Zhuocheng Xie

Tensile twinning accommodates -axis strain in Mg, yet the role of pre-existing twin boundaries in twinning-mediated plasticity in confined volumes remains unresolved. Combining micropillar compression with atomistic simulations, we uncover two deformation pathways in Mg bicrystals containing a longitudinal twin boundary. When the two crystallographically equivalent grains lack preferential activation sites, the high stress required for twin nucleation favors near-synchronous activation and symmetric deformation. By contrast, local defect heterogeneity lowers the activation threshold in one grain, leading to sequential twin activation, unequal growth, and mechanical symmetry breaking. The pre-existing twin boundary minimally influences yielding but substantially raises the post-yield flow stress by impeding twin growth and generating non-basal dislocations at the resulting intersections. Impingement of twins from both grains additionally creates a new boundary blocking basal slip transmission between the reoriented domains. These findings reveal how local defect heterogeneity overrides nominal bicrystal symmetry, while twin–boundary interactions govern post-yield plastic flow.

Defects are fundamental to the behavior and performance of structural materials, yet their treatment in alloy design has often been decoupled from thermodynamic considerations of phase stability. The emerging concept of "defect phases" — chemically and structurally distinct configurations at lattice defects — offers a unified framework that integrates defect chemistry, thermodynamic stability, and mechanical behavior. While grain-boundary (two-dimensional) defect phases have gained recent attention, this article expands the scope to include defect phases across all dimensionalities, with a particular emphasis on dislocations (one-dimensional) as mobile carriers of plastic deformation and sites of complex phase behavior. We discuss how point, line, and planar defects can host distinct defect phases, how these phases compete for solute atoms, and how their stability can be mapped using defect phase diagrams constructed in chemical potential space. Through selected case studies in metallic solid solutions and ordered intermetallics, including Laves, B2, and µ-phases, we illustrate how dislocation-based defect phases can influence plasticity, strengthen alloys, or even drive local transformations that modify mechanical properties. By bridging defect physics with materials thermodynamics, we advocate for a defect phase-informed design paradigm that connects atomic-scale phenomena to bulk processing and performance. Graphical abstract