Structure and compositional evolution in epitaxial Co/Pt core-shell nanoparticles on annealing

► We examined the alloying of epitaxial Co/Pt core-shell nanoparticles. ► Z-contrast imaging revealed formation of Co-shell/Pt-core structures. ► Locally ordered areas were formed at the core of the nanoparticles. ► Stacking of two orthogonal variants causes atypical image contrast. ► High-density {111} stacking faults were formed inside the Co-Pt nanoparticles.