Prof. Zhiwei Shan, Excutive Director of CAMP-Nano, attended the 2014~2015 Academic Seminar of Institude of Solid Mechanics in Tsinghua University on Dec 19(Friday), 2014, and he gave a report on the topic of “Extraordinary mechanical properties of micronanoscaled materials and their potential applications".
 Abstract: In this talk, I will review our recent progress in exploring the extraordianry mechanical behavior of micronanosclaed materials [1] which has attracted interests of researchers throughout the world in the past decade. We found that single crystal pillars fabricated through focused ion beam always contain high density of defects[1-5]. However, if the sample size is small enough, both face-centered-cubic and body-centered-cubic metal pillars can experience "mechanical annealing,’’ i.e., a phenomena referring to the reduction of dislocation density in the deforming volume, when dislocation generation is outweighed by dislocation annihilation through the free surface [1-3]. Most recently, we found that low amplitude cyclic straining can be used to eliminate those preexisting dislocations in submicron-sized single crystal aluminum samples at a stress level much lower than the critical stress necessary to activate them under monotonic loading.
For hexagonal-close-packed (HCP) metals, we found that when the sample size was reduced below 1000 nm or so, stress saturation and deformation mechanism transition occurred in a single crystal Ti alloy [4]. Most interestingly, our study on single crystal pure magnesium found that when submicron-sized samples were compressed normal to its prismatic plane, the reorientation of the parent lattice produced a new orientational relationship akin to that of conventional twinning, but without a crystallographic mirror plane. In addition, the migration of the boundary between the parent lattice and newly formed crystal is dominated by the movement of the semicoherent basal/prismatic interfaces undergoing basal/prismatic transformation via unit cell reconstruction. This newly discovered deformation mode is different from the well known dislocation or twinning mediated plasticity [5] and therefore has important implication on alloy design for HCP structured materials.