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Dislocation-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 in three different nominal sizes. 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 previous work Malyar et al. showed that the shear stress of a single and bi-crystalline pillar (having one CTB located in the centre 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.

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. This is in agreement with the current experimental results.

Twin boundaries usually consist of long coherent and much shorter incoherent sections. This is due to lower energy required for formation of coherent twin boundary. However, presence of incoherent twin boundary in ample nanotwinned materials has been observed and its role in mechanical behaviour should not be underestimated. Therefore, we plan to study the role of incoherent twin boundary with similar methodology to understand the dislocation-twin boundary interaction more thoroughly.

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

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