Size-dependent toughness and strength in defective brittle nanowires

Vacancy defects are ubiquitous in growing materials at the nanoscale. Yet their role in low-dimensional materials such as nanowires remains less understood. Here we report the observation of two mechanisms: elastic softening and stress localization that govern effective mechanical behavior of diamond and silicon carbide nanowires. They control different mechanical aspects of the nanowire at finite deformation. Elastic softening controls the effective mechanical properties such as stiffness in the linear regime of mechanical deformation, whereas stress-localization affects the effective toughness and strength of the nanowire. As a result, the condition for crack nucleation and the direction of crack growth are directly controlled by the stress-localization mediated by the higher-order elastic behavior of the lattice. With the increasing size of the defective regime, the nanowire shows softer effective elastic behavior that arises from the low-coordinated atoms forming the basis for the softening state of the defective regime. Results show that defect-size dependent effective stiffness is controlled by softening at the defective and surface regimes, whereas defect-size dependent toughness and strength are controlled by stiffening of the material by second-order elastic modulus.