Defects-driven abnormal thermal expansion behavior

Low thermal expansion alloys are typically developed by incorporating negative thermal expansion materials into metal matrices. However, the mismatches of lattice structure and opposite thermal expansion behavior between constituent phases frequently induce defects that interact with lattice vibrations, thereby complicating the expected thermal response. Here, we demonstrate that the mismatch induced defects can fundamentally overturn conventional predictions based on phase mixing rules. By incorporating LaFe9.8Co1.1Al2.1 particles into a magnesium matrix, we observe an anomalous thermal expansion behavior: low particle concentrations lead to an increase in the coefficient of thermal expansion, while higher concentrations result in a significant reduction in it. This inversion is driven by dislocation generation and immobilization at particle–matrix interfaces, as revealed by multiscale microstructural analyses. Our findings demonstrate that thermal volumetric behavior in composites can be actively tuned through defect engineering, providing a viable strategy for designing dimensionally stable structural materials under thermal loading.