Molecular Dynamics Simulation Study Of Plastic Deformation Mechanism Of Face-Centered-Cubic Metal Nanowires With Twin Structures

Twin-structured metallic nanowires?NWs?have attracted intensive interest because of their excellent mechanical properties.However,most of the previous studies were focused on NWs with coherent twin boundaries?CTBs?that were perpendicular to the NWs'longitudinal axis.For those twin-structure NWs with CTBs parallel to their longitudinal direction,their strengthening effect and deformation mechanism is still unclear.In this paper,the mechanical properties and deformation mechanisms of face-centered-cubic?FCC?Al,Ni,Cu,Ag,Pt and Au NWs with CTBs that are parallel to their longitudinal axis were investigated using MD simulation.The simulation results of metallic NWs with different twin thickness??6.3,?4.2,?2.8,?1.4,and 0.7 nm?stacking fault?SF?energies showed that both the strengthen and plasticity behaviors of these NWs can be significantly affected by the twin thickness.The main conclusions are below:?1?The systematical investigation showed that the strength of the NWs increase as the twin thickness decreases.For example,the yield strength of Al nanowire increased from?3.12 GPa?single-crystal?to?3.51 GPa?twin thickness?0.7 nm?.The yield strength of Ni increased from 10.47 GPa?single-crystal?to 13.05 GPa?CTBs thickness?0.7 nm?.The yield stress of Cu increased from?3.82 GPa?single-crystal?to 6.06 GPa?CTBs thickness?0.7 nm?.?2?We revealed that the deformation mechanism of the twin-structured NWs can be significantly affected by the twin thickness.As the twin thickness decreased into three atomic layers,the plastic deformation of Al and Ni NWs was transferred from dislocation behaviors to lattice distortion,that led to brittle-like necking.Meanwhile for Cu,Au,Pt and Ag NWs,their plasticity was transferred from dislocation behaviors to the lattice distortion accompanied by lattice rearrangement,which led to new grain formation in the twin-structure NWs.?3?We uncovered that the deformation mechanisms of these NWs were significantly affected by both the ratio between SF energy(?_sf)and unstable SF energy(?_usf),and the twin thickness.For the NWs with very high?_sf/?_usf values,the plastic deformation was controlled by extended/full dislocations with Burgers vectors of 1/2<011> that intersected with the CTB.However,for NWs with low?_sf/?_usfsf values,the plastic deformation was controlled by partial dislocations with Burgers vector of 1/6<112> that intersected with the CTB.?4?We discovered a new twin nucleation mechanisms,which the deformation twin nucleation through the extended dislocation overlap.With continued loading,these extended dislocations move to the free surface and erase the SFs,and then ultimately lead to the detwinning.?5?We constructed a new structured Al,Ni,Cu,Ag and Au NWs with quadruple-twin-structure.The MD simulation shows that these special structured NWs exhibit strong strain hardening.The plastic deformation was controlled by CTBs migration and atomic rearrangement at incoherent twin boundaries?ITBs?.