Experimental Study Of The Microstructural Evolution Of Electrodeposite Nano-twin Nickel

Nanotwinned metals material has the mechanical properties such as high strength,good ductility, physical properties of both good electrical conductivity and corrosionresistance, have great development potential and broad application prospect in theimprovement all aspects of performance of traditional engineering materials anddeveloping new structure materials. Recent years, the technology of nanotwin hasbecome a hot research topic in the field of frontier science. In this thesis, thenanocrystalline Ni thin films with high density nano-scale growth twins weresynthesized by direct electrodeposition technology. The comprehensivecharacterization of microstructure of electrodeposition nanotwinned Ni were analyzedusing X-ray diffraction, scanning electron microscopy, transmission electronmicroscopy; discussed the microstructure evolution after deformation and theinteractions between twin and defects; finally, the thermal stability of nanotwin wereanalyzed, main results are listed as follows:1) The five-fold twinning structure in electrodeposited nano-twin Ni wassystematically investigated by TEM. The remarkable diffraction pattern and HRTEMimages obtained from the cross-section observation demonstrate directly that theelectrodeposited nano-twin Ni has five-fold twinning structure with five {11l}twinned subcrystals and systematically analyzed the7.35°intrinsic structural gap. Inthis work, the7.35°gap was at least inset in two twin boundaries of the five-fold twin,the twin boundaries which share the7.35°gap always broaden and was decomposedinto other twins, so that, the grain present a irregular shape. cross-sectional TEMmicrograph revealed that the electrodeposited nano-twin Ni had columnar grainstructure with a strong {110} texture. By means of comprehensive structurecharacterization, a new space structural model of the five-fold twin was proposed.2) The structure of the Σ3{112}incoherent twin boundaries has been investigatedusing high resolution transmission electron microscopy. The Σ3{112}ITBs has arepeatable pattern involving units of three {111} atomic planes. ITBs can be createdby the glide of a set of partial dislocations with a repeatable sequence b2:b1:b3onevery {111}plane in half of a perfect crystal, b1is equal to1/6[11-2], a pure edgepartial dislocation, b2and b3are equal to1/6[-211] and1/6[1-21],respectively, both ofwhich are mixed partial dislocations with opposite sign of screw components. ITBs can be dissociated into two phase boundaries bounding a9R phase. Generally, the9Rphase was very thin because of the high stacking fault energy of Ni. The migration ofΣ3{112}ITBs led to detwinning, three or three integer times{111} atomic layerscollectively move in the migration process.3) discussed the microstructure evolution after deformation and the interactionsbetween twin and defects; dislocations can glide along twin boundary, whenencounter the Frank steps, can form the incoherent twin stairs, these twin steps can befurther slip; Dislocations can also through the twin boundary made the twin boundarycurve. Finally analyses the thermal stability of the twin, annealing treatment at400℃and higher temperatures, the grains growing up significantly and the twin densitydramatically reduce.