When Twins Collide: adventures in nanocrystals
While twins readily occur during deformation or annealing in many bulk materials, they occur especially frequently in face centered cubic nanocystalline metals. The high density of twins imply frequent twin collisions. We perform molecular dynamics simulations of grain growth in nanocrystalline nickel and observe interesting relationships between grain boundary migration and twinning and, especially, twin junction formation. We analyze the geometry of twinning and show that only a small set of twin junctions are possible, find examples of all of these in our simulations and measure the frequency of occurrence of each. Simple geometric ideas are used to rationalize these frequencies. Perhaps the most interesting aspect of our observations is the high frequency with which five-fold or pentatwins occurs. We clarify the mechanisms by which they occur. The surprise, however, is that they occur at all given that even though the grain size is small, the sample size is not. We then analyze penta-twin (i.e., disclination) elasticity to show why they occur so readily on the nanoscale but not in the bulk. Finally, we close the loop by showing how twin junctions and, especially penta-twins, feed back to retard grain boundary migration.
More about the Speaker: David Srolovitz is the Joseph Bordogna Professor of Engineering and Applied Science at the University of Pennsylvania (departments of Materials Science & Engineering and Mechanical Engineering and Applied Mechanics). At Princeton, Yeshiva and Michigan he was variously in the departments of Mechanical and Aerospace Engineering, Materials Science and Engineering, Physics, Applied Physics, and Applied Mathematics. He was also executive director of Singapore's Institute of High Performance Computing. He is a member of the National Academy of Engineering, Fellow of MRS, TMS, and ASM and the recipient of many awards, including the Materials Theory Award of the Materials Research Society. His research combines theory and simulation of defects in materials, microstructure and morphology evolution, and the mechanics of novel structures. He is the author of nearly 500 research papers and has an H-index of 76.