Adsorbate effects on ductile-brittle transition in crystals

Brittle or ductile failure of materials is typically governed by a competition between crack propagation and plastic deformation. For ductile crystals like aluminum, this framework shows that brittle fracture does not occur, regardless of preexisting cracks. However, it does not account for environmental effects that can promote embrittlement. We show that self-assembled organic monolayer adsorbates of varying chain length can induce a ductile-to-brittle transition in crystals under bending and shear. These adsorbates impose a surface stress that varies from compressive to tensile with increasing chain length, driving the transition. We develop an analytical model incorporating adsorbate-induced surface stress into the criterion for dislocation emission and compare it against crack propagation. This yields an embrittlement map demarcating regimes of ductile and brittle behavior in load-surface stress space. Our findings suggest a critical role for surface stress in environmentally assisted cracking, as well as opportunities for utilizing this stress to enhance manufacturing processes such as machining, polishing, and comminution.