| dc.description.abstract | While nanocrystalline materials offer numerous advantages in various applications due to their superior properties, the presence of grain boundaries inherent to these materials can have a detrimental impact on their efficiency in certain applications. Nonetheless, grain boundaries also provide an opportunity to manipulate the microstructure of nanomaterials for the benefit of developing and designing high performance materials. Although improved high resolution microscopy techniques and computational atomistic simulations have enabled the identification and quantitative characterization of grain boundaries in various materials at atomic scale, there are still areas of uncertainty and knowledge gaps regarding the structural transformation of grain boundaries and the impact related to their interaction with the free surfaces, which this thesis aims to address. This thesis provides new insight in the study of emergent grain boundaries on Cu (111), grain boundaries that emerge at the free surfaces of copper, using Scanning Tunneling Microscopy (STM). The STM is a powerful tool to visualize and analyse the structural characteristics of grain boundaries that appear at the surface in atomic scale detail, providing a novel perspective for the exploration of grain boundaries. Emergent [111] tilt grain boundaries have been investigated on the surface of a 50 nm thick nanocrystalline (NC) Cu (111) thin film and on the surface of an engineered Cu (111) bicrystal (BC). The microstructural evolution of the NC Cu thin film through thermal annealing and the effect of the annealing temperature were demonstrated. Following the optimization of the annealing temperature, the grain boundary rich texture of the Cu (111) surface was uncovered, and the grain boundaries were investigated. The out-of-plane grain rotation, within the framework of grain boundary restructuring phenomenon, has been shown to manifest as valleys and ridges at the triple junctions, for various grain boundaries on the surface of NC Cu thin films. Driven by energy minimization, the grain boundary restructuring mechanism occurs as the dislocation cores shift towards [112], to a lower energy configuration, resulting in so-called core-shifted grain boundaries. Through comparative STM analysis of various valley type grain boundaries, a general trend in the out-of-plane grain rotation angle and behaviour was demonstrated, as a function of the misorientation angle. While in general the out-of-plane rotation scales with the misorientation angle, the manifestation of its extent is found to be more localised for high-angle grain boundaries. The STM analysis of the engineered grain boundary on the Cu BC revealed frequent step dynamics at room temperature, due to reconfiguration of highly unstable sites. The transmission of a screw dislocation across the grain boundary was captured dynamically in real-time. At metastable sites, a polytype phase with a 9R-like periodicity was found, attributed to the facilitated emission of Shockley partial dislocation loops as part of the relaxation near the surface at the triple junction, at room temperature. The out-of-plane grain rotation seen on the NC Cu thin films was also observed on the surface of well-annealed Cu (111) BC, demonstrating it is the free surface that facilitates the restructuring in emergent grain boundaries in copper. Through high resolution STM measurements, the periodicity and misorientation angle have been confirmed in atomic detail. Finally, these experimental observations were compared and supported by the molecular statics simulations of [111], ?=13.17? core-shifted emergent tilt boundary, described as a wedge disclination at the free surface. The subsurface restructuring, i.e., core-shift, was illustrated and the optimized depth of rotation, i.e., extent of restructuring, corresponding to the energy minimum, was found to be determined by the interplay between the interfacial energy and the elastic stress. | en |
