The Effect of Internal Damage Accumulation on the Stress-Strain Response of a Metallic Glass

How the strain of a material changes with stress is a central concept in materials science. Ideally, stress-strain curves are represented in true quantities, which, however, is difficult in cases where strain distributes inhomogenously across the specimen volume. This is the case in metallic glasses (MGs), where highly localized shear-deformation is confined to nano-scale shear-bands (Adv. Func. Mat. 25 (2015) 2353). Consequently, literature presents engineering stress-strain curves, which for nominally identical tests reveal very large scatter in post-yielding parameters. Recent insights gleaned from x-ray tomography work (Acta Mater. 140 (2017) 206; Scripta Mater. 170 (2019) 29) revealed the presence of internal voids (shear-band cavities) just prior to fracture, but it remains unclear how they evolve as a function of plastic strain. In the present talk, we rely on a combination of x-ray tomography (XRT) and in-situ acoustic emission (AE) to track this internal damage accumulation, which allows us determining a true stress-strain curve of a MG. The data demonstrates how strain-softening coincides with the first detection of shear-band cavities that grow exponentially as a function of strain. A power-law scaling between the strain-dependent shear-band cavity area and boundary length is revealed. We capture the exponential shear-band cavity growth and the scaling between the cavity area and boundary length with a phenomenological model based on stable crack growth of mode II. These insights rationalize the large variability of plastic flow curves reported for MGs.