A new angle on stacking faults: Overcoming the edge-on limit in high-resolution defect analysis
The nature of stacking faults - whether intrinsic or extrinsic - plays a pivotal role in defect-mediated processes in crystalline materials. Yet, current electron microscopy 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 dichotomy by introducing a high-resolution scanning transmission electron microscopy (HRSTEM) method that enables full structural discrimination of inclined stacking faults, as demonstrated for various faults in fcc, $L1_2$, and sphalerite crystals. This approach eliminates a long-standing geometric constraint on high-resolution analysis, providing comprehensive access to stacking faults on all glide planes along the widely used [001] and [110] zone axes. We demonstrate the robustness of the method in a CoNi-based superalloy, achieving clear discrimination of fault types even for overlapping configurations and foil thicknesses exceeding 100 nm. The analysis of bounding dislocations, revealing the fault's formation mechanism, is also presented for inclined geometries. 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 stacking fault itself - thereby enhancing contrast for atomic-scale studies of long-range ordering, compositional fluctuations, and nanoclustering.