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

J Dong; Y Huan; B Huang; J Yi; Y H Liu; B A Sun; W H Wang; H Y Bai

doi:10.1016/j.xinn.2021.100106

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

Innovation (Camb). 2021 Apr 16;2(2):100106. doi: 10.1016/j.xinn.2021.100106. eCollection 2021 May 28.

Authors

J Dong^{1

2}, Y Huan³, B Huang⁴, J Yi⁴, Y H Liu^{1

2}, B A Sun^{1

5

2}, W H Wang^{1

5

2}, H Y Bai^{1

5

2}

Affiliations

¹ Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China.
² Songshan Lake Materials Laboratory, Dongguan, Guangdong 523808, China.
³ State Key Laboratory of Nonlinear Mechanics (LNM), Institute of Mechanics, Chinese Academy of Sciences, Beijing 100190, China.
⁴ Institute of Materials, School of Materials Science and Engineering, Shanghai University, Shanghai 200444, China.
⁵ College of Materials Science and Opto-Electronic Technology, University of Chinese Academy of Sciences, Beijing 100049, China.

Abstract

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.

Keywords: glass surface; metallic glasses; shear modulus; torsion.