In Situ TEM Study Of Deformation Behavior In Body-Centered Cubic Metallic Nanowires

Body-centered cubic(BCC)metallic nanowires possess unique properties,including ultra-high mechanical strength,superior high temperature performance and excellent anti-irradiation property,making them potential candidates as a fundamental building block of nanodevices used in harsh circumstances.However,the mechanical behavior of BCC metals usually demonstrates more complicated characteristics at microscale,which were usually attributed to the intrinsic crystallographic symmetry of BCC lattice and the nonplanar structure of its lattice defects.Besides,when the feature size of BCC crystals descends to tens of nanometers,the consequentially noticeable volume and surface effects would influence the plastic deformation of the nano-sized BCC metal significantly.Recently,great efforts,in both experiments and simulations,have been made to reveal the deformation mechanisms in nanosized BCC metals.However,much less is known about the mechanical behavior of nanosized BCC metals,in stark contrasts to the comprehensive understanding of deformation mechanisms in their face-centered cubic(FCC)counterparts.In this dissertation,the plastic deformations of the nanosized BCC meals were systematically investigated from atomic scale by conducting in situ mechanical testing of as-fabricated niobium(Nb)and tungsten(W)nanowires inside a transmission electron microscopy(TEM).The implications of phase transformation,twinning and dislocation activities on the mechanical behavior of BCC metallic nanowires were discussed from different aspects.The main findings of this dissertation include:1.Consecutively-occurred multiple reorientations and resultant superplastic deformation have been discovered in Nb nanowires.With the assistance of high-resolution TEM(HRTEM)technology,a superplastic deformation of Nb nanowires and its underlying mechanism was directly visualized by tracking the dynamic evolution of lattice structure during tensile loading.Upon tension,the deformation-induced phase transformation and resultant crystallographic reorientation were realized by a two-step process of[100]BCC-[110]FC-[111]BCC;while the crystal rotation induced by deformation twinning and dislocation slip revolved the Nb nanowire around the[111]axis with different angles.Given that the change of nanowire orientation may alter the deformation geometry and thereby stimulate other deformation modes in the subsequent deformation,the superplastic deformation of Nb nanowires originates from a synergy of consecutively-nucleated multiple reorientation processes that occur for more than five times via three distinct mechanisms,i.e.stress-activated phase transformation,deformation twinning and slip-induced crystal rotation,in which a remarkable elongation of more than 269%is achieved before fracture.Based on the atomistic observations and statistic measurements,our findings not only thoroughly analyzed the dynamics of phase transformation,deformation tuwinning and dislocation slip that sequentially-happened in Nb nanowires,but also,for the first time,revealed a superior mechanical property of BCC Nb nanowires through the close coordination of multiple deformation modes during the continuous change of orientations,rendering a novel initiative to optimize the mechanical properties of the BCC metallic nanowires2.The anti-twinning deformation behavior and its size dependence of BCC metallic nanowires were revealed.Using the in situ fabricated W and Nb nanowires as model systems,the anti-twinning deformation of nanosized BCC metals was discovered for the first time,via detail analysis of defect dynamics under different deformation modes and loading orientations.In BCC crystals,twinning usually results from sequentially positive shears along[111]axis,which took place on the neighboring{112}twin planes.In contrast to the ordinary {112}<111>twinning,the anti-twinning involves a reverse<111>twin shear that experiences a substantially higher resistance.Therefore,the anti-twinning deformation in BCC had been deemed impossible previously.However,our in situ nanomechanical tests revealed that the anti-twinning can be activated in the tensile deformation of the[110]-W,[111]-W and[121]-Nb nanowires respectively,which clearly demonstrated the existence of anti-twinning,like the dislocation and ordinary twinning,acting as one of the underlying plastic carriers in BCC metallic nanowires Moreover,the anti-twinning in BCC W nanowires showed a strong size-dependence.where the anti-twinning gradually transformed to the ordinary dislocation slip as the nanowire size increased from 20 nm to 45 nm.These discoveries not only advance our current understanding of the size-dependent abnormal plastic behavior in nanosized BCC metals,but also stimulate the reconsideration of the conventional twinning-anti-twinning asymmetry model of BCC structures.3.The effects of loading mode on the deformation mechanism of W nanowires was unveiled.Investigations into the plastic deformations of BCC metallic nanowires under complicated stress state would penetrate the implications that how loading geometries affect plastic deformation mechanism.To do so,the plastic deformation of<112>-W nanowires was conducted under both uniaxial and non-uniaxial stress,by controlling the angle between the loading direction and nanowire axis precisely.Under uniaxial tension and compression,the dislocation slip acted as the dominated plastic mode of<112>-W nanowires.However,under the combined tension and bending loading,the plasticity of W nanowire transferred from the ordinary dislocation to the deformation twinning,where a deformation twin band nucleated and then thickened gradually.Furthermore,by calculating the maximum geometric factors of slip and twinning systems,it has been confirmed that the distribution of critical resolved shear stress played a key role in the dislocation-to-twinning transition under non-uniaxial loading modes with different misalignment angles.Meanwhile,the interfaces of bending-induced twins are composed of numerous stepwise {112} twin boundaries,the projection of which shows as an overall interface on{110}plane when viewed along<111>zone axis.This observation provides a new experimental reference for the accurate analysis of deformation twin structure in BCC crystals.In conclusion,in situ nanomechanical tests have uncovered some unique defect dynamics in BCC metallic nanowires,in which four distinct plastic deformation mechanisms,including BCC-FCC-BCC phase transformation,dislocation slip,twinning and anti-twinning,have been identified and discussed from atomic scale.Based on experimental observation,the critical factors governing the plastic deformations of BCC nanowires,such as loading modes,loading orientations and crystal sizes,have been systematically discussed,which not only advances our current understanding about the mechanical behavior of nanosized BCC metals,but also provides a new initiative for further studies of nanosized BCC metals.