Effects Of Surface Nanocrystallization Induced By High Energy Shot Peening On Pure Copper And Its Alloy

Copper and its alloys are widely used in industry as metal material. However, copper alloys have lower strength, poor wear resistance. So its application is severely limited. The surface of metallic materials can form nanocrystalline by means of surface mechanical attrition treatment (SMAT) and this is one of popular research method about surface strengthening in recent years. The application of this technology in industry has a very broad application prospects. It combines the performance of nanocrystalline materials with traditional construction materials. Stacking fault energy(SFE) has a greater impact on deformation mechanism of face centered cubic metals. Lower stacking fault energy will inhibit dislocation glide and as a result the form of deformation is twin. High strain rate and temperature also can cause Twin plastic deformation beside SFE. The stacking fault energy of Cu is 78mJ/m2, and the stacking fault energy has a great change after adding alloying elements.In this paper, pure copper, Cu-2wt%Ti alloys and Cu-10wt%Ni alloys have been treated with different time of the surface mechanical attrition treatment (SMAT). The impact of different alloying elements dissolved in the copper alloy on stacking fault energy has been researched and the impact of different Stacking fault energy on deformation mechanism has been researched too. The difference between the surface mechanical attrition treatment process of aging Cu-2wt%Ti alloy and deformation mechanism of not aging Cu-2wt%Ti alloy has been studied. And we have researched the harden caused by dropping of stacking fault probability. And material wear resistance is improved. The hardness of different materials after surface nanocrystallization from surface to core has been measured by the microhardness tester. The materials was characterized by OM, XRD, SEM and so on. Cu-2wt%Ti after surface mechanical attrition treatment was analyzed after ageing. the pure copper samples after surface nanocrystallization was annealing and Thermal stability of nanocrystalline was studied.The results show that:(1) the surface grain of pure copper was refined after surface nanocrystallization, but obvious deformation twins was not been found. the main deformation way was dislocation slipping. After surface nanocrystallization, along with the extending of time for surface nanocrystallization, the number of deformation twins increased obviously in Cu-2wt%Ti alloy. The intersecting of deformation twins was not obvious and the main deformation way was dislocation slipping. After dislocation pile-up, the main deformation way was twinning, but twinning can change phase difference and after this the main deformation way was dislocation slipping.(2) Both Cu-2wt%Ti nano-samples in solid solution state and the samples treated by solution and aging, a mass of deformation twin crystal and crossover twin crystal were observed as well as a phenomenon of layering. With the introduction of Ti, Stacking Fault Energy of Cu was brought down, which led to the twinning of crystal. Layering occured because big crystal particles tended to twin but small ones were easy to slip under the different plastic deformation mechanisms of them. Cu-2wt%Ti in the solid solution state possessed higher stacking fault energy afer the aging treatment, which can be certified by nano change at the same time, reduction of deformation twin crystal and drop of intersecting of deformation twins.(3) Pure copper, Cu-10wt%Ni alloys and Cu-2wt%Ti alloys occupied higher hardness in their surface than that in the heart atfer the surface nanocrystallization. Besides, their corrosion resistance is a bit lower than before. In contrast, they all have an increased abrasion performance. The solution strengthening which derived from soluteatoms solutioned in the Cu-10wt%Ni alloys, made the alloys more wear resistant than pure Cu. Other than the solution strengthening, the decreaseing of stacking fault energy in the Cu-2wt%Ti also made its wear resistant higher than pure Cu. As a result the fault strips widened and the twins can form easily and The twins and stacking fault both can block motion of dislocations(4) After the surface nanocrystallization for 45min and 60 min, respectively, the samples of Cu-2wt5Ti alloys went on to be aged in temperature of 400℃which is similar to its recrystallization temperature or within the range of its recrystallization temperature. Therefore, the aging procedure was carried out in the meanwhile of recrystallization and high Ti concentration in the surface was due to the spinodal decomposition, which occurred in the surface layer wihtin a certain range.(5) After the nanorization of pure copper sheet for 30min and 60 min, respectively, at the temperature of 450℃, its surface indeed had a tendency of growth although only in the nanometer level which can be found by XRD. In addition, the bigger microscopic strain made the steel more stable, and also restrained the growth of the nanocrystal in it.