|Nano-mechanical deformation and kinking non-linear elastic behavior of hexagonal materials |
Advisor: Prof. Michel W. Barsoumm, Drexel University,
3:00 PM, CCR 101
In this presentation we address the increasing need to understand nano-mechanical deformation of solids for applications in various micro/nano devices. The elastic-plastic deformation of materials under spherical nanoindenter is demonstrated; and, also an optimized and robust technique to obtain indentation stress-strain curves from spherical nanoindentation experiments is discussed. The stress-strain curves have been important for understanding dislocation nucleation and propagation in different materials, besides determining elastic modulus, hardness, and yield point. The dependency of dislocation nucleation and propagation on initial defect concentration, defect size and microstructure has also been explained by probing different orientations of materials with spherical tips of different radii.
Recently, we have shown that many materials, named kinking non-linear elastic (KNE) solids, deform by forming dislocation-based kink bands. It has been postulated that a sufficient condition for a material to belong in that group is high plastic anisotropy, which may occur due to high c/a ratio as in hexagonal crystal structures. Previously, there was little available in literature on kinking non-linear elastic deformation of materials under contact loading. In this presentation, with the help of cyclic nanoindentation experiments, we demonstrate the KNE behavior in ZnO, LiNbO3, and Al2O3. This has been proved by excellent agreement between the experimental results and our theoretical micro-scale model based on reversible dislocation motion in solids. We also show the dependency of room-temperature creep deformation in materials on different dislocation-based structures (kink bands and pile-ups) by obtaining indentation stress-strain response over extended period of time on different orientations of the materials.
In summary, we demonstrate that the wealth of information that can be obtained from indentation stress-strain analysis cannot be overemphasized and these curves will be important in understanding the mechanical deformation behavior across graded microstructures, thin films, nanomaterials with different grain sizes, etc. This work also illustrates that kinking plays a much more important role in the mechanical deformation of many technologically important materials than hitherto been appreciated.