Breakdown of the Hall–Petch relationship in a Mg–0.8Mn alloy grain: Role of grain-boundary-mediated deformation

The breakdown of the Hall–Petch relationship in Mg alloys is closely associated with the transitions in dominant deformation mechanisms, but a physically consistent understanding of this behavior in dilute Mg alloys remains lacking. In this study, a Mg–0.8Mn (wt%) alloy with a wide grain size range (0.4–44 μm) was produced by high pressure torsion (HPT) followed by short-time annealing, enabling a systematic investigation of grain-size-dependent mechanical behavior. Grain growth kinetics exhibit a transition in the apparent activation energy during annealing, reflecting a shift in boundary migration mechanisms that governs the observed grain size spectrum. The as-prepared alloy exhibits a pronounced breakdown of the Hall–Petch relationship, which can be divided into three distinct regimes separated by two critical grain sizes (∼5.3 μm and ∼0.9 μm). With decreasing grain size, the dominant deformation mode transitions from twinning-assisted deformation to dislocation slip–dominated plasticity, and eventually to a regime where grain-boundary-mediated deformation becomes progressively dominant. Electron microscopy analyses (i.e., EBSD and TEM) provide direct evidence for such transitions, including the suppression of deformation twinning, the evolution of dislocation substructures, and the activation of grain-boundary-related deformation features in the ultrafine-grained (UFG) regime. Compared with pure Mg, the critical grain size for the onset of inverse Hall–Petch behavior is reduced. This shift is primarily attributed to Mn-induced precipitation strengthening, which increases the resistance to dislocation motion and thereby shifts the transition toward grain-boundary-mediated deformation. An optimal strength–ductility synergy is achieved at an intermediate grain size (∼2.3 μm). The present work establishes a unified framework linking Hall–Petch breakdown to deformation mode transition in Mg alloys, which is expected to provide guidance for the design of high-performance lightweight materials.