Molecular Dynamics Study of Crystal Plasticity During Nanoindentation in Ni Nanowires

Aerospace Engineering - Daytona Beach College of Engineering 3-2009 Molecular Dynamics Study of Crystal Plasticity during Nanoindentation in Ni Nanowires V. Dupont Embry Riddle Aeronautical University, [email protected] F. Sansoz The University of Vermont Follow this and additional works at: https://commons.erau.edu/db-aerospace-engineering Part of the Aerospace Engineering Commons Scholarly Commons Citation Dupont, V., & Sansoz, F. (2009). Molecular Dynamics Study of Crystal Plasticity during Nanoindentation in Ni Nanowires. Journal of Materials Research, 24(3). Retrieved from https://commons.erau.edu/db- aerospace-engineering/3 This Article is brought to you for free and open access by the College of Engineering at Scholarly Commons. It has been accepted for inclusion in Aerospace Engineering - Daytona Beach by an authorized administrator of Scholarly Commons. For more information, please contact [email protected]. Molecular dynamics study of crystal plasticity during nanoindentation in Ni nanowires V. Dupont and F. Sansoza) School of Engineering, The University of Vermont, Burlington, Vermont 05405 (Received 31 July 2008; accepted 28 October 2008) Molecular dynamics simulations were performed to gain fundamental insight into crystal plasticity, and its size effects in nanowires deformed by spherical indentation. This work focused on <111>-oriented single-crystal, defect-free Ni nanowires of cylindrical shape with diameters of 12 and 30 nm. The indentation of thin films was also comparatively studied to characterize the influence of free surfaces in the emission and absorption of lattice dislocations in single-crystal Ni. All of the simulations were conducted at 300 K by using a virtual spherical indenter of 18 nm in diameter with a displacement rate of 1mÁsÀ1. No significant effect of sample size was observed on the elastic response and mean contact pressure at yield point in both thin films and nanowires. In the plastic regime, a constant hardness of 21 GPa was found in thin films for penetration depths larger than 0.8 nm, irrespective of variations in film thickness. The major finding of this work is that the hardness of the nanowires decreases as the sample diameter decreases, causing important softening effects in the smaller nanowire during indentation. The interactions of prismatic loops and dislocations, which are emitted beneath the contact tip, with free boundaries are shown to be the main factor for the size dependence of hardness in single-crystal Ni nanowires during indentation. I. INTRODUCTION in Ni nanopillars as small as 150 nm in diameter. Never- Quasi-one-dimensional metal nanowires1 are the theless, it remains crucial to characterize the influence of building blocks for nanoscale research in a vast variety sample size on dislocation activity at even smaller length of disciplines that range from biology to electromecha- scale (<100 nm) to achieve meaningful results on the nics and photonics.2–5 These nanomaterials have recently crystal plasticity of metal nanowires. Nanoindentation technique via pillar compression stimulated the interest of the mechanics community, be- 6,7,9–11,13 cause most experimental evidence shows a strong influ- method has enabled rapid progress in the exper- ence of the sample dimension on the mechanical imental investigation of nanomechanical properties and properties of metals at nanometer scale.6–11 Whereas the their size dependence in metals at the micron and submi- strength and ductility of metals in macroscopic samples cron scales. However, the nanopillar compression meth- are predominantly determined by the relevant micro- od has never been applied to samples less than 100 nm in structure length scale (e.g., grain size), which is often diameter due to complications in preparation and me- small relative to the sample size, a distinctive behavior chanical testing at such a small scale. By contrast, the of crystal plasticity emerges in metal nanostructures, use of nanoindentation tips to probe the radial elastic modulus and hardness of sub-100 nm nanowires has been where the material strength significantly increases as the 14–22 deformation length scale (diameter or volume) decreases. found successful in the past. Whereas a fundamental A microplasticity mechanism has been proposed to ac- understanding of dislocation activity during metal nanopillar compression has already been supplemented by count for the size scale dependence of small metallic 23–27 samples based on dislocation starvation, in which the atomistic simulations, the atomic mechanisms of density of mobile dislocations created from pre-existing plasticity and related size effects for metal nanowires dislocation sources is counter-balanced by the density of deformed by nanoindentation remain elusive. dislocations escaping the crystal at free surfaces.7,10,12 The aim of this work is to elucidate the processes of The in situ TEM compression experiments of Shan plastic deformation in metal nanowires subjected to et al.13 have recently confirmed this mode of deformation nanoindentations by atomistic simulation. To this end, large-scale molecular dynamics simulations are per- a)Address all correspondence to this author. formed to provide atomistic insight into the size e-mail: [email protected] dependence of mechanical properties in [111]-oriented DOI: 10.1557/JMR.2009.0103 single-crystal, defect-free Ni nanowires deformed by 948 J. Mater. Res., Vol. 24, No. 3, Mar 2009 © 2009 Materials Research Society V. Dupont et al.: Molecular dynamics study of crystal plasticity during nanoindentation in Ni nanowires spherical indentation. The significance of dislocation ac- both thin-film and nanowire models, the indentation tivity in the absence of free-surface boundaries is also was performed along the ½112 crystallographic direc- addressed by using nanoindentation simulations on sin- tion of the sample, and the bottom two atomic layers gle-crystal Ni thin films. A comparative study of the were fixed along the direction of indentation. In addi- effects of sample size and free surfaces between thin tion, the bottom two atomic layers in the nanowires film and nanowires is presented. The atomic mechan- were constrained along the ½110 direction, normal to isms of plastic deformation related to the emission of the direction of indentation, to prevent the wire from lattice dislocations and their absorption by free surfaces rolling during the deformation. The total number of during indentation are also analyzed. atoms ranged from 400,000 (12-nm-diameter nanowire) to 4.4 million (30-nm-thick film). The embed- II. SIMULATION METHODS ded-atom method (EAM) interatomic potential for Ni from Mishin et al.28 was used. This potential has been Parallel molecular dynamics simulations were per- fitted on ab initio calculations and experimental formed to study the nanoindentation of single-crystal Ni values, which improves the prediction of surface ener- thin films and nanowires deformed by a frictionless, gies and stacking fault energies for Ni. The simula- spherical tip of 18 nm in diameter (Fig. 1). Simulations tions were performed at 300 K by using a NVT were conducted on [111]-oriented single-crystal Ni integration scheme with a time step of 5 fs. Similar to nanowires with a cylindrical shape. The nanowires were past atomistic studies,29,30 the virtual indenter was 40 nm in length, and either 30 or 12 nm in diameter as modeled by a spherical, repulsive force of magnitude illustrated in Figs. 1(b) and 1(d), respectively. Periodic boundaries were only applied along the wire axis. To simu- FðrÞ¼Àkðr À RÞ2 ; ð1Þ late thin films, the whole simulation box (40 Â 40 nm) where r is the distance from an atom to the center of the was filled with atoms and periodic boundary conditions indenter, R is the radius of the indenter, and k is a force were applied to the directions parallel to the film surface constant (= 10 N/m2). A gap of 0.2 nm was initially [Fig. 1(a)]. Two films of 30 and 12 nm in thickness as imposed between the sample surface and the indenter. shown in Figs. 1(a) and 1(c), respectively, were investi- The indenter was displaced at a rate of 1 m/s. Despite gated. We have also verified that the boundary conditions being unrealistically high, such a velocity used in our of our thin-film models did not influence the results by simulations is relatively small in comparison with the simulating the nanoindentation of a larger model of thick- range of strain rates achievable by current molecular ness 12 nm and size 60 Â 60 nm (data not shown). For dynamics simulations.29,30 The final depth of indentation into the samples was 1.8 nm (400,000 steps). The atomic positions were recorded at 50 ps intervals (10,000 steps). The local lattice structure was determined from the for- mulation given by Ackland and Jones.31 In this paper, atoms in dark-gray color represent surface and noncoor- dinated atoms, whereas atoms in HCP configuration (stacking faults) appear in light-gray color. For most figures, in the following, atoms in FCC configuration have been omitted for clarity. The contact zone was defined by the atoms positioned within the boundary of the indenter (i.e., r<R). As such, the mean contact pressure pm was calculated as P p ¼ ; ð2Þ m A where P is the total load applied by the indenter to the contacted atoms, and A is the projected contact area, which was directly observed in the samples. In our atomistic simulations, it was found that the shape of the contact area was irregular, due to the discrete number of FIG. 1. Atomistic models of spherical indentation for thin films and atoms; but for simplicity, the contact area was approxi- nanowires. The diameter of the virtual indenter is 18 nm. (a) 30-nm- mated by an elliptical shape following the model pro- thick film. (b) 30-nm-diameter nanowire. (c) 12-nm-thick film. (d) 12- 18 nm-diameter nanowire.

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