Size scaling in bi-crystalline Cu micropillars containing a coherent twin boundary

The impact of a coherent twin boundary (CTB) on the size scaling of the shear stress in micropillar compression tests has been investigated through microcompression of bi-crystalline pillars containing a vertical CTB, as well as single-crystalline pillars in three different nominal diameters of 1, 3 and 5 µm. While both, single- and bi-crystalline pillar results follow the size scaling trend typically observed in micropillars, namely “smaller is stronger”, we could identify a size-dependent contribution of the CTB in the increase of the shear stress at 2% strain (τ2%). A probabilistic analysis was performed to quantify the magnitude of the effect and to separate the CTB contribution from the single crystal size scaling contribution of the strength increase. The CTB-related strengthening was most prominent for smaller pillars and tended to be small for larger pillar diameters. The behavior can be explained by attributing an excess dislocation curvature in the scaling law according to the double-hump dislocation line shape model for bi-crystals, which requires parallel alignment of the dislocation line and the Burgers vector at the CTB.