Surface compressive and softening effect on deformation mode transition in Ni-Nb metallic glassy thin films: A molecular dynamics study

摘要

Size-dependent deformation mode transition in metallic glasses (MGs) attracts a lot of interest due to potential application in micro-devices, but the underlying mechanisms are still unclear from the perspective of structure, e.g., how the chemical composition affects the deformation mode transition in a particular system is mysterious as well and needs to be addressed. Here, a series of NixNb100-x (x = 30, 50, 62, and 70 at.%) MG thin films has been studied by molecular dynamics simulations for better understanding the thickness-dependent tensile behaviors. The results show that the deformation mode transition from highly-localized to non-localized occurs as the film thickness (t) approaches the critical size, t(c), which strongly correlates with the chemical composition, i.e., a Ni-rich specimen with higher modulus has smaller t(c). It is revealed that the softening and compressive effect of surface layers with about 0.4 nm thickness in terms of Voronoi volume is the key factor for this transition regardless of composition. We illustrate the surface softening effect in various MG thin films by introducing a softening coefficient (Psi) parameter reflecting the structural difference between the surface layer and the internal part. It is found that the higher the Psi, the severer the surface softening effect, and the larger the t(c) in the Ni-depleted specimen, indicating the importance of Psi as an indicator for the deformation mode transition. Published by AIP Publishing.