Shock-induced breaking of the nanowire with the dependence of crystallographic orientation and strain rate
© Wang et al; licensee Springer. 2011
Received: 24 November 2010
Accepted: 5 April 2011
Published: 5 April 2011
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© Wang et al; licensee Springer. 2011
Received: 24 November 2010
Accepted: 5 April 2011
Published: 5 April 2011
The failure of the metallic nanowire has raised concerns due to its applied reliability in nanoelectromechanical system. In this article, the breaking failure is studied for the , , and  single-crystal copper nanowires at different strain rates. The statistical breaking position distributions of the nanowires have been investigated to give the effects of strain rate and crystallographic orientation on micro-atomic fluctuation in the symmetric stretching of the nanowires. When the strain rate is less than 0.26% ps-1, macro-breaking position distributions exhibit the anisotropy of micro-atomic fluctuation. However, when the strain rate is larger than 3.54% ps-1, the anisotropy is not obvious because of strong symmetric shocks.
In recent years, the metallic nanowires applied as nanoconnectors  and the active components of nanoelectromechanical system (NEMS) devices [2, 3] have attracted extensive interests owing to their special mechanical , thermal , electrical , and magnetic  properties. The approaches to investigate nanowires in experiments include using scanning tunneling microscopy (STM) [8, 9], atomic force microscopy (AFM) , transmission electron microscope (TEM) [11, 12], and mechanically controllable break junctions (MCBJ) [13, 14]. However, it is difficult to manipulate the deformation processes when the nanowires are applied in NEMS, because controlling the failure of the nanowires is a challenging thing due to their small scales. Hemker  proposed the reliability of NEMS would require a fundamental description of its deformation mechanism, which must be based on a corresponding solid understanding. In contrast, molecular dynamics (MD) simulation [16, 17], which solves Newton's equations of motion for a collection of interacting particle over a number of time steps, is an effective method to study the deformation and breaking failure processes of the metallic nanowires.
With the method of MD simulation, Koh and Lee  studied the strain-rate effects on the tensile structure of platinum nanowire. Meanwhile, they  gave the mechanical behaviors of gold and platinum nanowires under different strain rates, which indicated that the displayed crystalline-ordered deformation of the nanowires was governed by the formation of a main dislocation plane at low strain rate. Ikeda et al.  proposed amorphization in nickel nanowire induced by high strain rate. These studies indicate strain-rate effects on the deformation of the single-crystal metallic nanowires. For the single-crystal materials, we noticed that the plastic response in copper could occur rapidly [21, 22]. According to this point, studying the deformation and breaking failure of the copper single-crystal nanowires shall be of vital importance for developing and processing the nanoscale systems based on metallic nanowires. In addition, anisotropies in single-crystal materials will give rise to the dependence of crystallographic orientation. For example, Tsuru and Shibutani  showed copper had a much larger anisotropic factor than aluminum in terms of the load-depth relation and stress distribution. Bringa et al.  proved single-crystal copper had a marked anisotropic behavior in shock wave propagation. It is known that crystallographic orientation is related with structural anisotropy and symmetric stretching at different strain rates will generate different mechanical shocks, but we do not know which factor will dominate the deformation and the breaking failure mechanism of the nanowires? In order to make the question clear, we focused on the MD simulation investigation of the tensile deformation and breaking failure of the single-crystal copper nanowires under the effects of crystallographic orientation and strain rate.
The applied strain rate and its absolute rate of the nanowires
rate (% ps-1)
rate (% ps-1)
Where is the stress tensor of atoms α in the tensile direction (z-axis), Ω i is the volume of i atoms, m is the mass, and is the velocity component of atom i in the z- direction. ϕ, F, ρ, and ƒ are parameters from EAM potential , which corresponding to the pair potential, the embedded energy, the electron density between the atom i or j and all other atoms, the electron density in r ij between atomic i and j, respectively. The first and second terms in the right side of the above equation represent the thermal effect and the atomic interactions, respectively. All the presented MD simulations and visualization process were performed with the self-developed software NanoMD , the reliability of algorithms has been validated not only by a large amount of theoretical simulations [25, 26, 37–41], but also with the comparison to the experimental measurements [42, 43].
It is shown in Figure 1 that macro-breaking position distributions of the nanowires are from the micro-atomic fluctuation of three crystallographic orientations at different strain rates and the ways of atomic fluctuation are related with deformation mechanism of the nanowires. With the MD simulations of the nanowires at the strain rates from 0.01 to 7.69% ps-1, Videos S1-S3 in Additional files 1, 2 and 3 are selected to exhibit the representative deformation behaviors of the  single-crystal copper nanowires at the strain rates of 0.01, 1.54, and 6.16% ps-1, respectively. At low strain rate of 0.01% ps-1 (Video S1 in Additional file 1), the nanowire slips along (111) planes after the elastic deformation. In general, for the FCC closed pack structure, Burgers vectors exit along the <110> direction and induce the structure slip and reconstruct themselves along (111) planes. The slippage mechanism had been discussed in detail by Finbow et al. , who gave that the overall dislocation associated with slippage had a Burgers vectors given by (a 0/2) [ ] in the nanoscale wire. The process can be better described as a uniform slip in the [ ] direction of one (111) plane relative to the neighboring one. Slippage retains the crystalline order in the plastic deformation, and the linear atomic chains tend to occur near the middle of the nanowire because of symmetric stress. At 1.54% ps-1 (Video S2 in Additional file 2), the obvious slippage allowing for reconstruction along (111) plane is not found and the nanowire shows superplasticic behavior with amorphous structures. At 6.16% ps-1 (Video S3 in Additional file 3), the nanowire is more likely to break near the two ends because of local melted structures.
In comparison with , the  single-crystal copper nanowires behave as different deformation behaviors at the strain rates of 0.01, 1.54, and 6.16% ps-1 (see Videos S4-S6 in Additional files 4, 5 and 6). As shown in Video S4 in Additional file 4, the  nanowire prefers to maintain the crystallographic structure at low strain rate of 0.01% ps-1. The neck appears abruptly with the strain increasing, and the nanowires break accompanying with a few atoms in the disorder movement. This behavior is in agreement with the observations of Tavazza et al.  Because the preferred slip directions are identical to the tensile direction so the system has no ability to get the atomic rearrangement at lower strain rates. The nanowires remain a better crystal structures during the weak mechanical shocks at low strain rates. Increasing the strain rate will result in the increasing of atomic thermal motion, which facilitates the ductility of the materials. The deformation behavior in Video S5 (Additional file 5) shows that a local lattice reconstruction becomes predominant after the first yield point and the necking takes place at the positions of the lattice reconstruction. Unlike at the low and middle strain rates, the  nanowire at the strain rate of 6.16% ps-1 (Video S6 in Additional file 6) exhibits superplasticity behavior with local disordered deformation. With the tension strain increasing, the  nanowire is more likely to break near the two ends of the nanowire due to the symmetric stress and local melted structures.
For the special deformation behaviors of the  single-crystal copper nanowires at the strain rates of 0.01, 1.54, and 6.16% ps-1 (see Videos S7-S9 in Additional files 7, 8 and 9), the difference from  and  is that the deformation mechanism is the partial lattice rotation for the  nanowires. After the relaxing, the nanowire retains relative order lattice at the low strain rate of 0.01% ps-1, and the disorder crystal structures becomes obvious at the strain rate of 1.54% ps-1. When the strain rate is at 6.16% ps-1, the local disorder structures distribute at the two ends of the nanowires with the strain increasing, not at one side of 0.01% or 1.54% ps-1. In the stretching processes of the nanowires, the disorder crystal structures increase obviously with the strain rate increasing. The effects of strain rates on deformation structures could be reflected by the maximum average potential energy per atom in Figure S1 (Additional file 10), which increases generally for each crystallographic orientation with the strain rate increasing. Within the simulated strain rates, the , , and  nanowires have the lowest energy, the intermediate energy, and the highest energy, respectively, which are consistent with lattice plane energies in FCC metals [46, 47].
From the above results, the single-crystal copper nanowires present various deformation behaviors at each crystallographic orientation. At low strain rates, clear slippage for  orientation occurs along the (111) planes. When the copper nanowires are stretched along the  orientation, the obvious lattice reconstruction becomes predominant after the first yielding point, whereas the  nanowires are partial lattice rotation in the deformation processes. However, at high strain rates, the nanowires always behave as local disorder structures at the two ends for each crystalline orientation. The dependence of deformation mechanism on strain rate and crystallographic orientation indicates that anisotropy behaves obvious at low strain rates, whereas unobvious at high strain rates due to strong symmetric stretching. Different deformation behaviors of the nanowires, which may be effectively evaluated by the mechanical property, are attributed to the different deformation mechanisms.
For the dependence of crystallographic orientation, Figure 3b, c and 3d give the first yield stress, strain, and Young's modulus as a function of strain rate, respectively. Strain rates applied on the , , and  nanowires are all from 0.01 to 7.69% ps-1, and the average statistical result is from 300 samples for each strain rate. For the  single-crystal copper nanowires, the first yield strain and stress both increase with the strain rate increasing. However, the first yield strain and stress are insensitive to the lower strain rate, whereas they are sensitive to the higher strain rate. Here, we named the divided range of strain rates as insensitive area (I), transitional area (II), and sensitive area (III). Young's modulus (Y) is defined by Sun  as the stress of a material divided by its strain in the elastic deformation region, which may be used to evaluate the mechanical strength of the nanowires. In the insensitive area of strain rates (I), the average Y fluctuates within its error, and it behaves as an increasing trend in the transition area of strain rates (II). While reaching the sensitive area of strain rates (III), Y abruptly increases in a line with the strain rate increasing, indicating the presence of the hardening effect. However, while comparing among , , and  crystallographic orientations, the mechanical properties of the  nanowires behave as different characters. It is attributed to the deformation mechanism and breaking behavior of the  nanowire at different strain rates, i.e., the  nanowire always prefers to maintain its crystallographic structure at low, middle, and high strain rates, respectively, (see Videos S4-S6 in Additional files 4, 5 and 6), so the mechanical property behaves insensitively to strain rates. In general, the stress and the Y of the  are not sensitive to strain rates within the range of strain rates, but the Y of the  indicates the largest mechanical strength.
The structural anisotropy and symmetric stretching under different strain rates could bring different deformation mechanisms and mechanical properties, which could give insight into mechanical breaking failure and operation of metallic nanowires. If we could predict the deformation behaviors and the final breaking positions of the nanowires, the breaking failure could be controlled and the nanowires also could be strengthened near the breaking positions to avoid failure.
Under the same simulation conditions of the  nanowires, the breaking position distributions of the  and  nanowires behave completely different at the strain rates from 0.01 to 7.69% ps-1 (to refer to strain rates in Table 1). The breaking position distributions in Figure 4c show that the  nanowires would like to break at two ends of the nanowire at the low simulated strain rates, and the symmetric property of the breaking position distribution becomes obvious with the strain rates increasing. However, the  nanowires in Figure 4d show the breaking position exhibits a single-peak distribution at the insensitive area of strain rate (I), and the symmetric distributions at the two ends of the nanowire gradually behave obvious with the strain rates increasing. From the influence of crystallographic anisotropy, we conclude the implied relationships in the scheme of Figure 1. When the symmetric stretching of the nanowire is applied at the low strain rates (less than 0.26% ps-1), micro-atomic fluctuation in equilibrium state brings the system enough ability to exhibit anisotropic characters of the crystal structures. Therefore, different deformation mechanisms for each crystallographic orientation exhibit various macro-breaking position distributions of the nanowires at low strain rates. When the symmetric stretching of the nanowire is applied at the high strain rates (larger than 3.54% ps-1), strong symmetric shocks dominate the deformation and breaking at the two ends of the nanowires. And the micro-atomic fluctuation has not enough ability to exhibit anisotropic characters of the crystal structures, thus, symmetric stretching results in macro-breaking position distributions at the two ends of nanowires.
For the microscopic behaviors of atoms under mechanical shocks, Holid and co-workers [49–51], Kadau et al. , and Bringa et al.  studied the shock wave propagation in solid materials and demonstrated its existence in nanoscale materials. Koh et al. [18, 19] and Liu et al.  used the strain wave propagation theory to predict the breaking position of the nanowires. The theory could be stated that, if shock is involved, the longitudinal shock wave velocity can be derived from the simplified wave equation given by U s = (Y/ρ)1/2, Y is the Young's modulus and ρ is the average density of solid materials, which is estimated to be 8,900 kg/m3 for copper, and the most probable breaking position of the nanowires could be predicted and interpreted with the shock wave propagation theory, using the shock wave propagation distance d = U s × t = (Y/ρ)1/2 × t, and t is the required time to attain atomic break.
The microscopic mechanism of shock wave propagation in solids is inherently complex because the plastic flow is governed by the creation and motion of defects in the deformation of nanoscale materials. Meanwhile, with the statistical analysis of samples, we can find it is difficult to calculate exactly the fixed breaking position using the above shock wave propagation equation, especially for the breaking in the distributions of , , and  crystallographic orientations at the insensitive area of strain rates, and the uncertainty of  at the transitional area of strain rates. From microscopic viewpoint, mechanical shock is induced by symmetric stretching at different strain rates, and different mechanical shocks and anisotropies could affect the micro-atomic fluctuation, which could induce different ways of shock wave propagation. Mechanical shocks can disrupt the lattice order in the tensile deformation of the nanowires, attributing the concentrated and dispersed energy in shock wave propagation which is converted to the atomic kinetic energy, so the atomic bonds will break when the activated atoms have enough energy to overcome the atomic cohesive energy. At low strain rates, symmetric stretching retains the relative order lattice in the equilibrium state. In this state, micro-atomic fluctuation in anisotropic crystal structures mainly affect shock wave propagation in the stretching processes of the , , and  copper nanowires. Different styles of shock wave propagation for each crystallographic orientation make macro-breaking position distributions with various characters. At high strain rates, symmetric stretching in non-equilibrium state brings a large stress gradient, which induces the difficulty of shock wave propagation, so shock waves overlap at the two ends of the nanowires, which tend to break at the two ends without anisotropic behaviors. Thus, macro-breaking position distributions at the two ends of the nanowires show symmetric characters at high strain rates.
In summary, we have simulated the , , and  single-crystal copper nanowires subjected to symmetric stretching at strain rates from 0.01 to 7.69% ps-1, and we have studied the deformation behaviors, mechanical properties, and their breaking position distributions. We find that: (i) at low strain rates, the , , and  crystallographic orientations behave as three deformation mechanisms, slippage, reconstruction, and rotation, respectively. Whereas, high strain rates easily induce their local melted structures at two ends of the nanowires; (ii) for the effect of strain rate on the mechanical properties of , , and  crystallographic orientations,  is obvious,  is not obvious, and  is between of them; (iii) the macro-breaking position distributions reflect the micro-atomic fluctuation during the symmetric stretching applied on the nanowires. When the strain rate is less than 0.26% ps-1, macro-breaking position distributions exhibit the structural anisotropy. However, the anisotropy is not obvious when the strain rate is larger than 3.54% ps-1 because of the strong symmetric shocks.
atomic force microscopy
mechanically controllable break junctions
most probable breaking position
scanning tunneling microscopy
transmission electron microscope.
This project was supported by the National Natural Science Foundation of China (Grant Nos. 20821063, 20873063 and 51071084), National Basic Research Program of China (973 Program, Grant No. 2007CB936302 and 2010CB732400), the Natural Science Foundation of Jiangsu Province (BK2010389) and Specialized Research Fund for the Doctoral Program of Higher Education (SRFDP, 20070284007).
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