Abstract 17 December 2014

Dislocation nucleation-limited deformation in Au nanowires

Cynthia A. Volkert, Institute of Materials Physics, Georg August University Göttingen, Germany

Nucleation remains one of the central puzzles of materials science and is often the rate-limiting and microstructure-determining step in transformations and processing. Since we rarely have the temporal and spatial experimental resolution necessary to directly interrogate nucleation, we must rely on comparing resultant microstructures with models to indirectly test our understanding. In this presentation I aim to describe the surface nucleation of dislocations by comparing experimental observations of the defects introduced by straining single crystal Au nanowires with predictions of classical nucleation theory and molecular dynamics simulations. The high crystal quality of the nanowires ensures well-characterized sites for nucleation on the wire surfaces and the small cross-sectional dimensions of the wires (between 20 and 300 nm) inhibits the generation of dislocations by dislocation reactions. We use in-situ tensile straining in scanning and transmission electron microscopes to investigate defect evolution and mechanical behavior of the initially defect-free Au nanowires. We observe that planar defects are formed along the length of the single crystal nanowires at a wire diameter-independent yield stress of approximately 1.2 GPa. The defects are stacking faults which may eventually thicken to twins by layer-by-layer growth. High-resolution transmission electron microscopy studies and predictions from classical nucleation theory support the idea that their formation is controlled by nucleation of partial dislocations at surface steps which move through and exit the wire. In wires containing longitudinal twin boundaries, the partial dislocations pile up against the boundary while under load. On unloading, they either reverse and exit the wires or join with trailing partial dislocations to form full dislocations. By using classical nucleation theory to extrapolate these observations from experimental time scales and strain rates to the much faster regime of molecular dynamics simulations, we obtain insights into the likely atomic scale mechanisms for dislocation nucleation.

 

 

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