EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Recent work using micropillar compression has shown that the stress for ideal dislocation slip transmission through a coherent twin boundary (CTB) in copper is similar to the stress required for dislocation cross-slip [1]. The difference in shear stress during deformation of single and bi-crystalline micropillars (Δ2%) can be as low as 7 MPa for 3 µm sized pillars. A double-hump dislocation curvature was proposed to explain this unexpectedly low difference, where an additional dislocation curvature in bi-crystalline micropillars is necessary to form the perfect screw dislocation required for cross-slip-like transmission. This alignment of dislocation near the CTB causes the dislocation line to form a double-hump shape.

The aim of the current study was to investigate the size scaling of CTB containing micropillars to validate or revise the double-hump theory. We employ focused ion beam (FIB) machining to mill more than 90 micron-sized single and bi-crystalline pillars with a single vertical Σ3(111) CTB in 3 different nominal sizes of 1, 3 and 5 µm diameter. Subsequently, in situ microcompression experiments inside a scanning electron microscope (SEM) as well as post-mortem imaging using SEM were performed.

It was found that bi-crystalline pillars follow the same size scaling laws as typically observed in micro-pillars, i.e. smaller pillars are substantially stronger. Importantly, Δ2% was observed to not remain constant over the diameter range of pillars. A thorough statistical analysis proved that Δ2% inversely scales with the pillar diameter, which is in agreement with the proposed double-hump dislocation curvature hypothesis. The developed probabilistic model can also be employed to predict the Δ2% across the whole diameter range.

[1] N. V. Malyar, B. Grabowski, G. Dehm, C. Kirchlechner, Dislocation slip transmission through a coherent Σ3{111} copper twin boundary: Strain rate sensitivity, activation volume and strength distribution function, Acta Mater. 161 (2018) 412–419. https://doi.org/10.1016/j.actamat.2018.09.045.