Molecular dynamics simulation of plastic deformation and interfacial delamination of NiTi/Ag bilayer by cyclic-nanoindentation: Effects of crystallographic orientation of substrate

Abstract

This paper presents a comparative study of plasticity and fracture behavior of the NiTi/Ag bilayer for the different crystallographic orientations of the substrate. Molecular dynamic (MD) simulation was used to determine the deformation mechanism, dislocation density, plastic energy dissipation and delamination of the NiTi/Ag bilayers near the interface, when NiTi aligned at (1 0 0), (1 1 1), (1 1 0), (3 2 1), (2 1 0) and (2 1 1) faces during the cyclic-nanoindentation test. The Griffith energy balance model was used to estimate the energy release associated with the delamination. The results of the simulation are suggested the dependence of deformation mechanism, energy release rate (Gin), interfacial toughness (Kin) and average plastic energy dissipation per unit cycle (Eplastic) of NiTi/Ag bilayer on the crystallographic orientation of the NiTi substrate. Prismatic dislocation loop and the nest-like organization of the twins were responsible for the plastic deformation of the NiTi/Ag bilayers when NiTi aligned at (211,010,111) and (100,210,321) faces, respectively. The values of Gin and Kin of the bilayers were in the range of 64.27–147.71 J/m2 and 77–192 J/m2, respectively. The values of Eplastic is increased in the following order: NiTi (2 1 1) < NiTi (3 2 1) < NiTi (1 0 0) < NiTi (2 1 0) < NiTi (1 1 1) < NiTi (0 1 0). The average dislocation densities produced for the indenter radius of 30 Å were in the range of 1.23 × 1019–1.84 × 1019 m−2. The results show that the NiTi (0 1 0)/Ag case has the larger dislocation density than other bilayers. Both of hardness and the interfacial toughness of the NiTi/Ag at different crystallographic orientations were consistent with the free energy data obtained from Jarzynski’s equality for nanoindentation results, which were in the range of 3.35 × 10−4−7.81 × 10−4 eV/atom. Due to high interfacial toughness and hardness plus lower plastic energy dissipation, the NiTi (2 1 1)/Ag bilayer may show the better results for engineering and medical applications.