| Exploring the effect of defect behavior on the deformation process of metallic materials is crucial to our deep understanding of the deformation mechanism of metals,and is of great significance for the design,improvement,and fabrication of engineering devices with excellent mechanical properties.The plastic deformation of face-centered cubic(FCC)metals with low stacking fault energy(SFE)is dominant by the nucleation of partial dislocations or twinning,while the deformation behaviors regarding the high SFE metals is still lacking.In this paper,with the advanced in situ nanomechanical testing system inside electron microscope,we present a systematic and in-depth research on the deformation behaviors in FCC-structured nickel(Ni)nanowires(NWs)with high SFE,including the orientation-dependent dislocation activities in single-crystal NWs,dislocation evolution and dislocation-twin boundary interaction mechanism,reversible structural evolution of grain boundaries(GBs)subjected to tension-compression cycles,etc.The specific results are summarized as follows:(1)In situ welding technology and high-resolution transmission electron microscopy(HRTEM)were used to study the deformation of FCC metallic nanostructures NWs with different orientations.The orientation-dependent dislocation and twinning-mediated plastic deformation in single-crystal Ni NWs can be well understood based on the Schmidt’s law and the geometric characteristics of dislocation motion.(2)We fabricated nano-twinned NWs with different twin orientation angles(θ)and twin boundary spacings(λ).We present a systematic experimental investigation of the ductility and underlying deformation modes in nano-twinned(nt)Ni NWs as a function of θ and λ.Compared with the corresponding single-crystal NWs,the nt NWs with θ ～0° were found to exhibit limited ductility,and twin boundaries(TBs)can act as effective barriers for dislocation propagation.In contrast,in the NWs with θ～20° and 55°,TB migration/detwinning caused by dislocation-TB interactions and partial dislocation motions dominate the deformation,which contributes to enhance the ductility of NWs.Regarding the NWs with θ ～90°,it is found that dislocations can continue to propagate across TBs,which has limited influence on NW stretchability.(3)The effect of λ on the plasticity of nt NWs with different θ is proposed.For the NWs with θ ～0°,20° and 90°,λ has little effect on the plastic ductility,because TBs could both act as obstacles for dislocations propagation and resources for dislocations nucleation.But for NWs with θ ～55°,reducing λ makes it easier for twinning partials on planes parallel to TB to nucleate,thus promoting the detwining process and improving the NW ductility.(4)NWs with special geometric shapes and twin structure were fabricated.The reversible TB slip is observed under the tension-compression cycles,which is induced by the propagation of full and partial dislocations.Besides,the deformation behavior of low-angle grain boundaries(LAGB)under shear stress was studied.It was observed that the plastic deformation was dominated by the reversible migration of GBs.Based on in situ observations at atomic scale,it was found that GBs are composed of full dislocations.The full dislocations dissociated into an extended dislocation under shear stress and the extended dislocations slip orderly along the slip plane,which dominants the reversible migration of GBs.The work of this paper enriches and develops the theory of plastic deformation in nano-structured FCC metal NWs with high SFE,which facilitates the developing and designing of nanodevices with improved ductility. |
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