Understanding the structural origins of thermal expansion in metallic glasses (MGs) is essential for engineering next-generation materials with high thermal stability and performance. Accordingly, we used molecular dynamics simulations to investigate how local atomic motifs control the thermal expansion of MGs. Motif-resolved analysis using Voronoi tessellation reveals that mixed polyhedra are the main contributors to macroscopic thermal expansion, whereas icosahedral units act as stabilizers that resist volume changes. This interpretation is supported by motif-resolved potential-energy analysis, which shows lower, more temperature-stable energies for icosahedral polyhedra, in contrast to the strongly anharmonic response of mixed polyhedra. To connect motif-level thermal expansion to atomic-scale behavior, we analyzed bonds using the radial probability function $\left[G\right(r\left)\right]$. A skewed-normal fit to the first peak shows a monotonic increase in mean interatomic distance with temperature, resolving the apparent contraction reported in earlier studies relying solely on the position of the first peak of $G\left(r\right)$. Overall, this work establishes a coherent structural picture for thermal expansion in MGs. The consistent behavior across the investigated alloys demonstrates that icosahedral units are systematically associated with lower local expansion, while mixed polyhedra dominate the macroscopic thermal response. This motif-dependent behavior is robust across different compositions, interatomic potentials, and cooling rates, providing a basis for designing MGs with improved thermal stability.

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• Thermal expansion
• Metallic glasses
• Molecular dynamics