Research On Twin Boundary Structure And Its Mechanical Response In Nanoporous Gold

Nanotwinned metals have ultra-high strength and ductility,mainly due to the specificity of the twin grain boundaries.Also surface effects are a key factor affecting the properties of nanometallic materials.The current study is missing the effect of surface effect on twin boundaries.Therefore,in this thesis,we investigated twin boundaries at different locations in nanoporous gold by in situ transmission electron microscopy with high spatial and temporal resolution.The effect of surface on twin boundaries and the mechanical response of twin boundaries during ligament deformation are obtained,adding insight into twin boundaries in eccentric cubic metals.In this thesis,the ligament deformation processes of incoherent twin boundaries and nanoporous gold ligaments that are at different locations of nanoporous gold ligaments were investigated by objective spherical aberration-corrected transmission electron microscopy and in situ high-resolution transmission electron microscopy as follows:(1)The study of the atomic structure of the incoherent twin boundaries at different positions of the ligament yields that the deflection of the incoherent twin boundaries in the direction perpendicular to the eutectic twin boundary is influenced by the free surface in addition to the dislocations and coherent twin boundaries.For the incoherent twin boundary,the displacement in the direction perpendicular to the coherent twin boundary represents its phase transition.The free surface limits the phase transition of the incoherent twin boundary in this direction.(2)In situ electron microscopy study of the deformation of nanoporous gold ligaments revealed that the nanoporous gold prepared by the de-alloying method has high internal stresses.When the high-energy electron beam is irradiated to the nanoporous gold ligament,it will produce the release of internal stresses leading to the fracture of the ligament.During the fracture of the ligament,the nucleation and slip of incomplete dislocations lead to the generation of twin crystals and the migration of twin boundaries.The presence of twin crystals improves the stability of the ligament during plastic deformation.When the ligament has incoherent twin boundaries at the surface,the plastic deformation behavior of the ligament occurs preferentially at the incoherent twin boundaries,and the deformation process is accompanied by the migration of the incoherent twin boundaries toward the direction of detwinning.