocation results in the formation of Shockley, stair-rod and Frank partial dislocations. Generations of those Shockley partial dislocations activate a series of additional slip systems in the crystal, while the sessile (stair-rod and Frank) dislocations provide dislocation locks preventing further dislocation migration. The Lomer dislocations are important in the plastic deformation of the nanotwinned crystal as the source of the experimentally observed strain-hardening effect. We also observed a new mechanism in which a 30° partial dislocation interacts with the twin boundary to generate three new Shockley partial dislocations during the twin-mediated slip transfer. The gliding of the high-density Shockley partial dislocations contributes to the experimentally observed ultrahigh ductility, while the sessile dislocations formed through the dissociation of the Lomer dislocation contribute to the experimentally observed ultrahigh strength.
Section snippets
Methods
The stacking fault energy in fcc metals, , and the equilibrium separation between pairs of Shockley partial dislocations, are known to be sensitive to the choice of interatomic potential employed in atomistic simulations. Therefore, the first step in simulations of nanotwinned Cu is the identification of an interatomic potential function that produces realistic values of the stacking fault energy. While many interatomic potentials are available for pure Cu, we chose the EAM potential
Dislocation nucleation and evolution
The initial simulation cell contains a regular array of coherent twin boundaries, a pair of “low-angle tilt” grain boundaries, as shown in Fig. 1, and no dislocations. This microstructure is similar to the as-sputtered microstructures reported in Ref. [13]; in particular, both the simulations and experiments contain low-angle (∼9°) boundaries and a set of parallel coherent twin boundaries (∼15 nm apart). In the present study, we examine the dislocation nucleation mechanisms and the subsequent
Dislocation–twin interaction mechanisms
Dislocations can be blocked where they first make contact with twin boundaries. As more of the dislocation encounters the twin boundary, a long, straight dislocation segment forms at the intersection of the slip plane and the twin boundary. Because the dislocations intersecting the twin boundary are straight in fcc systems, we need only focus on three types of lattice dislocation: (i) a screw dislocation ( on plane for example); (ii) a 60° dislocation with a 30° leading partial and a 90°
Discussion
Many different dislocation generation mechanisms operate in coarse-grained materials. For example, a classic Frank–Read source can generate dislocation after dislocation, thereby providing a continuous supply of dislocations to sustain plastic deformation. However, when the grain size is on the nanometer scale, most of the dislocation generation mechanisms that operate in coarse-grained materials shut down, leaving a dearth of operative dislocation sources and resulting in what has become known
Conclusions
We present MD simulation results for the uniaxial tensile loading of Cu with a microstructure similar to that produced by a pulsed electrodeposition or a magnetron-sputtering technique (i.e. an ultrafine-grained microstructure containing a high-density of growth nanotwins). The plastic deformation of the initially dislocation-free sample is initiated by dislocation nucleation from the grain boundaries. The applied load couples to multiple slip systems producing both Schmid and non-Schmid
Acknowledgements
The authors grateful acknowledge useful discussions with Prof. Peter Gumbsch. All simulations were performed using Lammps Molecular Dynamics Simulator [19], [20] and visualized in Atomeye [21].
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