Employing molecular dynamics simulations we observe formation of a complex metastable carbide ${\mathrm{Fe}}_7{\mathrm{C}}_3$ on the surface of ${\mathrm{Fe}}_3$C mediated by a zigzag interface that is almost ideally coherent. We show that formation of dynamic quasiparallel Fe chains and their reconstruction into Fe rings induced by C interstitials are two important atomistic mechanisms underlying the formation of this metastable carbide. Our study elucidates the atomistic mechanism underlying the formation of this specific metastable carbide in the Fe-C system, and suggests that the unveiled role of coherent zigzag interfaces may offer useful perspectives for understanding and inspiring future investigation of metastable phase formation in related systems.

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• Phase transitions
• Solid-solid interfaces
• Surface growth
• Molecular dynamics