Overcoming geometric embrittlement in Mg–Li foils through grain refinement and grain-boundary–mediated deformation

Ultrathin metallic foils deform under plane-stress conditions, where the absence of through-thickness constraint and a low thickness-to-grain-size ratio (t/d) promote early necking and severely limit uniform elongation. Here, we demonstrate that high-ratio differential speed rolling (HRDSR) mitigates these geometric limitations in Mg–10Li alloy foils by refining grains to the nearly ultrafine regime, thereby increasing t/d and activating grain-boundary–mediated deformation. Foils 100 µm thick with grain sizes of 1.1 µm (t/d ≈ 91) exhibit elongations exceeding 30 % at 10⁻⁵ s⁻¹, whereas coarse-grained counterparts (29.4 µm, t/d ≈ 3.4) of the same thickness fail abruptly with < 1 % uniform strain under identical conditions. Micro-pattern formability tests confirm homogeneous deformation and high surface fidelity in ultrafine-grained foils, in sharp contrast to severe strain localization and pattern collapse in coarse-grained samples. Strain-rate jump tests on the ultrafine-grained foils reveal an elevated strain-rate sensitivity (m ≈ 0.23) and low activation volumes (15–30 b³) at low strain-rates, suggesting that deformation is governed by a combined contribution of grain boundary sliding (GBS) and dislocation climb creep (DCC). A unified constitutive framework captures the transition from DCC at moderate strain-rates to GBS at low rates. The present findings demonstrate that refining Mg–Li alloys to a quasi-ultrafine-grained regime effectively overcomes the intrinsic ductility limitations imposed by plane-stress geometry, thereby enabling their practical application in flexible electronics, bioresorbable implants, and lightweight energy-storage systems.