DEFORMATION MECHANISM AND THERMALSTABILITY OF NANOCRYSTALLINE PURE METASL

纳米金属是一种晶粒尺寸在10nm左右的多晶材料，由于其在极小的晶粒中具有新特性而受到研究人员的关注。本文采用日本发展起来的气相沉积法制备了不同粒径的纳米金样品。对这些试样在不同温度下退火后，通过x射线衍射测量内部应变和晶粒尺寸，STM观察，弯曲试验测量屈服应变等。结果表明，初始晶粒尺寸为~ 10nm且择优取向较强的样品在800K下退火1h后，具有很高的热稳定性，几乎没有晶粒长大。另一方面，弱择优取向的试样表现出相对较低的热稳定性，在570K退火后已经显示出一定的晶粒长大。晶粒尺寸越大，热稳定性越差。弯曲试验的结果再次表明，在晶粒生长开始之前，弹性极限一直保持在极高水平。热稳定性不仅在实际应用中很重要，这种异常高的热稳定性从基础的角度来看也是值得关注的。我们提出了这种高热稳定性的起源是晶粒尺寸的狭窄分布和晶界存在许多含氦气的孔隙。为了证明这一观点，本研究扩展到纳米铜，因为从熔体的蒸汽压来看，铜似乎比金更合适。这项研究仍在进行中。目前得到的结果如下：对于初始晶粒尺寸为20 ~ 100nm的纳米晶铜，除了（III）平行于膜面优选取向的样品（在金中发现）外，还出现了（100）织构的样品（在金中没有发现）。与金相比，迄今获得的所有样品都是热不稳定的。与压实法制备的样品相似，在500K退火后，它们显示出相当大的晶粒生长。我们目前还没有成功地找到条件来制备具有类似金的高热稳定性的纳米铜。少

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