Effects Of Surface Mechanical Attrition Treatment On Cu-Ti Alloy

Recent years have witnessed an explosion of interest in the nano-structured materials which have the unique micro-structure and excellent mechanical properties. Surface Mechanical Attrition Treatment (SMAT) can not only be prepared no pollution, no defects and ideal nanostructure layer on the materials surface, but also to avoid the binding interface defects between the solid nano-deformation layer and the substrate. Cu-Ti alloy is a typical age-hardening alloy. The aging temperature of Cu-Ti alloy is higher than that of the other copper-based alloys. And the microstructure of Cu-Ti alloy is relatively stable after aging treatment, so they are able to service at a high temperature. In addition, Cu-Ti alloy is well used in the electronics industry as well as musical instruments, accessories and other fields because it has higher thermal and electrical conductivity. But its broader application is limited by its low intensity and relatively poor wear and corrosion resistance. The purpose of this paper is to optimize and strengthen the surface of typical age-hardening Cu-Ti alloy by SMAT, and to explore the mechanism of nano-structured deformation as well.In this paper, Cu-2wt.%Ti and Cu-4wt.%Ti alloy is treated by solid solution, quenching and surface mechanical attrition for45min and60min,respectively. Then the tests of microstructure, X-ray diffraction spectra, micro-hardness, corrosion performance, impedance, friction and wear performance are characterized to explore the changes in the nano-deformation layer of Cu-2wt.%Ti and Cu-4wt.%Ti alloy. And the aging treatment is carried after the treatment above. The aging time differs from30min to240min. Then, the microstructure, micro-hardness, X-ray diffraction are characterized to investigate its aging behavior.Results for this study:(1) By means of Surface Mechanical Attrition Treatment, the deformation layer with hundreds microns thick is formed in the surface of Cu-2wt.%Ti and Cu-4wt.%Ti alloy. The nano-sized deformation layer of SMAT60min is thicker than that of SMAT45min. The interlocking twins plays the main role in the deformation layer, which divided the substrate into small triangles and quadrilaterals. By means of SMAT for60min and45min, the surface grain size Cu-4wt.%Ti alloy reached17.12nm and40.72nm, respectively.(2) The stratification layer of Cu-2wt.%Ti and Cu-4wt.%Ti alloy after SMAT is obvious, differs a significant way from the gradient microstructure reported in previous work. The thickness of the total stratification layer generated by SMAT for45min and60min in Cu-4wt.%Ti alloy plates can be identified to be about50μm. This discrepancy has been attributed to the difference of deformation mechanism. Cu-2wt.%Ti and Cu-4wt.%Ti alloy belong to the lower stacking fault energy metals, at the beginning, twins play the main role in the process of deformation, but when the surface grain size changed to nano-scale, dislocation gliding will play the main role in the process of deformation.(3) By means of SMAT for45min and60min, the micro-hardness of Cu-2wt.%Ti reached to200MPa and220MPa, it increased by60%and76%compared with the matrix. By means of SMAT for45min and60min, the micro-hardness of Cu-4wt.%Ti reached to230MPa and245MPa, it increased by64%and75%compared with the substrate. The nano-structured surface layer plays a significant role in the strengthening of Cu-Ti alloy.(4) By means of SMAT, the resistance Rct of Cu-4wt.%Ti alloy decreased, the inductor and inductor resistance played reversely. The current of Cu-2wt.%Ti and Cu-4wt.%Ti alloy after SMAT increased in the solution of0.05mol/L H2SO4and0.1mol/L CuSO4. The corrosion resistance of Cu-Ti alloy is deteriorated by SMAT.(5)The friction co-efficients in both Cu-2wt.%Ti and Cu-4wt.%Ti samples decrease obviously in the initial period and tend to steady-state values with the increasing of loading. Under a load30N, wear resistance of the SMAT Cu-2wt.%Ti is obviously enhanced over that of the untreated sample. There is almost no difference of wear resistance between the two kinds of samples when the load is exceeded over60N. Under a load50N, wear resistance of the SMAT Cu-4wt.%Ti is obviously enhanced over that of the untreated sample. There is almost no difference of wear resistance between the two kinds of samples when the load is exceeded over50N. The wear resistance of both the SMAT Cu-2wt.%Ti and Cu-4wt.%Ti alloys is obviously enhanced over that of the untreated samples.(6) The metallographic microstructure of the SMAT Cu-2wt.%Ti and Cu-4wt.%Ti alloys is obviously changed after aging for different time at400℃.The microstructure of the sample aging for180min had the most significant change, the grain shows polygon, twin interlocking shows increasing, the etching deepened. After aging240min, the grains were obviously refined, there were new twins in the inner grain, and there were new grain growing in the boundaries of large-angle grain, which may occur during the overlap of aging and re-crystallization.(7) The micro-hardness of SMAT Cu-2wt.%Ti and Cu-4wt.%Ti alloys after aging at400℃decreased firstly and then increased to the peak, and finally tends to a steady trend. The reasons for the decline of the surface hardness may attribute to the growing of the surface grains after aging treatment, the peak appears just in the boundary layer, where maybe related to the high-density twins.