13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- NWs either with GBs, TBs or pre-existing defects. For example, Cao et al. [10] demonstrated that, polycrystalline Cu NWs exhibit tensile deformation behaviour distinctly different from their single crystal counterparts. A serial of investigation on the twinned Cu NWs under tensile deformation has been conducted by Cao and his group members [11]. Au and other metal NWs with twin boundaries have also been extensively studied by Sansoz and his group members [12]. The influence of different pre-existing defects on the mechanical properties of metal NWs under tension [13, 14], compression [15] have also been investigated. It is noticed that majority of current studies on polycrystalline NWs have emphasized on the mechanical properties under uniaxial loading conditions, and the study of their mechanical performance under torsion is still lack in the literature. Thus, we perform a comprehensive investigation of the influence from GBs or TBs on the torsional properties of Cu NWs using large-scale MD simulations in the present work. 2. Computational details Large-scale MD simulations were carried out on bamboo-like polycrystalline Ag NWs with square cross-section, which were constructed either with GBs or coherent TBs. In detail, the bamboo-like NW with GBs were generated according to the work by Cao et al. [10], i.e., the <100> crystalline direction is chosen as the misorientation axis, and the GBs separating individual grains are ∑5(310)36.9° symmetric high-angle tilt GBs, which has a high density of coincident atomic sites across the interface (see Fig. 1a). This structure has also been experimentally observed [16] and studied using numerical simulations [10] by previous researchers. We considered a serial of such NWs that contain different numbers of GBs with an identical size (4.34 nm×4.57 nm×46.64 nm) and constant grain boundary spacing (GBS). Specifically, GBs numbers including 0, 1, 3, 4 and 9 were considered which leads the GBS ranging from 4.57-11.43 nm (that is comparable to the experimentally investigated grain size of 5-40 nm). Similarly, for the bamboo-like NW with (111) coherent TBs, a serial of NWs that contain different numbers of TBs (i.e., 0, 1, 3, 4 and 9) and a constant twin boundary spacing (TBS) were considered, with the TBS ranging from 6.26-15.65 nm (see Fig. 1b). The size of the NW was uniformly chosen as 5.90 nm×6.13 nm×63.87 nm. We modelled Cu using the embedded-atom-method (EAM) potential developed by Foiles et al. [17]. This potential was fitted to a group of parameters, including cohesive energy, equilibrium lattice constant, bulk modulus, and others [18]. During each simulation, the NW was first created assuming bulk lattice positions, and then relaxed to a minimum energy state using the conjugate gradient algorithm, i.e. the length of the NW was allowed to decrease in response to the tensile surface stress. We then used the Nose-Hoover thermostat [19, 20] to equilibrate the NW at a constant temperature 10 K (NVT ensemble) for 400 ps at a time step of 4 fs while holding the newly obtained length of the NW fixed. Finally, a pair of constant torsional loads was applied to the two ends of the NW as schematically shown in Fig. 1a. No periodic boundary conditions were utilized at any point during the simulation process. The overall simulation methodology to study the torsional properties of the NWs is identical to that used previously for mono-crystalline Cu NWs [21]. All simulations were performed using the open-source LAMMPS code developed at Sandia National Laboratories [22]. In order to recognize the defects in NWs, the centro-symmetry parameter (CSP) [23] is utilized, which increases from
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