Molecular dynamics study of crystal plasticity during nanoindentation in Ni nanowires
- PDF / 1,279,055 Bytes
- 9 Pages / 584.957 x 782.986 pts Page_size
- 104 Downloads / 251 Views
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 -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 1 ms1. 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 1
Quasi-one-dimensional metal nanowires are the building blocks for nanoscale research in a vast variety of disciplines that range from biology to electromechanics and photonics.2–5 These nanomaterials have recently stimulated the interest of the mechanics community, because most experimental evidence shows a strong influence of the sample dimension on the mechanical properties of metals at nanometer scale.6–11 Whereas the strength and ductility of metals in macroscopic samples are predominantly determined by the relevant microstructure length scale (e.g., grain size), which is often small relative to the sample size, a distinctive behavior of crystal plasticity emerges in metal nanostructures, where the material strength significantly increases as the deformation length scale (diameter or volume) decreases. A microplasticity mechanism has been proposed to account for the size scale dependence of small metallic samples based on dislocation starvation, in which the density of mobile dislocations created from pre-existing dislocation sources is counter-balanced by the density of dislocations escaping the crystal at free surfaces.7,10,12 The in situ TEM compression experiments of Shan et al.13 have recently confirmed this mode of deformation a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0103
in Ni nanopillars as small as 150 nm in diameter. Nevertheless, it remains crucial to characterize the influence of sample size on dislocation activity at even smaller length scale (
Data Loading...
