Intrinsic Mechanical Properties of Nanocrystalline Metals

此次授予约翰霍普金斯大学的续奖将支持马教授研究通过机械研磨工艺制备的新型块体纳米晶材料。动态再结晶的纳米晶表面层或原位固结的样品将接近于“理想”的纳米晶变形行为的机制研究。该奖项的重点将是在不同应变率（包括应力松弛和跳跃测试）和不同温度下对这些纳米晶材料进行机械性能测试。人们对揭示纳米晶材料的新型力学行为有着越来越浓厚的兴趣。分子动力学和计算机模拟表明，当金属的晶粒尺寸降低到小于30 nm时，传统的位错机制不再占主导地位，并且与大量存在于晶界处的原子相关的几种不寻常的变形机制将接管。迄今为止，这些机制还没有在实验上得到很好的建立。有了这个奖项，关键参数，如激活体积，应变速率敏感性，和温度依赖性将得到结论的速率控制变形机制在真实的纳米晶金属。这项研究的结果将产生重大的科学影响，因为它们将为纳米晶制度中的基本结构-性质关系提供急需的见解。由于拟议的研究将建立知识基础，使纳米材料能够用于需要高性能的新世纪技术，因此预计还将产生更广泛的重大影响。学生将接受关于这些新材料的系统培训，培训结果将在大学网站上公布。另一个具有更广泛影响的组成部分是该项目预期将促进的国际交流。

This renewal award to Johns Hopkins University will support Professor Ma to study new type of bulk nanocrystalline materials prepared by a mechanical attrition process. The dynamically recrystallized nanocrystalline surface layer or in situ consolidated samples would be close to being "ideal" for mechanistic studies of nanocrystalline deformation behavior. Emphasis of this award will be on mechanical property tests with these nanocrystalline materials at different strain rates (including stress relaxation and jump tests) and at different temperatures. There is strong and growing interest in uncovering the novel mechanical behavior of nanocrystalline materials. Molecular dynamics and computer simulations revealed that when the grain size of a metal is brought down to less than 30 nm, the conventional dislocation mechanism is no longer dominant and several unusual deformation mechanisms associated with the large fraction of atoms residing at the abundant grain boundaries would take over. These mechanisms have not been well established experimentally so far. With this award, key parameters such as activation volume, strain rate sensitivity, and temperature dependence would be obtained to conclude on the rate-controlling deformation mechanisms in real nanocrystalline metals. Results from this research would have significant scientific impact, as they would provide much-needed insight into the fundamental structure-property relationship in the nanocrystalline regime. Significant broader impact is also expected because the proposed studies would build the knowledge base that will enable nanomaterials to be used in the technologies of the new century that demand high performances. Students will receive systematic training on these novel materials, and results will be disseminated by the University web site. Another component with broader impact would be the international exchange that this project is expected to foster.