AIMS Materials Science, 4(1): 102-117. DOI: 10.3934/matersci.2017.1.102 Received: 08 November 2016 Accepted: 27 December 2016 Published: 05 January 2017 http://www.aimspress.com/journal/Materials Research article Deformation mechanism of kink-step distorted coherent twin boundaries in copper nanowire Bobin Xing 1, Shaohua Yan 1, Wugui Jiang 2, and Qing H. Qin 1,* 1 Research School of Engineering, Australian National University, Acton, ACT 2601, Australia 2 School of Aeronautical Manufacturing Engineering, Nanchang Hangkong University, Nanchang 330063, China * Correspondence: Email: [email protected]; Tel: +61-0261258274; Fax: +61-0261255476. Abstract: In the construction of nanotwinned (NT) copper, inherent kink-like steps are formed on growth twin boundaries (TBs). Such imperfections in TBs play a crucial role in the yielding mechanism and plastic deformation of NT copper. Here, we used the molecular dynamic (MD) method to examine the influence of kink-step characteristics in depth, including kink density and kink-step height, on mechanical behavior of copper nanowire (NW) in uniaxial tension. The results showed that the kink-step, a stress-concentrated region, is preferential in nucleating and emitting stress-induced partial dislocations. Mixed dislocation of hard mode I and II and hard mode II dislocation were nucleated from kink-step and surface atoms, respectively. Kink-step height and kink density substantially affected the yielding mechanism and plastic behavior, with the yielding stress functional-related to kink-step height. However, intense kink density (1 kink per 4.4 nm) encourages dislocation nucleation at kink-steps without any significant decline in tensile stress. Defective nanowires with low kink-step height or high kink density offered minimal resistance to kink migration, which has been identified as one of the primary mechanisms of plastic deformation. Defective NWs with refined TB spacing were also studied. A strain-hardening effect due to the refined TB spacing and dislocation pinning was observed for defective NWs. This study has implications for designing NT copper to obtain optimum mechanical performance. Keywords: molecular dynamic simulation; nanotwins; dislocations; defective twins 103 1. Introduction Nanotwinning is believed to be a feasible strategy for improving the strength and ductility of metals. Nanotwin lamellae found in metals can be categorized into three main groups: growth twins, deformation twins, and annealing twins [1–4]. Growth twins are extensively fabricated via electrodeposition or magnetron sputtering, and two types of TBs are usually observed in growth twins, namely coherent twin boundaries (CTB) and incoherent twin boundaries (ITB). Despite the density of CTBs outweighing that of ITBs, they both play important roles in affecting mechanical behavior in NT metals [5]. Extensive studies have focused on the mechanical performance of CTB-dominated nanotwinned structures [6–10]. Strengthening phenomena have been observed in several experiments and MD work [11–14], in which grain refinement led to a significant improvement in peak stress before the nucleation event. It is well-established that the presence of CTBs that obstruct the transmission of dislocation often favors a substantial increase in strength. Moreover, yield strength is inversely proportional to CTB spacing (denoted by λ), where a decrease in λ increases the likelihood of interaction of CTB and dislocations. At this stage, the mechanisms of TB-strengthening phenomenon are tentatively understood via existing experiments and simulations. After all, most of the twin lamellae used for MD modeling were assumed to be defect-free, based on in-situ observations of CTBs. Thus, yield stress can be approximated as the stress required for nucleation of the first dislocation and the yielding mechanism is attributed to Schmid factor analysis based on the loading direction. To the best of the authors’ knowledge, there are few studies of defective NT metals or TBs with kink-like steps. The presence of kink-steps tends to trigger multiple slip systems and tailor mechanical performance. This type of defective TB has been identified using high-resolution inverse pole figure orientation mapping (IPFOM) [15], the converse of a perfectly flat TB. The density of kink-steps was suggested to be an average of one kink per 10-nm length, which is within the scale of current MD simulation. The kink-step can be represented by ITB segments, in which the movement of a kink-step corresponds to detwinning [16,17]. The influence of interaction between kink-step and screw dislocation on hardening and ductility is well-investigated [18]. Those investigations greatly shape our understanding of TBs and deformation mechanisms. Kink migration, a softening behavior, is believed to be the dominant mechanism when a TB is stretched in the twin growth direction [15]. The motion of the ITBs facilitates the achievement of better ductility in the NW and a decrease in yield strength. Investigations on ITB/CTB junctions in bicrystal models were carried out by researchers [19,20]. The ITB migration was caused by external deformation or thermally driven force, where the impact of dislocation interaction was rarely discussed. And the migration mechanism was found to closely relate to shear stress exerted on ITBs. However, insufficient atomistic investigation was performed to explore the significance of the propensity of kink-steps in NT NWs, in which the presence of TB strengthens the NW. In particular, some questions remain with regard to the yielding mechanism and the plastic mechanism of defective NT copper NW: Does the presence of a kink-step always weaken the material? Is high kink density beneficial to mechanical behavior? How does kink-step migrate with the presence of dislocations? Does the defective NW with kink-steps strengthen by virtue of confined TB spacing (λ)? To answer the questions cited above, we present a molecular dynamic simulation study to access the influence of kink-steps on the mechanical behavior of parallel-twinned defective NWs. We AIMS Materials Science Volume 4, Issue 1, 102-117. 104 first compare the mechanical behavior of NWs with perfect TBs and with defective TBs with different kink-step density. Then, the influence of kink height on yielding mechanism and plasticity is discussed. The mechanism for kink migration is discussed. Lastly, the mechanical behaviors of defective NWs with different values of TB spacing are compared, focusing on details of TB-mediated strain hardening and dislocation pinning. 2. Molecular Simulation The MD simulations in this work were performed using LAMMPS [21]. The embedded-atom-potential developed by Mishin [22] was utilized to describe the interplay among the Cu atoms, because the potential accurately predicts stacking faults and twin formation energies. The spurious influence of fixed ends in the axial direction was eliminated by imposing periodic boundary conditions. In contrast, free boundaries were applied to the sidewalls to simulate a nanowire. The [112] direction was arranged along the length dimension, while the TB planes were allocated in the [111] and [110] lateral plane, implying y axis and z axis, respectively. The twin plane was set parallel to the NW axis in the whole work, sitting in the very middle of the NW. That arrangement allowed stabilization of the kink-step throughout the relaxation. All tested specimens were 36 × 8 × 8 nm3 in size. The kink-steps were added on the TBs by introducing a displacement field in the (111) direction. The displacement field was varied from 0 nm (perfect) to 1.8 nm (nine-atom-layer kink). To investigate the effect of kink density on defective NWs, four distinct kink-step densities, namely 20, 10, 8 and 5 [112] lattice spacings, were created simultaneously, which are equivalent to 17.7 nm, 8.8 nm, 7 nm and 4.4 nm. After relaxation, the resultant kink-steps were found to be similar to the structural units in ITBs, a finding that was consistent with previous studies [15]. Furthermore, to highlight the influence of additional twin plane, defective NWs attained twin spacing of 3.1 nm and 1.8 nm were created, with kink height of two [111] lattice spacing and kink density of 1kink/17.7nm unchanged. Prior to the equilibrium process, molecular static simulations were employed using the conjugate gradient method. Each configuration was then equilibrated to a temperature of 1 K and zero pressure in the loading direction for 20 ps under an isothermal-isobaric ensemble with a Nose-Hoover thermostat. The time-step was set as 2 fs. The equilibrated specimen was then deformed in tension along the length direction at a strain rate of 108 s−1, with the environmental temperature controlled at 1 K under canonical ensemble (NVT). High strain rate in MD was found to induce amorphous deformation [23] and low strain rate in MD is computational demanding. The low temperature allowed exploration of the incipient plasticity mechanism of the system without any thermal impact. Virial theorem was implemented to yield the stress within the NWs, and the tensile stress was defined by the average of all atomistic stresses. Deformation potential energy is calculated by the difference between initial potential energy after relaxation and potential energy during deformation. To discern the preferable defects activated in the progress of deformation, the visualization tool Ovito was utilized [24]. It analyzes the generated atomic-level configurations and identifies dislocation activities using the common neighbor analysis (CNA) [25]. The perfect FCC Cu atoms are colored in green, while the HCP atoms are colored in red. Other atoms are grey. Partial dislocation <112>-{111} is indicated by the other atoms colored grey. A twin boundary is represented by a single layer of hexagonal close packing (HCP) atoms, while an intrinsic stacking AIMS Materials Science Volume 4, Issue 1, 102-117. 105 fault is plotted by two adjacent layers of HCP atoms. 3. Results In presenting the results, the stress-strain curve and deformation potential energy of perfectly twinned NWs and defectively twinned NWs with different number of kink-steps are first compared.