an antiphase boundary

Some alloys are not just a random mix of two kinds of atom; they are ordered, meaning the A atoms and B atoms sit on their own regular, alternating sub-lattices, like a perfectly laid checkerboard of black and white squares. Now imagine two such checkerboards growing toward each other and meeting a bit out of step, so that black squares meet black instead of black meeting white. Along that seam the tidy alternation is broken. That out-of-step seam is an antiphase boundary (APB).

An antiphase boundary is a planar defect specific to ORDERED alloys (superlattices) such as Cu3Au or Ni3Al. On either side the crystal is perfectly ordered, but the two domains are shifted relative to each other by a vector that is a lattice translation of the disordered structure but NOT of the ordered one, so across the boundary you find wrong neighbours (A next to A, B next to B) that the ordering wanted to avoid. That costs energy, the APB energy, because it creates unfavourable bonds. The regions of consistent order separated by APBs are called antiphase domains.

APBs are central to the strength of ordered intermetallics like the Ni3Al that stiffens jet-engine superalloys. A dislocation moving through an ordered crystal creates an APB in its wake, so dislocations travel in bound pairs (superdislocations) that drag an APB ribbon between them; the energy cost makes these alloys unusually strong, and even stronger as temperature rises. Honest note: an APB is not a grain boundary. The crystal lattice runs continuously and unrotated straight through it; only the chemical ordering pattern is out of step.