Macroscopic low-friction via twinning assisted lattice reconstruction in magnesium

One-fourth of the global energy losses are spent to overcome friction, making it particularly important to reduce and minimize friction between contacting materials. Hexagonal close-packed (HCP) metals are an important class of structural materials. It has not been possible to reduce their friction, primarily because of friction induced dislocation slip and twinning. Here, we find particularly low friction when sliding perpendicular to the a-axis on the basal plane in HCP Mg single crystals. This is in contrast to the common belief that friction is small along the preferred dislocation slip direction (a-axis). This macroscopic low-friction stems from twinning assisted lattice reconstruction sharing a common rotation axis, confirmed by atomistic simulations and strain energy analysis. While sliding along the a-axis and other directions,〈c + a〉 dislocation activity accounts for high frictional resistance. By unambiguously decoupling the contributions of dislocation slip and twinning, this dis covery reveals potential opportunities in mitigating the energy dissipation at tribological interfaces of HCP metals, e.g. through crystallographic texture design.