PhD Proposal by Sandra Stangebye

Primary tabs

THE SCHOOL OF MATERIALS SCIENCE AND ENGINEERING     GEORGIA INSTITUTE OF TECHNOLOGY      Under the provisions of the regulations for the degree   DOCTOR OF PHILOSOPHY   on Tuesday, February 9, 2021 10:00 AM   via     BlueJeans Video Conferencing  https://bluejeans.com/548202584   will be held the     DISSERTATION PROPOSAL DEFENSE for    Sandra Stangebye      “Investigating Microstructural Effects on Stress-Induced Grain Boundary Migration in Freestanding Ultrathin Metal Films”      Committee Members:    Prof. Joshua Kacher, Co-Advisor, MSE Prof. Olivier Pierron, Co-Advisor, ME Prof. Naresh Thadhani, MSE Prof. Hamid Garmestani, MSE Prof. Ting Zhu, ME    Abstract:                   The demand for smaller, smarter and faster devices has motivated continued research into understanding the mechanical behavior of small-scale materials used to create micron-sized features for devices such as flexible or stretchable electronics or micro electromechanical systems (MEMS). Nanocrystalline (nc) and ultrafine-grained (ufg) metal thin films show increased strength when compared to their coarse-grained equivalents, and as a result, have been proposed as viable solutions to high-strength MEMS materials. Their increased yield strength is generally attributed to the high volume of grain boundaries (GB) which impede conventional dislocation glide. Unfortunately, the increase in strength is accompanied by a decrease in ductility which limits the direct benefit of nanostructured metals. Grain boundaries play an increasingly important role in the deformation of nc/ufg metal thin films and are thus a key player in understanding and improving the mechanical properties within this grain size regime. Specifically, GB migration leading to grain growth has been documented as one of the primary means of deformation, however, the exact microstructural features that lead to or promote this mechanism are not well understood. Boundary migration is influenced by many factors, including grain size, GB structure, and GB curvature. As the grain size decreases, lattice dislocation-based deformation is limited as the grain size is not capable of carrying such dislocations which may increase the driving force for alternative deformation mechanisms, such as GB migration. However, the number of triple junctions (TJs) increases with decreasing grain size which may lead to added restriction on GB migration. Additionally, the specifics of the GB structure are expected to heavily influence both the boundary and TJ migration behavior. It is therefore necessary to understand how the GB structure and grain size both influence the migration behavior of a GB network.   The objective of this research is to determine the dominant factors that influence deformation-induced GB migration in nc and ufg metal thin films. Specifically, the goal is to determine the effect of GB structure and grain size on stress-induced GB migration in nc and ufg Au thin films. The central hypothesis of the proposed research is that stress-induced GB migration is a function of the specifics of the GB structure and local stress state. This will be completed by combining orientation mapping with in situ TEM straining to document the stress-induced migration behavior across boundaries of different, and known, structure. The orientation mapping will be completed prior to the in situ TEM deformation to obtain information regarding GB structure (misorientation/axis of rotation and/or Coincident Site Lattice S value). During the in situ TEM straining, GB migration behavior will be identified and measured in terms of migration velocity, and the stress and strain levels in which migration occurs. The GB structure for the specific boundaries that undergo migration will be identified by comparing the TEM micrograph with the orientation map obtained prior to the deformation. Experiments will be completed on specimens with two distinct grain size regimes (nc and ufg) to investigate the effect grain size has on GB migration behavior. Results will be reported in terms of how the migration, or lack of migration, changes across GBs of different structures (misorientation angle, rotation axis, CSL value), GB length/curvature and grain sizes.


  • Workflow Status: Published
  • Created By: Tatianna Richardson
  • Created: 01/26/2021
  • Modified By: Tatianna Richardson
  • Modified: 01/26/2021