Nanocrystallization during nanoindentation of a bulk amorphous metal alloy at room temperature

It is known that nanocrystallites can form in shear bands produced during severe bending or high-energy ball milling of thin ribbons of a metallic glass. We present direct experimental evidence that highly confined and controlled local contact at the ultrafine scale in the form of quasi-static nanoindentation of a bulk glassy metal alloy at room temperature can also cause nanocrystallization. Atomic force microscopy and transmission electron microscopy show that nanocrystallites nucleate in and around shear bands produced near indents and that they are the same as crystallites formed during annealing without deformation at 783 kelvin. Analogous to results from recent experiments with glassy polymers, our results are reasoned to be a consequence of flow dilatation inside the bands and of the attendant, radically enhanced, atomic diffusional mobility inside actively deforming shear bands.