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报告内容摘要：
     In contrast to generally disordered grain boundaries (GBs), twin boundaries (TBs) are coherent and strong. Replacing general GBs by TBs - so that high strength and large deformability can be realized in polycrystals – has been a focus of metallurgical society for about one century. Recent success in manufacturing ultra-fine grained Cu with embedded nanotwins revealed that such nanotwinned metals are indeed strong; their yielding strength can be as high as 1 Gpa, which is 10 times stronger than their conventional counterparts. Intriguingly, such nanotwined Cu strengthens as TB spacing decreases to 15nm, then softens if TB spacing narrows further.
      Here he will report his explanation about the softening mechanism in such nanotwinned metals. We found that nucleation of dislocations parallel to TBs governs the strength of such materials, resulting in their softening below a critical TB spacing. Large-scale molecular dynamics simulations and a kinetic theory of dislocation nucleation in nanotwinned metals show that there exists a transition in deformation mechanism, occurring at a critical TB spacing for which strength is maximized. The simulations indicate that the critical TB spacing for the onset of softening in nano-twinned copper and the maximum strength depend on the grain size: the smaller the grain size, the smaller the critical TB spacing, and the higher the maximum strength of the material.

报告人简介：
      1992年9月--1997年7月，2138cn太阳集团古天乐，学士；1997年9月--2000年7月，中科院力学研究所，硕士；2001年9月—2006年2月，麻省理澳门太阳娱乐网站官网，博士；2006年3月—2008年12月，布朗大学，博士后；2009年1月—2010年1月，阿拉巴马大学（美国），助理教授；2010年2月—今，中科院力学研究所，研究员（百人计划）。