The stability of Lomer–Cottrell jogs in nanopillars Christopher R. Weinberger a,⇑ and Wei Cai b a Sandia National Laboratories, P.O. Box 5800, MS1411, Albuquerque, NM 87185-1411, USA b Department of Mechanical Engineering, Stanford University, CA 94305-4040, USA Received 20 October 2010; revised 15 November 2010; accepted 20 November 2010 Available online 26 November 2010 Single arm spiral sources, or truncated Frank–Read sources, have been used frequently to interpret the size dependent plasticity in micropillars. The basis for these sources is strong pinning points which have been proposed to exist based on immobile Lomer– Cottrell jogs. Here, we show, using molecular dynamics of face-centered cubic nanopillars, that Lomer–Cottrell jogs are not as immobile as initially thought and that they do not provide strong pinning points for single arm sources. Ó 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Dislocations; Micropillar; Jog; Plastic Deformation Experiments on pillars ranging in sizes from a few hundred nanometers to tens of micrometers have shown that the flow stress in the pillars increases as pil- lar size decreases [1–3]. The plastic deformation of these pillars must occur through the activation of dislocation sources, which increase in strength as the pillar diameter decreases. Two competing mechanisms have emerged to explain the experimental observations: dislocation star- vation [4] and single arm sources [5]. The dislocation starvation model maintains that mobile dislocations es- cape the crystals, leaving them starved of mobile disloca- tions, and plasticity continues with nucleation from the surface. The single arm source model claims that trun- cated Frank–Read sources control plasticity and the strengthening is caused by shorter source lengths in smaller volumes. However, this model requires strong pinning points and the nature of these pinning points is unclear. The single arm source model is primarily supported by dislocation dynamics (DD) simulations [6,7] and some experiments [8]. These DD simulations require strong pinning points, which are created by dislocation segments, or junctions, which are assumed to be immo- bile. In order to provide a theoretical basis for single arm sources, Lee and Nix [9] investigated the creation of strong pinning points from Lomer–Cottrell (LC) junctions in face-centered cubic (fcc) micropillars. They concluded that LC junctions can indeed form single arm sources by transforming into LC jogs but rely on the immobility of the jog. In this letter, we investigate the stability and mobility of LC jogs and evaluate their po- tential as single arm sources. In order to ensure that our atomistic models are appropriate to simulate the dynamics of Lomer–Cottrell jogs, we first need to verify that the interatomic poten- tials used in this study reproduce known properties of these dislocations. For example, Mills et al. [10] have shown that different embedded atom model potentials will predict both compact and planar cores for Lomer dislocations. To this end, we have computed the core structure of both Lomer dislocations, the compact core as shown in Figure 1a, and the Lomer–Cottrell disloca- tion, which has a dissociated core structure shown in Figure 1b. The core structure for the predicted Lomer dislocation matches observations in aluminum [11] and the Lomer–Cottrell dislocation core structure shows the expected dissociation. These dislocations are known to have high Peierls stresses [12] and thus low mobility on their {0 0 1} glide planes, which is accurately pre- dicted by the potentials we use [13,14] exhibiting Peierls stresses in the GPa range. However, LC jogs, which are the subject of investigation in this letter, are different from the Lomer and LC dislocations because they have two end nodes, one of which is always constricted [9], about which the dislocation arms can rotate. To investigate the stability of LC jogs, we use molec- ular dynamics simulations. This is done by introducing a jog into a dislocation that spans a nanopillar, as shown in Figure 2a. Most of the nanopillars in our simulations have a diameter of 16 or 30 nm, while a few have a 1359-6462/$ - see front matter Ó 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2010.11.037 ⇑ Corresponding author. Tel.: +1 505 284 0896; e-mail addresses: crweinb@sandia.gov; caiwei@stanford.edu Available online at www.sciencedirect.com Scripta Materialia 64 (2011) 529–532 www.elsevier.com/locate/scriptamat