13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- zero for perfect FCC lattice to positive values for defects and for atoms close to free surfaces. Figure 1. (a) A polycrystalline Cu NW containing three GBs with a nearly square cross-section. (b) A polycrystalline Cu NW containing three TWs with a nearly square cross-section. Both NWs were divided into three sections, including mobile region, and two rigid boundary regions ‘E’. 3. Results and discussion Following studies will focus on the impacts from GBs or TBs on the critical angle, torsional rigidity and the plastic deformation of NWs. The critical angle refers as the angle when plastic deformation or dislocations begin to emit. While the torsional rigidity (GIp) can be obtained from the strain energy (ΔE) versus torsional angle (φ) curve through the relation of ΔE=(GIp)φ 2/(2L) [21]. 3.1. Polycrystalline nanowire with GBs Mono-crystalline NW and polycrystalline NWs contain different numbers of GBs were firstly studied. Fig. 2 shows the changing trend of ΔE along with the increase of φ for these different cases. It is evident from Fig. 2 that due to the presence of GBs, the ΔE-φ curve appears a significant difference from that of the perfect NW. In detail, for the perfect NW, ΔE-φ curve exhibits a parabolic portion before yielding, after which the strain energy received an obvious reduction, and the critical angle approximates 1.627 rad. Whereas, the ΔE-φ curves for NWs with GBs show a considerably shortened parabolic portion, and this parabolic portion is apparently different from that of the perfect NW. In other words, a much smaller critical angle is observed for NWs with GBs. Particularly, ΔE-φ curves for NWs possess more than 1 GB exhibit a relatively flat fluctuation part after passing the critical angle. Selected atomic configurations are presented to investigate the deformation process of the NW. Fig. 3 shows the atomic configurations of different NWs at different torsional angle values. Basically, for the perfect NW, the crystal structure retains its original crystal structure (see Fig. 3a1) until the torsional angle arrives 1.634 rad, which shows a relatively long elastic deformation period. After yielding, stacking faults (SFs) begin to nucleate around the centre of the NW as shown in Fig. 3a2. It is observed that, the plastic deformation has concentrated around the central area of the NW (see Fig. 3a3), which involves with the nucleation and propagation of both intrinsic and extrinsic SFs.
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