朱运田
南京理工大学
地点：教学楼102
时间：2015-12-18 14:00
It has been over 30 years since Prof. Gleiter first raised the concept of nanocrystalline materials, which has since become a major research field in the international research community. The processing of nanocrystalline materials by severe plastic deformation made it possible to produce nanostructured materials that are 100% dense and contamination free. This provided the needed samples for studying the intrinsic mechanical properties and deformation physics. It is generally found that nanostructured materials have very high strength, however, they are also found to have very low ductility. Mechanical instability, i.e. necking, occurs soon after tensile yielding, leading to very limited uniform elongation and making the nanostructured materials unsuitable for structural applications. The low ductility is primarily caused by the low strain-hardening rate, which in turn was caused by the poor capability to store and accumulate dislocations in nanostructured materials. A few strategies were proved effective in increasing the ductility without sacrificing the strength, including twinning, stacking faults, and nano-particle dispersion. To utilize these mechanisms for high ductility, it is critical to design the nanostructures including grain sizes because there is grain size effect on the deformation mechanisms. Finally, we need to start thinking about the post nanomaterial era, which is coming soon. Gradient and heterogeneous structures with a mixture of nano and micro grains have recently demonstrated mechanical properties that are superior over both coarse-grained and nanocrystalline materials, and this is becoming a hot research field in the near future.
Before joining the NCSU faculty in 2007, Yuntian Zhu was the team leader of the Nanomaterials Team in MPA-STC, Los Alamos National Laboratory. Zhu’s research in recent years has focused on three major nano-related areas: gradient structured metals, nanostructured materials, and carbon nanotubes (CNT) and CNT nanocomposites. He and his colleagues has found that the gradient structures can produce a synergetic strengthening and extra strain hardening to produce a superior combination of strength and ductility that is not accessible to their coarse-grained counterparts, which is caused by an intrinsic internal mechanical incompatibility. They also experimentally observed, for the first time, a new twinning mechanism in nanocrystalline fcc metals that does not generate macroscopic strain, an inverse grain size effect on twinning, and an optimum grain size for twinning. In addition, his group recently developed several strategies to simultaneously increase the strength and ductility of nanostructure/ultrafine-grained metals and alloys, which is a significant breakthrough because previous attempts to improve ductility always sacrifice the strength. In the area of carbon nanotubes, his group has recently synthesized 4-cm long carbon nanotubes and fabricated super strong and stiff CNT nanocomposites. These achievements have won his team two Nano50 Awards. He recently received the China ThousandPlan Scholar award (2011), the TMS SMD Distinguished Scientist/Engineer Award (2012), the NCSU Alumni Distinguished Research Award, the NCSU Alcoa Foundation Distinguished Engineering Research Award (2014), ASM International Albert Sauveur Award (2014), TMS Leadership Award (2015). TMS Leadership Award (2015), IUMRS Sômiya Award (2015). In addition, he is a founding Editor-in-Chief of Materials Research Letters, and is among Highly Cited Researchers 2014 by Thomson Reuters. More information about Dr. Zhu’s research group can be found at http://www.mse.ncsu.edu/zhu/ .