Stacking faults from a different angle - Overcoming the edge-on limit in high-resolution defect analysis
The nature of stacking faults (SF) - whether intrinsic or extrinsic - plays a pivotal role in defect-mediated processes in crystalline materials. Yet, transmission electron microscopy (TEM) techniques for their reliable analysis remain limited to either conventional fringe-contrast imaging of inclined faults or atomic-resolution imaging of edge-on configurations. Here, we overcome this long-standing geometric constraint by introducing a high-resolution scanning TEM method that enables full structural discrimination of inclined SFs, as demonstrated in fcc, $L1_2$, and sphalerite crystals. This approach complements edge-on analysis and provides access to SFs on all glide planes along commonly utilized zone axes. We demonstrate the method's robustness in a CoNi-based superalloy by discriminating fault types even in overlapping configurations and for foil thicknesses exceeding 100 nm, and extend its application to analyzing bounding partial dislocations in inclined geometries to reveal the fault formation mechanism. Probe propagation simulations reveal that fault-induced de-channeling is key to contrast formation and is strongly governed by the fault's depth within the sample. Leveraging this effect, we further establish a route to artificially generate ultrathin TEM lamellae - bounded by the SF itself - enhancing contrast for atomic-scale studies of long-range ordering and compositional fluctuations.