Nanotwinned materials are known for their high strength and ductility, i.e. damage tolerance. The fundamental origin of the damage tolerance is yet not fully clear: While the high strength can be explained by the obstruction of dislocation motion caused by numerous twin boundaries, the origin of the unexpected ductility remains unclear. Classically, damage occurs as soon as the geometric softening due to localization cannot be countered by strain hardening. Since high strength materials typically do not exhibit pronounced strain hardening, slip localization – hence failure – sets in at low strains. But why doesn´t that happen in nanotwinned materials?

To shed light on the damage tolerance we are using nanoindentation with spherical indenters to measure the statistical pop-in behavior of copper single crystals and in the vicinity of coherent Sigma3 twin boundaries (TBs). The pop-in, i.e. a sudden burst in displacement, occurs at the transition from elastic to plastic deformation. We further correlate the pop-in force to the maximum shear stress beneath the indenter, which we interpret as the dislocation source activation stress. Our experiments show that the stress for operating a dislocation source in the vicinity of a TB is substantially lower than the one for dislocation source operation in the grain. Furthermore, the narrow distribution of pop-in stresses near TBs suggest that dislocation nucleation at TBs is omnipresent.

Based on our experiments we propose, that slip localization in nanotwinned materials is suppressed by a unique dislocation multiplication process occurring at imperfections of the otherwise coherent Sigma3 TBs. This dislocation multiplication process can only operate few times at a single imperfection, which suppresses slip localization and facilitates damage tolerance.