Surface Nanocrystallization And Characterization Of Metal

Nanostructure layer was produced on the surface of metallic material by surface nanocrystallization method. It is worthy to be studied because of improved surface characteristic of materials obtained by this method.In this thesis, Surface Mechanical Attrition Test (SMAT) was used to get deformed layer on three types of material, including stainless steel304L,316L and2205. The microstructure features were systematically characterized by optical microscopy, X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The microhardness was examined by Vicker’s hardness tester and nanoindentation. Small punch test (SPT) was carried out to measure strength of304L and316L.The nanocrystallization mechanism were clarified.The results showed that the microstructure with grain size varying at a range of nanoscale to microscale was obtained along the depth of surface layer of304L and316L.The size of304L nanocrystalline was about80nm, and the size of316L nanocrystallined was90nm. In addition, the size of the grain decreased gradually from center to the surface. However, there was no nanocrystal grain observed in the structure of2205. The microhardness of nanostructured surface layer was enhanced significantly. On the top surface nanostructured layer,the microhardness reached450HV,which was about three times than that of the coarse grained matrix.The nanocrystallization mechanism was interpreted through TEM on different depth of the sample. The nanocrystalline formed in304L is mainly due to twin intersections, while in316L is mainly due to dislocation movement and mechanical twinning. The mechanical properties of the nanocrystalline layer were able to be measured by SPT. The results of SPT also indicated that the strength of the material processed was improved.