In Situ Strain Apparatus Of Metallic Nano-Materials And Its Research On Deformation Behaviours Atatomic Scale

This thesis aims to investigate the correlations of atomic scale microstructureand the physical properties of low dimensional, small scale ordered and disorderedstructures by the in-lab developed atomic resolution in situ transmission electronmicroscopy (TEM) techniques for controllable mechanical testing platform.The experimental method and device: An invention relates to an apparatus andmethod for measuring mechanical properties of nano-materials by in situ highresolution TEM was developed. More specifically, this invention pertains to a TEMsample support or grid that allows the dynamic and real-time measure of themechanical-microstructural relationships of individual nanostructure under stressfield by in situ TEM at atomic lattice resolution level.Using these techniques together with advanced spectroscopy and atomic scaleelectron microscopy, studying the novel properties of the periodical andnon-periodical condensed matter in the confined system is one of the key issues ofthe important branch, Metallic Physics of Solid Physics. The achievements are asfollows:Single-crystal Au system: For the first time, we report an exceedingly largeplasticity of~150%uniform elongation and231%total elongation of a singlecrystalline Au nanowire (with an initial diameter of~55nm) by in situ transmissionelectron microscopy tensile experiment. The super-plasticity attributed to thecontinuous nucleation, moving and escaping of bubble-like stacking fault ribbons(SFR). These SFR are highly efficient plasticity mechanism which carried out110%of the plastic deformation in the early stage. The SFR can penetrate each otherwithout forming sessile-locks and leading to strain hardening. Quantitativeevaluation of dislocation density during each strain stage indicates that thedeformation of Au nanowire is sensitive to strain-rate. When the uniform elongationexceeded150%and the diameter of the nanowire approached33nm, thedeformation mode switched to be nucleating twins across the nanowire andstress-concentration emerged and caused necking and quick fracture of the nanowire.Quantitative plastic contribution of kinds of dislocation bubbles has also been givenduring each strain stage.Nanocrystalline Au system:(1) In situ atomic-scale observations and directlycharacterization the atomic-scale plastic deformation mechanisms and fractureprocess of nano-crystalline gold thin films. Grain rotation accompanying the tensile fracture processes is one of the plastic deformation mechanisms when the grain sizeis in the20nm range. Grain rotation in polycrystalline materials is caused bycrossover plastic deformation mechanisms after the nanocrystalline metals reachpeak strength. Cooperative grain rotation was revealed as a necessaryaccommodation route that mediates individual grain rotations. Grain boundarydisclination nucleation for accommodating inter-grain rotation plasticity was directlydynamically observed in situ. The GB dislocation/disclination-mediated grainrotation process could possibly increase the strength and ductility of nanocrystallinemetals.(2) The strain-induced deformation-twinning processes through incoherent twinboundary (ITB) migration were revealed with the dynamic and atomic scale directobservation in the polycrystalline Au nanofilms. With the dynamic and atomic scaledirect observation, the quantitative strain maps of the ITB region were consecutivelyevaluated. Using these strain maps, ITB propagation assisted twinning process canbe accomplished by contraction and relaxation of9R phase region. The compressivestrain field dynamically evolves as the propagation of the deformed-ITB in which, astrong compressive strain filed can be as high as5.8%, corresponding to a resolvedshear stress of0.584GPa. In this condition, the cooperative glide of triple Shockleypartial dislocations (bA, bB, bC) can be actuated. The novel atoms screw-rotationinduced twinning mechanism (SRTP) is proposed by in situ observation and atomicstructural model. These results present new insights into stress-induced deformationtwinning mechanisms in face centered cubic metals to release local stressconcentration inside the shear confined to9R phase region, causing macro strain of0.707.Furthermore, the mechanical properties and atomic mechanism on elastic strainlimit of metallic glass film. The10%super elastic strain limit was revealed in binaryNiNb amorphous nano-pillars ranging from tens to hundreds of nanometers by insitu TEM observations. The rearrangement of loosely bonded atomic clusters (oratoms) and the expansion of interatomic distance upon elastic deformation areresponsible for the super elastic deformation.