Role of Defects on the Deformation Behavior and Mechanisms in Large-scale Nickel Nanowire Subjected to Different Loading Methods: A Molecular Dynamics Simulation Study

Abstract

Metallic nanowires (NWs) are one of the essential building blocks in electromechanical devices, interconnects, and other nanodevices due to their unique mechanical, electrical, optical, and magnetic properties. The reliability of these devices depends mainly on the mechanical response of the constituents. NWs have a large surface-to-volume ratio compared to bulk materials, and hence the free surfaces are expected to play an essential role in the plastic deformation at smaller length scales. Also, the plastic deformation is observed to be heterogeneous at small length scales characterized by localized strain. Experimental studies are expensive as it requires expensive setups to maintain high precision equipment. Also, there could be uncertainty in measurements that may arise from specific issues, such as the gripping of NWs and rotation of the NWs. Therefore, a numerical simulation study is an alternative to conducting experiments. A computational tool like molecular dynamics (MD) numerically solves the N-body problem of classical mechanics and has been widely used to gain insights into the deformation behavior and mechanisms of perfect and defect nanoscale materials. In the present study, large-scale MD simulations are carried out to investigate the effect of defects (linear and voids), defects interactions (linear-void), and temperature effect (10 K-1200 K) on the mechanical properties and deformation mechanisms in single crystal nickel (Ni) NW (~925625 atoms). The NWs are tested for different loading methods, i.e., tensile, bending, and torsion at different temperatures and loading rates. The microstructural evolution (dislocations, stacking faults, twins, and twin boundaries) is analyzed and correlated with the deformation behavior. Finally, an experimental study is carried out to correlate the deformation relief patterns with those observed in MD simulation studies. The experimental studies are carried out on Ni-28W (28 wt.%W) single crystal alloy, subjecting it to compression test at room temperature. The major findings from the above mentioned studies are discussed in the following paragraph. The MD simulation tensile deformation results at a temperature of 10 K and strain rate of 108 s-1 show that the defects lower the yield stress and is more prominent in the presence of internal void (~30 % decrease). Several intrinsic and extrinsic parallel stacking faults (SFs) are generated after yielding by slip occurring on planes. Crack propagation and crack-defect (void) interactions studies carried out at a temperature of 10 K and strain rate of 109 s-1 on [010], [1-10], and [111] axial crystallographic orientations of Ni NW show that crack does not propagate in the [111] orientation. The crack velocity is 313 m/s in [010] orientation. With the decrease in crack-void spacing from 20 Å to 5 Å, the average crack velocities (207 m/s-120 m/s) decrease in [010] orientation due to crack tip blunting arising from the crack-void coalescence. The tensile and creep behavior of nickel NWs containing single and multiple-voids (void diameter = 30 Å) carried out in the strain rate range of 108 s-11010 s-1 and temperatures of 10 K  1200 K show that the load-bearing capacity, the elastic modulus of the NWs decrease with an increase in the number of voids and temperature. The stress exponent (n) estimated from the steady-state regions of the creep curves is in the range of 0.8 – 2.77, which confirms the diffusive mechanism. The activation energies for creep are estimated and found to be 42.5 kJ/mol for perfect NW, 40 kJ/mol for single void, and 38 kJ/mol for multi-void nickel NW. So, it can be concluded that multi-void NW is less creep resistant. In the torsion studies, the critical torsional angles are lower in the NWs containing defects as compared to the perfect NW. Finally, the experimental and simulated compressive stress-strain curves of Ni-28W (28 wt.%W) alloy show a qualitative similarity.