|Ph.D Student||Doron Shilo|
|Subject||Interaction of Dynamic Deformation Fronts with Dislocations|
in Single Crystals
|Department||Department of Materials Science and Engineering||Supervisor||Professor Emeritus Zolotoyabko Emil|
This research thesis explores an interaction between individual dislocations and deformation fronts, which are generated by surface acoustic waves (SAW) and propagating cracks. Interaction of high-frequency acoustic waves (phonons) with dislocations has crucial importance for heat conductivity and acoustic attenuation. Unfortunately, the existing methods provide only averaged information over a whole ensemble of dislocations with neither spatial nor temporal resolution. In order to overcome these limitations we developed a new imaging technique - high frequency stroboscopic X-ray topography, which allowed us to visualize both individual acoustic wave fronts and vibrating dislocations on the same image. X-ray images taken from LiNbO3 crystals excited by 6-12 mm SAWs revealed remarkable distortions of the acoustic wave fronts in the vicinity of dislocation lines. Simulations of the deformation contrast showed that these distortions are caused by the dynamic deformation fields of vibrating dislocations. Comparison between simulated deformation maps and x-ray images allowed us to determine the local velocities of vibrating dislocations and their viscosity coefficients. We found unexpectedly high velocity values (not far from the sound velocity) and low viscosity coefficients (2-3 orders of magnitude smaller than those previously measured in materials).
It was commonly believed before our studies that in brittle materials dislocations have no noticeable influence on crack propagation. We showed that interactions of a dynamic crack front with individual dislocations do result in strong perturbations on the crack surface. The developed theoretical model predicted substantial crack front deflections in close vicinity (~ 10 nm) of dislocations. Surface monitoring, by atomic force microscopy and high resolution scanning electron microscopy, revealed height perturbations in samples with induced dislocations, which were completely missing in dislocation free samples. The perturbation height was in the range of 2 - 20 nm, in excellent agreement with the prediction of the theoretical model.