Kim Sunghwan, Ryu Seunghwa
Department of Mechanical Engineering, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon, 34141, Republic of Korea.
Sci Rep. 2017 Oct 18;7(1):13472. doi: 10.1038/s41598-017-13410-3.
Despite the significance of surface effects on the deformation behaviours of small-scale metallic glasses, systematic investigations on surface states are lacking. In this work, by employing atomistic simulations, we characterise the distributions of local inhomogeneity near surfaces created by casting and cutting, along with internal distributions in pristine and irradiated bulk specimens, and investigate the effects of inhomogeneity on the mechanical properties. The cast surface shows enhanced yield strength and degrees of shear localisation, while the cut surface shows the opposite effects, although the fraction of vibrational soft spots, known to indicate low-energy barriers for local rearrangement, is high near both surfaces. Correspondingly, plastic deformation is initiated near the cut surface, but far from the cast surface. We reveal that improved local orientational symmetry promotes strengthening in cast surfaces and originates from the effectively lower quenching rate due to faster diffusion near the surface. However, a significant correlation among vibrational soft spots, local symmetries, and the degree of shear localisation is found for the pristine and irradiated bulk materials. Our findings reveal the sensitivity of the surface state to the surface preparation methods, and indicate that particular care must be taken when studying metallic glasses containing free surfaces.
尽管表面效应对于小尺寸金属玻璃的变形行为具有重要意义，但目前仍缺乏对表面状态的系统研究。在这项工作中，我们通过原子模拟，表征了铸造和切割产生的表面附近局部不均匀性的分布，以及原始和辐照块状样品的内部分布，并研究了不均匀性对力学性能的影响。铸造表面表现出增强的屈服强度和剪切局部化程度，而切割表面则表现出相反的效果，尽管已知表明局部重排低能垒的振动软点分数在两个表面附近都很高。相应地，塑性变形在切割表面附近开始，但远离铸造表面。我们发现，改善的局部取向对称性促进了铸造表面的强化，这源于表面附近扩散更快导致的有效较低淬火速率。然而，对于原始和辐照块状材料，发现振动软点、局部对称性和剪切局部化程度之间存在显著相关性。我们的研究结果揭示了表面状态对表面制备方法的敏感性，并表明在研究含有自由表面的金属玻璃时必须格外小心。
