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Dislocation-coherent twin boundary interactions in FCC metals: Size Scaling

The thorough, mechanism-based, quantitative understanding of dislocation-grain boundary interactions is a central aim of the Nano- and Micromechanics group of the MPIE. For this purpose we isolate a defined grain boundary in a micron-sized sample. Subsequently, we measure and compare the mechanical properties with respect to single crystalline samples. [1-8]

Coherent twin boundaries (CTB), as the most common grain boundary in FCC metals, are of immense importance for simultaneous strength and ductility. In order to study their interaction with dislocations, Focused Ion Beam (FIB) machining is employed to mill micron-sized bi-crystal pillars with a defined single Σ3(111) CTB. Subsequently, in situ microcompression experiments are performed. The pre- and post- mortem imaging of the pillars is done using Scanning Electron Microscopy (SEM).

In our previoues work Nataliya Malyar et al. showed that the shear stress of a single and bi-crystalline pillar (having one CTB located in the center of the pillar) is marginal different (7 MPa) [1]. In order to explain the unexpectedly low difference, they proposed a “double-hump” dislocation shape, in which the difference in shear stress needed for bi-crystals is explained by the dislocation curvature near the CTB. The curvature is needed to form a perfect screw dislocation required for cross-slip-like dislocation transmission through the CTB.

Copper micropillar with a coherent S3 twin boundary (arrows). The slip steps meet at the boundary.

The aim of this project is to study the size scaling in the dislocation-CTB interaction. This can be used to indirectly confirm the double-hump theory, because as the theory suggests, the higher curvature in smaller samples results in significantly higher stress differences. Whereas in bigger samples, the difference between the single and bi-crystals is expected to be minimal.

See also: „Influence of microstructural constraints on slip transfer in nanotwinned Ag"

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