In-situ Tem Investigation On The Plastic Deformation Mechanisms Of Nanocrystalline Gold

It's well know that the strength of polycrystalline materials increases as the decrease of grain size,thus the strength of nanocrystalline materials is usually higher than their coarse-grained materials.However,the simulations and theories predict that as the grain size is below critical values of ~ 15 nm,the strength of nanocrystalline materials decrease as the grain size decreasing.The simulations and theories proposed that this is because that when the grain size is below the 15 nm,the lattice dislocations behaviors in nanocrystalline will be replaced by grain boundary behavior during plastic deformation,such as grain boundary sliding,grain rotation,and grain boundary migration.However,there is no direct quantitative evidence that is indeed the case.More importantly,there is few of the quantitative study on the plastic deformation of nanocrystalline metals,and most of previous studies were focus on Cu,Ni,Al and other metals.While for Au nanocrystalline,there is only rare experimental evidence revealed how their grain boundary accommodated the plasticity during the deformation.In order to solve above mentioned issues,the tensile test of metallic gold nanocrystals was carried out in a transmission electron microscope by using a home-made tensile device.(1)Indeed,there is a transition from the lattice dislocation to the grain boundary mediated plasticity,indicating the strength of the nanocrystalline can't be increased unlimited by the grain size decreasing.During the plastic deformation,there are large number of grain boundary migration and grain rotation behavior were observed.We uncovered that the plastic deformation is coordinated by the grain rotation and grain boundary migration.(2)The grain size effect on the grain boundary mediated plasticity and dislocation behavior of Au nanocrystalline were investigated.For larger grains(d > ~ 20 nm),movements and interactions of lattice dislocations were frequently observed upon deformation.For smaller grain sizes(d < ~20 nm),the GB migration grain rotation process were the dominant contributor to the observed plasticity,while GB sliding was rarely observed.Statistics results also show that the grain boundary migration in Au nanocrystalline is higher than grain rotation,which is different from previous reports.(3)Based on our experiments results and the calculated strain rate,we explained why the grain boundary migration is more frequent than the grain rotation in Au nanocrystalline.The in-situ atomic-scaled observations show that the grain rotation is realized by the climbing of dislocation in the low-angle grain boundary.(4)We also uncovered that there is an ultra-large pseudoelasticity strain of 86% at the fracture tip of Au nanocrystals.The in-situ atomic-scaled observations show that the ultra-large pseudoelasticity strain is resulted from the inward diffusion of surface atoms.