Prof. Christoph Kirchlechner
19/11/2026
אודיטוריום ע"ש דויד וואנג, בניין מידן, קומה 3
13:30
“If one excludes single crystal research, one could claim that the balance of the field of metallurgy relates to interfacial properties.” John P. Hirth’s statement captures a persistent challenge: although interfaces govern the response of most engineering materials, the tools to probe their individual mechanical behaviour have long been limited. This lecture introduces how nanoindentation and in situ micropillar compression can isolate individual interfaces and reveal the mechanisms by which they initiate, transmit, accommodate and, finally, also form during plastic deformation.
First, pop-in statistics from spherical nanoindentation reveal that coherent twin boundaries act as dislocation sources. Mechanistically, the effect can be explained by the dislocation emission through the dissociation of pre-existing Frank partials at the boundary. While initially being proposed based on experiments, it was meanwhile confirmed by MD simulations. Hence, interfaces can act as sources of dislocations!
But how efficient are interfaces in blocking dislocation slip? The transmission of dislocations across a twin boundary is examined in copper bicrystal micropillars. Correlating mechanical response with local information from synchrotron Laue diffraction reveals repeated slip transmission with little sustained dislocation storage at the interface. These observations place experimental bounds on the stress required for transmission and challenge the assumption that even a coherent boundary necessarily acts as a strong barrier. In the talk, scenarios for twin boundaries acting as strong and weak barriers will be presented and discussed.
At elevated temperature, however, interfaces are not necessarily obstacles or static objects of deformation anymore: grain-boundary sliding adds an additional deformation mechanism. The temperature and size dependence of the response points to a dislocation-mediated mechanism, where dislocation slip in the grain followed by dislocation dissociation and motion at/in the grain boundary substantially contributes to plasticity.
Finally, during deformation new interfaces can form – a process called deformation twinning. The stress required, its size dependence as well as the slip morphology is used to identify responsible mechanisms: Submicron pillars favor twins consistent with a single-slip, three-layer mechanism, whereas larger pillars exhibit additional twin variants or – in case the stress is too low for twinning – deform predominantly by dislocation slip. Together, these observations can be used to answer a fundamental question of twinning: Is it a stress, or a strain-controlled phenomenon?
Across these examples, controlled experiments turn interfaces from presumed obstacles into observable participants in plasticity: they generate dislocations, transmit them, move under load and emerge anew during deformation. In the talk, the current understanding of plasticity at interfaces and the contributions of in situ micromechanics will be reviewed and discussed.