Proc. Natl Acad. Sci. USA http://doi.org/p8z (2013)

Despite being substantially more resistant to plastic deformation than crystalline metals, metallic glasses are rarely used as structural materials because of their high susceptibility to fatigue. Indeed, because glasses lack the typical obstructions to crack propagation that crystalline materials have (such as microstructural defects or grain boundaries), relatively low cyclic stresses can cause small cracks to propagate within the material. Cheng-Cai Wang and colleagues now provide real-time insights into the fatigue cracking at room temperature of notched metallic microbeams inside a high-resolution transmission electron microscope. They show that with increasing strain cycles the surface of the notches roughens through the formation of nanoscale intrusions and extrusions that act as crack-nucleation sites (which is analogous to surface roughening in crystalline metals), and that enhanced atomic diffusion in regions of high deformation (such as near crack tips) causes the formation, growth and coalescence of nanocrystals, which can provide resistance to crack propagation. Monotonically strained samples show instead smoother notch surfaces and smaller nanocrystals.