a stacking fault

Close-packed crystals are built by stacking flat layers of atoms in a strict repeating order, like stacking layers of oranges where each layer nestles into the dimples of the one below. There are two natural stacking recipes. FCC metals follow the sequence ABCABCABC..., cycling through three offset positions; HCP metals follow ABABAB..., alternating just two. A stacking fault is a local slip-up in that sequence — one layer landing in the wrong position, so the perfect order is briefly interrupted before it recovers.

For example, an FCC crystal that should read ...ABCABCABC... might instead read ...ABCAB|ABCABC..., where a C was skipped. That little glitch is a thin sliver where the local stacking momentarily looks HCP-like inside an FCC crystal. It is a planar (two-dimensional) defect, and it always sits sandwiched between a pair of partial dislocations — the ribbon of fault is exactly the region a full dislocation has split into two partials to spread across, because two smaller Burgers vectors store less energy than one big one.

How wide the fault ribbon spreads depends on a material property called the stacking-fault energy (SFE). A high-SFE metal like aluminum keeps its faults very narrow; a low-SFE metal like brass, austenitic stainless steel, or silver spreads them wide. This single number quietly controls a lot of behavior: low-SFE metals cross-slip with difficulty, so they work-harden fast, twin readily, and develop different deformation textures. So a defect that sounds like a mere mistake in stacking turns out to be a design lever for how a metal strengthens and deforms.