실시간 투과전자현미경 변형 실험을 통한 서브마이크론 FCC 단결정의 전위 소성 변형 거동 연구

Abstract

For the last two decades, the experimental observation of size effects has attracted tremendous amounts of interests, reported in the mechanical strength of thin metal films and small samples, such as single-crystalline pillars, or whiskers. In order to applicate the outstanding strength and unique deformation behavior of nanomaterials, fundamental understanding of underlying physical origin is required. However, there has been only a limited number of studies, especially experimental observations, due to practical difficulties in characterization of a single nanostructures. The present study is focused on dislocation-mediated plasticity of face-centered cubic (fcc) single crystals at a submicron scale by utilizing in-situ transmission electron microscopy (TEM) deformation test, which is able to simultaneously apply mechanical stress to small volume and observe the changes in microstructure. Because of its simple and well known dislocation structures and reactions, fcc crystals are chosen as a model system for the study. First, onset of plasticity in a dislocation-free crystals was investigated. Nucleation processes of the first dislocation remain unclear despite lots of attention because of its important role in following plasticity. In this study, detailed dislocation activity during the initial deformation of dislocation-free Au [110] nanowire was experimentally observed in a first time. Strain gradient deformation which accommodates the stress localization by emission of geometrically necessary dislocations (GNDs) was occurred by nucleation of several prismatic dislocation loops through successive cross slip of glide loops. Atomic scale nucleation process of prismatic dislocation loops was investigated by molecular dynamics (MD) simulation showing that formation of sessile dislocations during cross slip plays an important role of the process. Those prismatic dislocation loops don't attributed to strain hardening though a high density of typical GNDs formed during nanoindentation increases the hardness of thin film by formation of obstacles for dislocation glide motion. After initial deformation by prismatic dislocation loops, deformation behavior under various uniaxial stress condition was studied to analyze effects of loading conditions. It is observed that deformation mode of Au nanowires was altered by changing loading directions from compression to tension. Due to difference in the magnitude of Schmid factors acting on partial and full dislocations, the nanowires deformed by slip under compressive stress, and by deformation twinning under tensile stress. Furthermore, by performing cyclic loading, reversible plastic deformation by twinning and consecutive detwinning was observed in tension and compression, respectively. This reversible twinning-detwinning process accommodates large strains, around 40 %, that can be beneficially utilized in applications requiring high ductility in addition to ultra-high strength. Finally, effects of internal defects structures that may induced during specimen preparation was investigated. Recently developed and widely used specimen fabrication technique for nanomaterials, focused ion beam (FIB), inevitably generates surface defects, such as interstitials or their aggregates dislocation loops. In-situ TEM compression of nanopillars revealed that the initial plastic deformation and subsequent plastic flow are significantly altered by the presence of FIB-induced dislocation loops, as they actively respond to the applied stress through glide and expansion of the loops. With further straining, the dislocation loops escaped the pillar, leaving slip steps, which are potential dislocation nucleation sites, at the pillar surface and/or dislocation debris via the interaction with other mobile dislocations. Furthermore, the effects of a single high angle grain boundary vertically inserted in a nanopillars were studied. The boundary reduced the length of single-armed dislocation sources increasing flow stress of the bicrystal pillars. The present direct TEM observations contribute to the understanding of the roles of internal defect structures in the deformation behaviors of submicron pillars. To summarize the plastic deformation of submicron-sized fcc single crystals were studied by combination with in-situ TEM deformation tests and MD simulations. The present in-depth analysis on dislocation nucleation process and their effects on plastic deformation provides fundamental understanding for a wide range of applications of nanomaterials.