DEFORMATION MECHANISM AND THERMALSTABILITY OF NANOCRYSTALLINE PURE METASL

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Nanocrystalline metals are polycrystals with grain sizes of the order of 10nm and draw researchers' attention as new materials because of their novel properties expected from extremely small grains. In the present work, nanocrystalline gold specimens with various grain sizes were prepared by the Gas Deposition Method which was developed in Japan. On these specimens, after annealing at various temperatures, measurements of internal strains and grain sizes by x-ray diffraction, STM observations, measurements of yield strain by bending test, etc. were performed. It was found that specimens with initial grain size of 〜10nm and strong preferred orientation had a very high thermal stability and showed almost no grain growth after annealing at 800K for 1h. On the other hand, specimens with weak preferred orientation showed relatively low thermal stability, showing some grain growth already after annealing at 570K. Also, specimens with larger grain size tended to have lower thermal stability. … More Results of bending tests indicated again that an extremely high elasticity limit was kept until a beginning of a grain growth. The thermal stability is not only important in practical applications, such abnormally high thermal stability is of interest from a fundamental view point. We have proposed that origins of this high thermal stability are a narrow distribution in grain size and presence of many pores containing helium gas in grain boundaries.To testify this idea, present study was extended to nanocrystalline copper, since it seemed that copper was more suitable than gold in view of vapor pressure of melt. The study is still in progress. The results obtained at present are as follows: For nanocrystalline copper with initial grain sizes of 20〜100nm, other than specimens with preferred orientation of (III) parallel to film plane (which was found for gold), specimens with (100) texture (which was not found for gold) appeared. All the specimens so far obtained were thermally unstable compared to gold. Similar as specimens prepared by the Compaction Method they showed considerable grain growth after annealing at 500K. We are not successful yet at present in finding conditions to prepare nanocrystalline copper with high thermal stability similar as gold. Less