We demonstrate a nonzero twist-angle stabilization mechanism in lattice-mismatched 2D heterobilayers, which results from the geometric alignment between the flake edges and the vectors determining the moiré pattern supercell. Using atomistic simulations of graphene flakes on hexagonal boron nitride, with diameters of up to $\sim 2500\AA$, we identify twist angles corresponding to global minima in the twist-angle dependence of the interlayer interaction energy at $\sim 0.{61}^{\circ }$ for armchair and $\sim 1.{89}^{\circ }$ for zigzag-edged flakes, tunable via in-plane strain. This locking mechanism, which relies on energy barriers that are an order of magnitude larger than those of nearby metastable twist angles, provides a geometric route to precision twist-angle control of two-dimensional heterostructures and to understand the self-orientation of macroscopic flakes.

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• Interatomic & molecular potentials
• Graphene
• Hexagonal boron nitride
• Moire materials
• Twisted heterostructures
• Molecular dynamics