DEFORMATION MECHANISM AND THERMALSTABILITY OF NANOCRYSTALLINE PURE METASL

纳米金属是一种晶粒尺寸在10 nm量级的多晶体，由于其极细小的颗粒所具有的新奇性质，作为一种新材料引起了研究者的关注。本工作采用日本开发的气体沉积法制备了不同晶粒度的纳米金样品。对这些试件在不同温度下进行了退火处理，用X射线衍射法测量了内部应变和晶粒度，用STM观察了内部应变，用弯曲试验法测量了屈服应变等。结果表明，初始晶粒度为~10 nm、择优取向较强的样品具有很高的热稳定性，在800K热处理1h后，几乎没有晶粒长大。另一方面，具有较弱择优取向的样品热稳定性较低，在570K热处理后已有一定的晶粒长大。此外，颗粒尺寸较大的样品热稳定性往往较低。…更多的弯曲试验结果再次表明，在晶粒开始长大之前，保持了极高的弹性极限。热稳定性不仅在实际应用中很重要，而且从根本上讲，这种反常的高热稳定性是很有意义的。我们提出了这种高热稳定性的原因是晶粒度分布较窄以及晶界存在许多含有氦气的气孔。为了验证这一观点，本研究扩展到纳米晶铜，因为从熔体的蒸汽压来看，铜似乎比金更适合。这项研究仍在进行中。结果表明：对于初始晶粒度为20~100 nm的纳米铜，除了(III)择优取向平行于膜面的样品(金)外，还出现了(100)织构的样品(金没有)。到目前为止，所有获得的样品与金相比，都是热不稳定的。与用压实法制备的样品类似，在500K下热处理后，它们都显示出相当大的晶粒长大。目前，我们还没有找到条件来制备类似于金的热稳定性很高的纳米晶铜。较少

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