1 Size-Dependent Plasticity in Twinned Metal Nanowires F. Sansoz 1 and C. Deng 1 1 School of Engineering and Materials Science Program, University of Vermont, Burlington, VT 05405, USA 1. Introduction Face-centered cubic (FCC) metal nanowires, such as gold nanowires, are basic building blocks for numerous applications of nanotechnology ranging, for example, from electronics to self-assembly and medicine [1,2]. Therefore, it is critically important to gain fundamental insight into their mechanical behavior at the nanometer scale, in order to build more robust nanoscale devices. In particular, past atomistic simulation studies have proved that coherent twin boundaries can be strong obstacle to crystal slip in nanoscale FCC metals [3-8]. However, meaningful results related to FCC metal nanowires can only be achieved if the influences of microstructure and sample size on crystal plasticity and fracture behavior are fully understood. In this paper, particular focus is placed on the effects of nanoscale twins, which are commonly grown during synthesis in FCC metal nanowires [9-12], on the tensile plastic behavior and fracture of Au nanowires. Both tensile yield and fracture at room temperature in [111]-oriented Au nanowires were investigated using molecular dynamics (MD) simulations. The nanowires exhibited same diameter (12.3 nm), but different number of coherent (111) twin boundaries per unit length along the wire axis, from 0.12 to 0.71 nm -1 . Further details on the simulation procedure are provided in the next section. We show in Section 3 that the crystal plasticity and fracture behavior of Au nanowires strongly depends on the density of preexisting twin boundaries. The underlying mechanisms of plastic deformation and fracture in twinned gold nanowires at atomic scale are also addressed. 2. Simulation methods Parallel MD simulations were performed using LAMMPS molecular dynamics simulator [13] with an embedded-atom-method (EAM) potential for gold developed by Grochola et al. [14]. This potential enables the prediction of realistic values for the stacking fault energy of gold (γ SF prediction = 43.4 mJ⋅m -2 ) as compared to experimental data (γ SF experimental ~ 32-46.4 mJ⋅m -2 ) [14]. The structure of the nanowires was considered to be perfectly cylindrical and oriented along the [111] crystallographic direction. A periodic boundary condition was imposed along the nanowire axis, while the nanowire was kept free in the other directions. The periodic length and diameter of each cylinder was fixed at 33.6 nm and 12.3