Mechanistic understanding of Mg17Al12 mediated microstructural refinement and strengthening in Mg-Al Alloys – a comprehensive review

This review presents an in-depth mechanistic analysis of the microstructural refinement and strengthening pathways in Mg-Al alloys, with a focus on the influence of Mg 17 Al 12 precipitates. The Hall-Petch relationship, with a slope exceeding 150 MPa·µm 1/2 in most AZ-series Mg alloys, underscores the pivotal role of grain size in dictating yield strength. This high sensitivity to grain size is particularly affected by Al content and crystallographic texture of Mg alloys. This necessitates sophisticated thermomechanical treatments such as severe plastic deformation, twin-induced recrystallization, and rapid solidification for effective grain refinement. The Mg 17 Al 12 phase, characterized by its discontinuous or interconnected morphology in as-cast alloys (e.g., AZ80, AZ91), exerts a strong influence on ductility and fracture resistance. Controlled heat treatments enable modulation of Mg 17 Al 12 morphology, volume fraction, and crystallographic orientation, transitioning from coarse, brittle eutectics to fine, dispersed precipitates with semi-coherent interfaces. These changes fundamentally alter stress accommodation and interaction with dislocations, impacting both precipitation hardening and grain boundary strengthening. However, limited age hardening arises from sluggish solute diffusion and modest precipitation strengthening in the Mg matrix, imposing constraints on high-temperature performance. The review critically evaluates orientation relationships between Mg 17 Al 12 and the α-Mg matrix, detailing their effect on interfacial energies, nucleation sites, and deformation mechanisms. Recent advances in microalloying, process design, and hierarchical microstructure control are discussed to optimize the strength-ductility synergy for automotive applications. Proposed strategies integrate advanced grain refinement, targeted precipitate engineering, and alignment of Mg 17 Al 12 precipitates to achieve application-specific mechanical performance in Mg-Al alloys.