Effect of orientation and mode of loading on deformation behaviour of Cu nanowires

Abstract

Molecular dynamics simulations have been performed to understand the variations in deformation mechanisms of Cu nanowires as a function of orientation and loading mode (tension or compression). Cu nanowires of different crystallographic orientations distributed uniformly on the standard stereographic triangle have been considered under tensile and compressive loading. The simulation results indicate that under compressive loading, the orientations close to <100> corner deform by twinning mechanism, while the remaining orientations deform by dislocation slip. On the other hand, all the nanowires deform by twinning mechanism under tensile loading. Further, the orientations close to <110> and <111> corner exhibit tensioncompression asymmetry in deformation mechanisms. In addition to deformation mechanisms, Cu nanowires also display tension-compression asymmetry in yield stress. The orientations close to <001> corner exhibits higher yield stress in tension than in compression, while the opposite behaviour (higher yield stress in compression than in tension) has been observed in orientations close to <110> and <111> corners. For the specific orientation of <102>, the yield stress asymmetry has not been observed. The tension-compression asymmetry in deformation mechanisms has been explained based on the parameter α M , defined as the ratio of Schmid factors for leading and trailing partial dislocations. Similarly, the asymmetry in yield stress values has been attributed to the different Schmid factor values for leading partial dislocations under tensile and compressive loading.

Role of axial twin boundaries on deformation mechanisms in Cu nanopillars

“…It can be seen that the yield stress asymmetry for twinned nanopillars is different from that of the perfect nanopillars (higher the number of twin boundaries, lower the asymmetry), which indicates that the twin boundaries can also influence the tension-compression asymmetry. Previous studies have shown that the asymmetry in yield stress arises due to different values of Schmid factor for leading partials under tensile and compressive loading [29]. Further, Salehinia and Bahr [30] have shown that the asymmetry in the strength of nanowires decreases with increasing defect density, which is in agreement with the present findings.…”

“…With increasing strain, the extended dislocations belonging to different slip systems interact and results in the formation of a stacking fault tetrahedron (Figure 5b). The nucleation of trailing partials under compressive loading is favoured due to its high Schmid factor value as compared to leading partial [29]. Different from compressive loading, the deformation under tensile loading of perfect <112> Cu nanopillars occurs through the slip of Shockley partial dislocations ( Figure 5(c-d)).…”

Atomistic mechanisms of twin-twin interactions in Cu nanopillars

“…This condition is readily met in <100> nanopillar deforming under compression, <110> and <111> nanopillars under tension. As a result, numerous studies have reported twinning in these orientations and loading conditions [2,8,9,10]. Further, when twinning occurs on limited twin systems, nanopillars undergo complete reorientation [10,11], which also leads to sequential reorientation [12], shape memory and pseudo-elasticity.…”

“…As a result, numerous studies have reported twinning in these orientations and loading conditions [2,8,9,10]. Further, when twinning occurs on limited twin systems, nanopillars undergo complete reorientation [10,11], which also leads to sequential reorientation [12], shape memory and pseudo-elasticity. However, when twinning occurs on multiple twin systems, it is possible that the twins belonging to different twin systems interact and form a more complex twin-twin junctions.…”

Role of twin boundary position on the yield strength of Cu nanopillars

“…In nanopillars/nanowires, the surface mediated plasticity has been identified as a dominant deformation mechanism [2,3,4], while in bulk materials dislocation multiplication, pile-up, cross-slip and other forest mechanisms are known to play the dominant role. The yielding in FCC nanopillars occurs through the nucleation of Shockley partials or extended dislocations from the surface and it can be influenced by a variety of factors like nanowire size, shape, orientation, temperature and strain rate [1,3,5,6,7,8]. As a result, the yield strength of the nanopillars is also sensitive to all these factors.…”