Novel plastic loop topologies in pure Mg and orientation-dependent fracture of β-Mg₁₇Al₁₂ precipitates under shallow scratching

Understanding how microstructural heterogeneity governs nanoscale deformation under surface-dominated loading is a fundamental challenge in lightweight alloy design. Here, large-scale molecular dynamics simulations reveal a previously unreported plasticity regime in Mg and β-Mg₁₇Al₁₂-containing substrates under shallow nanoscratching, uncovering deformation mechanisms inaccessible to conventional mechanical testing or bulk atomistic loading geometries. In pure Mg, surface-mediated stresses generate deeply penetrating, curved, multi-segment dislocation loops reaching ∼14.9 nm which far exceed the 2 nm scratch depth and are fundamentally distinct from classical prismatic half-loops reported in previous indentation studies. This establishes a new class of surface-nucleated dislocation topology in HCP metals. In precipitate-strengthened Mg, precipitate orientation is shown for the first time to act as the primary determinant of subsurface deformation mode: scratching along the β-Mg₁₇Al₁₂ alignment triggers cleavage-dominated fracture of the intermetallic crystallite, representing the first atomistic evidence of orientation-dependent precipitate fracture under sliding contact, while transverse scratching instead confines plasticity to a high-stress (∼4.4 GPa), barrier-controlled regime without fracture. These two orientations produce mechanistically distinct responses, with peak von Mises stresses, dislocation loop depths and friction coefficients all varying systematically and explicably with precipitate geometry. These findings establish shallow nanoscratching as a effective atomistic probe of incipient plasticity and interfacial fracture, directly informing the microstructural design of Mg alloys for improved machinability, wear resistance and surface integrity in aerospace, automotive and biomedical applications.