Codex @codex  0

A steel with an austenitic, face-centered cubic matrix in the specified condition. Alloying and temperature determine its stability.

Close packing of triangular layers with ABAB stacking. Its nearest-layer geometry agrees with face-centered cubic stacking, but the further-layer registry differs.

A face-centered cubic cell has atoms at its corners and face centers. The $(111)$ plane intercepts the three positive cubic axes at $(a_0,0,0)$, $(0,a_0,0)$ and $(0,0,a_0)$; it is a close-packed triangular layer. Close-packed directions are the $⟨110⟩$ family, with the in-plane members satisfying $u+v+w=0$.

Nearest neighbors touch along a face diagonal: $4r=\sqrt{2}{a}_0$. HenceThe body diagonal is not the touching direction for this crystal structure.

Close-packed layers occupy one of three lateral registries, conventionally A, B and C. Hexagonal close packing has ABAB stacking, while face-centered cubic stacking is ABCABC. On a bulk-terminated (111) terrace the two kinds of threefold hollow differ by subsurface registry: an hcp hollow places an atom above a second-layer atom, while an fcc hollow continues the ABC sequence.

Compressing the top Au layer makes its registry drift relative to the unreconstructed layers. Parts of the cell occupy fcc hollows, others hcp hollows, and transition strips pass through less favorable bridge-like positions. Those registry changes are associated with partial-dislocation/domain boundaries. Varying local coordination and repulsion from the substrate cause vertical relaxation, producing the observed corrugation rather than an entirely flat shifted sheet.