Brittle-to-ductile transitions in glasses: Roles of soft defects and loading geometry

Abstract: Understanding the fracture toughness of glasses is of prime importance for science and technology. We study it here using extensive atomistic simulations in which the interaction potential, glass transition cooling rate, and loading geometry are systematically varied, mimicking a broad range of experimentally accessible properties. Glasses’ non-equilibrium mechanical disorder is quantified through A_g, the dimensionless prefactor of the universal spectrum of non-phononic excitations, which measures the abundance of soft glassy defects that affect plastic deformability. We show that while a brittle-to-ductile transition might be induced by reducing the cooling rate, leading to a reduction in A_g, iso-A_g glasses are either brittle or ductile depending on the degree of Poisson contraction under unconstrained uniaxial tension. Eliminating Poisson contraction using constrained tension reveals that iso-A_g glasses feature similar toughness, and that varying A_g under these conditions results in significant toughness variation. Our results highlight the roles played by both soft defects and loading geometry (which affects the activation of defects) in the toughness of glasses. Impact statement: Glasses are non-crystalline materials that find an enormous range of industrial and technological applications. They are typically formed by rapidly cooling liquids, resulting in arrested out-of-equilibrium states lacking the long-range order of their crystalline counterparts. The emerging disordered structures, which vary with the formation cooling rate, give rise to large variability in material properties. Among these, the fracture toughness—quantifying materials’ ability to resist catastrophic failure in the presence of a crack—is of prime importance; understanding its physical origin and range of variability is a major challenge with far-reaching implications. To address this challenge, we employ cutting-edge and extensive computer simulations of glasses, spanning a range of material properties that is comparable to that of real-life glasses. We focus on the failure resistance and show that it is controlled by both the abundance of soft defects inside the glass, which are responsible for glasses’ plastic deformability, and by the loading configuration of the fracture test employed, which affects the imposed deformation geometry. These two physical factors control together a transition from ductile-like (gradual, accompanied by extensive plastic deformation) failure to brittle-like (abrupt, accompanied by little and localized plastic deformation) failure. Graphic abstract: [Figure not available: see fulltext.].