Influence of anisotropic elasticity on the mechanical properties of fivefold twinned nanowires

Abstract

Previous atomistic simulations and experiments have shown an increased Young's modulus and yield strength of fivefold twinned (FT) face-centered cubic metal nanowires (NWs) when compared to single crystalline (SC) NWs of the same orientation. Here we report the results of atomistic simulations of SC and FT Ag, Al, Au, Cu and Ni NWs with diameters between 2 and 50 nm under tension and compression. The simulations show that the differences in Young's modulus between SC and FTNWs are correlated with the elastic anisotropy of the metal, with Al showing a decreased Young's modulus. We develop a simple analytical model based on disclination theory and constraint anisotropic elasticity to explain the trend in the difference of the Young's modulus between SC and FTNWs. Taking into account the role of surface stresses and the elastic properties of twin boundaries allows to account for the observed size effect in Young's modulus. The model furthermore explains the different relative yield strengths in tension and compression as well as the material and loading dependent failure mechanisms in FTNWs.

Study of the Mechanical Behavior of Radially Grown Fivefold Twinned Nanowires on the Atomic Scale

“…Apart from these fi ve TBs of the fi vefold twin, another four TBs (TB6, TB7, TB8, and TB9) intersecting with TB2 formed during the growth process and lots of steps and defects were thus located on TB2. Different from the disclination model, [ 28,37 ] the 7.5° misfi t was mostly shared by TB2 and TB4 thus broadening these two TBs, as shown in Figure 3 a. This was also confi rmed by the fast-Fouriertransform (FFT) image (inset in Figure 3 a), which showed that the spots of TB2 and TB4 were not as sharp as the ones of TB1, TB3, and TB5.…”

“…Previous result have shown that the Schmid factor for the leading partial dislocation is a factor of two larger under tension than under compression, meaning that less stress is required to nucleate dislocations under tension compared to under compression. [ 28 ] Therefore, the stresses under compression are larger than those under tension, which has been proven in cyclic deformations. [ 51 ] So, the formation of full dislocations, which mainly nucleated from the steps on the twin boundaries, dominated the plastic deformation behavior, as was reported before.…”

“…The mechanical performance in these two regions are usually asymmetric. [28] With increasing bending stress in the tension region, a lot of SFs are emitted from or near the twin center, which glide parallel to TB1. These Shockley partial dislocations that glide parallel to TB1, leading to TB1 migration, have been reported before.…”

“…So, there was a unique stress put on the surface, whereby local stress concentrations were easier to create in sites with surface undulations that in turn became the dislocation nucleation sites. [19][20][21][22][23][24][25][26]28,29] However, in our test, we loaded a bending force, which is similar to that used in AFM nanoindentation. [10,27] The bending force is more complex than the uniaxial tensile force, resulting in two stressed regions of the nanowire: the tension and compression regions, the real stress state may therefore be more complex than we expect.…”

“…[ 51 ] So, the formation of full dislocations, which mainly nucleated from the steps on the twin boundaries, dominated the plastic deformation behavior, as was reported before. [ 28 ] Full dislocations gliding along two crossing {111} planes provide an opportunity for the formation of an L-lock. These L-locks are stable and sessile as they can glide in neither of the slip planes of the reactant dislocations.…”