Investigation On Surface Nanocrystallization And Diffusion Behavior Of Copper Plate

Nanostructured metallic materials have been the subject of considerable research due to their unique microstructures and appealing mechanical properties.In recent years,surface mechanical attrition treatment(SMAT) which is used to fabricate bulk nanostructured materials(NSM),cannot only enable the fabrication of a porosity-free and contamination-free nanocrystalline surface layer which is induced by the severe plastic deformation at very high rates,but also avoid bonding interface between the nanostructured surface layer and matrix,has attracted the growing interest of specialists in materials science.In this paper,the nanostructured surface layer was obtained on pure copper plates by using SMAT.Microstructure features of various sections in the surface layer and properties of the SMAT samples were systematically characterized by using optical microscope(OM),X-ray diffraction(XRD), transmission electron microscope(TEM),microhardness testing machine.In order to investigate the effect of nanocrystallite on atomic diffusion kinetics, the copper samples surface was cleaned and electroplated with a layer of pure nickel and Scanning electron microscope(SEM)is used to analyze diffusion behavior of Ni atoms in the pure copper.In addition,Titanium(Ti) ions were implanted into the surface layers of the copper samples.Ti distribution is measured by making use of Auger electron spectroscopy (AES).The main conclusions can be drawn as follows:1.Equiaxed nanocrystallites with random crystallographic orientations are formed on the top surface layer of the copper plates,and the average grain size is about 10nm on the top of surface.Appropriate SMAT duration of copper plates is 30min.Grain size of the sample could not be reduced infinitely with the treatment duration increased.2.During SMAT,severe plastic deformation occurs randomly on the surface layer,and decreases gradually along the depth of the treated samples, which results in the gradient variation of the microstructures.With the depth increasing,the size of grains increases depending on the gradient variation of microstructures.3.After the SMAT,the micro-hardness of the surface layer in the copper samples is evidently enhanced,and is twice more than that of the original sample.The hardness decreases gradually with the increase in depth. The increased micro-hardness on the surface layer of nanocrystalline copper may be attributed to the refinement of grains and work-hardening.However, to certain extent,the polarization curves show that the corrosion properties of SMATed copper plates are downgraded.The main reason was that numerous grain boundaries and enhanced grain boundary energies by means of SMAT led to intensified reactivity of the atoms.Moreover,much stress existing on the surface of the sample induced stress corrosion for SMATed sample.This was also responsible for worsen corrosion.Besides,higher roughness of the samples via the SMAT was another reason.4.The nanostructured grains still maintain without obvious grain growth coarsening at 300℃.The diffusion phenomenon of Ni atoms in the SMAT copper can be obviously observed at 100℃after diffusion annealing, otherwise there is hardly diffusion of Ni atoms at 100℃in its coarse-grained counterpart.It concluded that the increase of diffusion rate was attributed to lower diffusion activation energy and higher coefficient which was induced by the non-equilibrium defects in nanocrystalline(such as high density dislocations and dislocation cells),especially,a large number of grain boundaries.5.The distributions of Ti concentration after ion implantation in the coarse-grained and SMAT copper plates agree with the Gaussian distribution, and the implantation depths of both are not different.However,compared with that in the coarse-grained sample,Ti concentration in the SMAT sample is enhanced.Enhanced Ti concentration was attributed to the interaction process of the implantated atoms and the non-equilibrium defects(such as high density dislocations and dislocation cells)in nanocrystalline induced by SMAT.In addition,this phenomenon may be explained by an increased volume fraction of grain boundary and an enhanced reactivity of surface atoms due to the nanostructured layer generated by the SMAT.