Unusually thick shear-softening surface of micrometer-size metallic glasses

The surface of glass is crucial for understanding many fundamental processes in glassy solids.A common notion is that a glass surface is a thin layer with liquid-like atomic dynamics and a thickness of a few tens of nanometers.Here,we measured the shear modulus at the surface of both millimeter-size and micrometer-size metallic glasses(MGs)through high-sensitivity torsion techniques.We found a pronounced shear-modulus softening at the surface of MGs.Compared with the bulk,the maximum decrease in the surface shear modulus(G)for the micro-scale MGs reaches~27%,which is close to the decrease in the G upon glass transition,yet it still behaves solid-like.Strikingly,the surface thickness estimated from the shear-modulus softening is at least 400 nm,which is approximately one order of magnitude larger than that revealed from the glass dynamics.The unusually thick surface is also confirmed by measurements using X-ray nano-computed tomography,and this may account for the brittle-to-ductile transition of the MGs with size reductions.The unique and unusual properties at the surface of the micrometer-size MGs are physically related to the negative pressure effect during the thermoplastic formation process,which can dramatically reduce the density of the proximate surface region in the supercooled liquid state.