Characterizing Plastic Deformation Mechanisms in Metal Thin Films using In Situ Transmission Electron Microscopy Nanomechanics

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Stangebye, Sandra
Kacher, Josh
Pierron, Olivier N.
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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. The 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. NC and UFG metals also exhibit an increase in strain-rate sensitivity and decrease in measured activation volume compared to their coarse-grained equivalents, both of which imply different atomistic mechanisms control the deformation. There remains a lack of quantitative characterization of these deformation mechanisms which hinders material design towards exception mechanical properties. In this work, the plastic deformation mechanisms that govern the mechanical properties of NC and UFG metal thin films are investigated through in situ transmission electron microscopy (TEM) nanomechanical experiments. This technique allows for the simultaneous observation of the active deformation mechanisms and quantification of the mechanical properties during monotonic and stress-relaxation experiments. Experiments were performed on NC Al and UFG Au specimens with different microstructures (grain sizes, thickness, texture), including irradiated UFG Au. A variety of deformation mechanisms have been identified, including dislocation nucleation and absorption at GBs, inter- and intragranular dislocation glide, and GB migration. It was found that the radiation damage in the irradiated UFG Au served as effective pinning points for transgranular dislocation glide, however, stress-assisted GBM was still active and effectively removed radiation damage as the defects were absorbed by the GB during migration. This resulted in defect-free (‘cleaned’) regions that can support unrestricted dislocation glide, suggesting that stress-assisted GBM is a healing mechanisms for radiation damage in UFG metals. The measured activation volume was found to increase with increasing grain size, decreasing stress level, and the addition of radiation damage. These values were compared with existing models to suggest that there is likely a competition between active displacive- and diffusive-type deformation mechanisms and that the contribution of the two depends on the microstructure. Furthermore, stress-assisted GB migration was studied in detail to investigate how the local microstructure influences boundary migration. This is completed by combining orientation mapping with in situ TEM straining to document the stress-induced migration behavior across boundaries of different structure.
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