Strain Rate Induced Amorphization in Metallic Nanowires

Abstract

Using molecular dynamics simulations with a many-body force field, we studied the deformation of single crystal Ni and NiCu random alloy nanowires subjected to uniform strain rates but kept at 300 K. For all strain rates, the Ni nanowire is elastic up to 7.5% strain with a yield stress of 5.5 GPa, far above that of bulk Ni. At high strain rates, we find that for both systems the crystalline phase transforms continuously to an amorphous phase, exhibiting a dramatic change in atomic short-range order and a near vanishing of the tetragonal shear elastic constant perpendicular to the tensile direction. This amorphization which occurs directly from the homogeneous, elastically deformed system with no chemical or structural inhomogeneities exhibits a new mode of amorphization.[ S0031-9007(99) We used molecular dynamics (MD) simulations to strain fcc crystalline nanowires at a uniform rate along the ͗001͘ crystallographic direction [2][3][4]. Here, to realize an infinite nanowire we applied 1D periodic boundary condition in the c direction (an initial length of ten fcc unit cells, ഠ4 nm, while the a and b directions are five fcc cells long, ഠ2 nm). We studied the deformation behavior at constant strain rates (0.05% to 5% ps 21 ) and constant temperature (300 K). The tensile strain component´3 3 was applied uniformly (in increments of 0.5%) to obtain the specified strain rate. The average stress components s ij in the specimen were computed at and following each applied strain increment. At the strain rates reported here, the stress distribution relaxes between strain increments to a homogeneous stationary equilibrium state of uniaxial tensile stress. We observe a crystal to glass transformation at 300 K above a critical strain rate.Such large strain rates are observed experimentally only in shock wave and high velocity impact studies, where it has been difficult to control temperature or to obtain details about dynamic structural changes. Experiments at such high strain rates lead to shear localization arising from adiabatic heat dissipation and local thermal softening of the material, which results in highly nonequilibrium systems that are difficult to study experimentally. The MD simulations follow the effects of loading and loading rate independently from those arising from heat dissipation and concomitant temperature increases.To illustrate the effect of strain rate on the detailed deformation, Fig. 1 displays snapshots of MD simulations on an fcc NiCu random alloy nanowire (at 300 K) deformed to 100% strain at strain rates from 0.5% and 5% ps 21 in the z direction. For ᠨ 0.5% ps 21 cooperative shear events within the crystal produce coherent shear bands, which are often coherent "twins." Multiple coherent shearing events finally lead to necking before failure. For ᠨ 5% ps 21 , the behavior is fundamentally different. No coherent shear bands or twins form as the system is strained. Instead, the specimen transforms homogeneously to an amorphous state at strains of only 0.15. This homogeneously disordered material un...

The uniaxial tensile deformation of Ni nanowire: atomic-scale computer simulations

“…The structural transformation takes place in nanoscale metallic wires under uniaxial strain [12,14,[17][18]23]. At lower strain rates, the Ni nanowire shows superplasticity [12,13], and at sufficiently high strain rates, it can transform continuously to an amorphous metal at constant temperature [13,14,23]. The Au nanowires, before breaking under tensile stress, can get as thin as one-atom chains, and as long as five suspended atoms [18,21].…”

“…It should be emphasized that the loading process of the tensile strain is different from those described in Refs. [13,14], where the tensile strain with increments of 0.5% in its total length after some time steps. In the present work, the tensile strain is increased by 5 Â 10 À6 , 2 Â 10 À6 , 5 Â 10 À7 in each time step along the Z-direction for the strain rate of 0.25%/ps, 0.1%/ps, and 0.025%/ps, respectively.…”

“…In the present paper, MD simulations have been performed with the quantum corrected SuttenChen (Q-SC) type many-body force field modified by Kimura et al [24] in which the parameters were optimized to describe the lattice parameter, cohesive energy, bulk modulus, elastic constants, phonon dispersion, vacancy formation energy, and surface energy, leading to an accurate description of many properties of metals and their alloys [25][26][27]. We have simulated the tensile process of Ni nanowire at room temperature.…”

“…However, measuring the mechanical and electrical properties of a nanowire is a difficult task due to the small dimensions [11]. In recent years, the deformation of nanowires have been studied by molecular dynamics simulations using either embodied-atom-method (EAM) [12][13][14][15] or effective-medium theory (EMT) [16,17] potentials as well as first-principles method based on the density functional theory (DFT) [18][19][20][21]. These studies focus on the correlation of the force (associated with the changes in the bonding of the nanowire) and the conductance for the metallic Au, Ni, and Na nano-contacts [17,22] and Au, Ni, Al, Na, and SiSe 2 nanowire [12,16,20,23].…”

“…These studies focus on the correlation of the force (associated with the changes in the bonding of the nanowire) and the conductance for the metallic Au, Ni, and Na nano-contacts [17,22] and Au, Ni, Al, Na, and SiSe 2 nanowire [12,16,20,23]. The structural transformation takes place in nanoscale metallic wires under uniaxial strain [12,14,[17][18]23]. At lower strain rates, the Ni nanowire shows superplasticity [12,13], and at sufficiently high strain rates, it can transform continuously to an amorphous metal at constant temperature [13,14,23].…”