Role of grain boundaries and substrate in plastic deformation of core–shell nanostructures

Abstract

The mechanical properties of metallic systems at the nanoscale can be modified by varying their structure and composition. One example of a deformation-resistant structure is a core–shell nanostructure (CSN). In the present work, we use molecular dynamics simulations to perform nanoindentation and retractions to understand plastic deformation of core–shell nanostructures. The core consists of aluminum (Al), the shell is amorphous silicon (a-Si), and the substrate is either crystalline Al or a-Si. The unique aspect of this work is that we study the deformation behavior of CSNs that contain symmetric and asymmetric grain boundaries in the core with two different orientations; comparisons are made to deformation in a CSN with a single-crystal core. Nanoindentation on CSNs with 5 nm and 10 nm core radii shows that the elastic stiffness with and without a grain boundary is similar when the substrate material is the same. Five-nanometer-core-radius CSNs with a-Si and Al substrates and an asymmetric tilt grain boundary in the core show 100% recovery from dislocation plasticity, but damage within the core leads to reduction of ~ 50% of the atoms that no longer being identified as FCC crystal structure, which makes these CSNs non-deformation-resistant. CSNs with Al substrate (5 nm and 10 nm core radii) obtained 100% recovery from plasticity and retained its crystal structure after unloading when the core is single crystal or contains a symmetric tilt grain boundary. Moreover, a single-crystal core can withstand nanoindentation to 100% of the shell thickness, whereas a symmetric tilt grain boundary core can only withstand nanoindentation about 80% of the shell thickness and still have 100% recovery from plasticity. Therefore, CSNs with single-crystal core are more reliable for deformation-resistant behavior than those that contain grain boundaries.