Extended defects, ideal strength and actual strengths of finite-sized metallic glasses

Initial condition dependence is the key to understanding the difference between ideal strength and actual strength of both crystalline and amorphous materials. Besides intrinsic structural heterogeneities in metallic glasses (MGs), a class of “extended defects” based on the “connected atomistic free volume” (CAFV) is proposed to define the microstructure (initial condition), which is crucial to understanding the strength. To explore these concepts and theories, deformation of finite-sized MG samples with different populations of pre-existing extended defects (damages) are simulated using a nanometer-scale shear transformation zone (STZ) model based on microelasticity and the kinetic Monte Carlo method. A “smaller is stronger” effect on the peak stress of simulated true stress–strain curves is seen in samples with pre-existing damage introduced as post-activated STZ clusters. Samples with “chemically contaminated” surface STZs also exhibit a size effect on the peak stress, and depending on whether the surface STZs are softer or harder than the bulk STZs, smaller can be either weaker or stronger.