Investigation Of The Atomic-scale Deformation Mechanism Of Ultra-small Sized Ag Nanowires Through In Situ Transmission Electron Microscopy

Due to their small characteristic size and high specific surface area,nano-materials have different physical and chemical properties from macro-bulk materials.This makes the study of mechanical behavior of nanomaterials an important issue in nanomaterials science.At the same time,nanomaterials,especially silver nanowires,are widely used in integrated circuits,solar panels,flexible electronic devices and other electronic devices because of their excellent electrical conductivity,thermal conductivity,mechanical properties and catalytic activity.In 2019,folding screen mobile phone was officially released.Among them,silver nanowire composite flexible substrate has been applied to flexible touch screen,which achieves both sensitive and accurate touch control and high bending resistance.It can be seen that the nano-materials,as components of electronic devices,must be able to work under the external fields such as force,temperature change,vibration and so on.Moore’s law reveals the speed of information technology progress and points out the trend of highly integrated and miniaturized electronic devices,which challenges the size of nano-components in electronic devices.At the same time,the size of nanomaterials in devices will be smaller,even only a few nanometers,so it is an urgent requirement to study the plastic deformation behavior and mechanism of small-sized nanomaterials.Transmission electron microscopy(TEM)with high spatial and information resolution is an important technique to characterize the micro-atomic structure and elemental analysis of materials at atomic scale.However,due to the narrow sample space,it is a challenging task to study the structure evolution of nanomaterials under external force and to conduct in situ atomic scale in TEM.In this paper,the plastic deformation behavior of ultra-fine Ag nanowires was observed in situ at atomic scale by high resolution transmission electron microscopy(HRTEM)using the unique in-situ nanomechanical testing device of our institute.The evolution of atomic scale structure(full dislocation,partial dislocation,twinning,surface atom diffusion,rigid slip and phase transition)during deformation of ultrafine silver nanowires was systematically studied,and the deformation mechanism was carefully studied.The main contents and results of this paper are as follows:For ultra-fine Ag nanowires with diameter less than 10 nm,the plastic deformation is controlled by both incomplete dislocation slip and surface atom diffusion.The uniform extension of nanowires is achieved by stacking faults and twins formed by incomplete dislocation slip.The diffusion of surface atoms reduces the diameter of nanowires,but has no effect on the elongation of nanowires.We find that in plastic deformation of nanowires with small size,incomplete dislocations nucleate randomly on the surface and emit to form twins.In situ transmission electron microscopy(TEM)showed that L-C dislocation locks were formed by the reaction of incomplete dislocations with stacking faults.The unlocking mechanism of L-C dislocation locks cut under high stress was also observed.Surface atom diffusion is the main factor to decreasing the diameter of nanowires during tensile deformation of superfine Ag nanowires,and the stacking fault step promotes surface self-diffusion.It is found for the first time that rigid slip occurs when small-size nanowires are deformed.For Ag nanowires,when the diameter is less than 2.96 nm,rigid slip is more likely to occur.In addition,we observed the phase transition behavior in compression and tensile deformation of ultrafine Ag nanowires.The phase transformation process is realized by shear deformation on the {111} face-centered-cubic(FCC)plane.Compression deformation of the [001] oriented Ag nanowires results in bending and twisting of the root.During the bending process of nanowires,the triangular regions of compression and tension will be formed,and polycrystalline transition will take place,resulting in the formation of triple twins.It is found that there is a high stress zone near the matrix in the compression region,and the phase transition of FCC-BCT-FCC(T)occurs,which can be explained by the fact that the shear stress can not be released during the phase transformation process,thus forming an unstable body-centered tetragonal(BCT)phase.The compression region is far away from the matrix region,and the phase transition FCC-BCT-HCP-FCC(T)occurs.The unstable BCT phase produced by the phase transformation transforms into a stable HCP phase.