Advanced TEM study of homogeneous flow and size dependent mechanical behaviour in highly ductile Zr65Ni35 metallic glass films

Abstract

Metallic glasses (MGs) exhibit outstanding mechanical and functional properties for numerous applications. However, although intensive research on the deformation and fracture mechanisms has been performed on metallic glasses for more than 20 years, the fundamental mechanisms governing the mechanical behaviour as well as the recently observed mechanical size effects in this class of materials are still not fully understood. Recently, amorphous Zr 65 Ni 35 (%at) freestanding thin film MGs (TFMGs) deposited by magnetron sputtering have been deformed using a “lab‐on‐chip” technique based on MEMS technology [1]. The results have shown that the ductility of the films is highly enhanced compared to bulk MGs and other TFMGs in the literature, and the plastic deformation occurs homogenously, i.e., without the observation of mature shear bands until fracture. In order to unravel the origin of these remarkable mechanical properties, the films have been investigated in‐depth using advanced transmission electron microscopy (TEM). Quantitative nanobeam electron diffraction (NBED) was used to investigate the relationship between the local atomic order and the activation of “shear transformation zones” (STZs) [2]. The basic principle of NBED is shown in Figure 1a, consisting of a coherent electron beam with diameter of around 0.4 nm in order to produce two‐dimensional diffraction patterns from atomic clusters with comparable size. Figures 1b, 1c and 1d exhibit NBED patterns with strong Bragg reflections which is the signature of a locally ordered region (i.e., atomic clusters) while in the NBED pattern of Figure 1e, only a diffuse background with speckles without Bragg reflections can be observed. High resolution HAADF‐HSTEM and EELS revealed a heterogeneous microstructure with Ni‐rich and Zr‐rich regions exhibiting different atomic densities with characteristic length of 2‐3 nm (Figure 2). Such behaviour can be attributed to the sputter deposition process involving very high cooling rates compared to bulk MGs. The results raise several fundamental questions that will be addressed: Does the nucleation of the STZs occur preferentially in regions with specific enriched chemical composition and atomic density? How will this affect the interaction between the STZs? How such features can be used to explain the exceptional high plastic deformation levels, the absence of shear bands and the delay of fracture in the Zr 65 Ni 35 TFMGs used in the present work?