Effect of shell thickness on mechanical behavior of Al/Ti core-shell nanowires during three-point bending and unloading

Abstract

The molecular dynamics models of Al and Al/Ti core-shell nanowires (NWs) are established using the large-scale atomic/molecular massively parallel simulator (LAMMPS) to simulate the loading and unloading of the three-point bending of NWs and to investigate the effect of Ti shell thickness on the mechanical behavior of core-shell NWs during loading and unloading. The results show that the Ti shell thickness considerably affects the mechanical properties of the NWs during the bending process. As the shell thickness of the NWs increases from 0 Å to 10 and 20 Å, their Young's moduli and yield strength first decreases and then increases; this is attributed to the unique evolution of the core-shell structure during the bending process. When the shell thickness is 0 Å, the yielding mechanism of the Al NWs involves the slip of the face-centered cubic (FCC) lattice plane to generate the hexagonal close-packed (HCP) stacking fault. During the subsequent plastic deformation, the FCC lattice plane continues to slip, resulting in the generation and annihilation of Shockley dislocations and HCP stacking faults. Finally, the Al NWs become completely amorphous at the cross section at the midpoint. The yielding mechanism of core-shell NWs involves the plastic deformation of the Ti shell at the bottom of the NW, followed by the continuous outward expansion of the plastic deformation region with increasing loading; this leads to the formation of a fan-shaped region at the bottom of the NW. Finally, the NWs are divided into different components according to their structure and atomic energy, and the driving force for each component during NW unloading is determined. Moreover, the strengthening effect of the shell thickness on the recovery performance of the NWs is investigated. The results show that Body Al provides the main driving force for the shape recovery of Al NW during the unloading process, and Surface Ti and Body Ti provide the main driving force for the shape recovery of the core-shell NWs. Larger the shell thickness, stronger the NW recovery performance during unloading. This study may facilitate the understanding of the unique mechanical behavior of the core-shell NWs.