Understanding the complex structures in nanoglasses
Authors: Wang Di , Mu Xiaoke , Wang Chaomin , Feng Tao , Kobler Aaron , Kübel Christian , Hahn Horst , Gleiter Herbert 
Published: December 2016

In recent years nanoglass materials have attracted a lot of interests due to their special physical properties, which differ significantly from traditional bulk amorphous materials of the same composition [1,2]. For example, a Sc
75
Fe
25
nanoglass exhibited a remarkable plasticity whereas the corresponding ribbon glass was brittle [3]. It has been suggested that these special properties originate from the interfacial regions between amorphous nano domains (analogous to grain boundaries changing the properties of nanocrystalline materials), which have either a different atomic configuration or chemical composition, or both compared to the domain core. Due to the amorphous structure of the materials, the structural variations between the core of a glassy grain and the interfacial region are difficult to distinguish, especially when the composition is similar. By STEM‐EDX/EELS spectrum imaging and by EFTEM imaging, such composition variations were confirmed for a number of nanoglass systems synthesized using various methods [2], for example inert gas condensation (IGC), magnetron sputtering and ultra‐high vacuum (UHV) cluster deposition. On the other hand, radial distribution function (RDF), which can be extracted from electron diffraction, has proven very sensitive to the small difference in atomic configurations [4], e.g., interatomic distances and coordination numbers. The newly developed STEM‐RDF mapping has been shown to be capable of resolving the different amorphous structures at nanoglass core and at the interface, respectively.

In this presentation, we investigated the Sc
75
Fe
25
nanoglass primary particles synthesized by IGC as well as the pellet pressed at 6 GPa. The primary particles were directly collected by a carbon coated TEM grid and the pellet sample was processed by FEI strata focused ion beam (FIB) for TEM observation. From the STEM‐EDX mapping on the primary particles (Fig. 1), a Sc‐rich shell is clearly resolved. Quantification of the integrated spectra from the out part and from the core of the particle reveals the Sc:Fe atomic ratio being around 4:1 and 2:1, respectively. After consolidation under high pressure, the inhomogeneity in primary particles remains and leads to Sc‐rich interface between the areas originated from the particle cores. Additionally, by performing STEM‐nanobeam diffraction on the pressed Sc
75
Fe
25
sample, RDF mapping was obtained and two different types of RDFs are distinguished, indicating that there exist two major components, one with higher Fe‐Sc coordination number (red curve in Fig.2), and the other one with higher Sc‐Sc coordination number (green curve in Fig. 2). By multiple linear least square (MLLS) fitting, corresponding component maps are constructed and shown in Fig.2. In the color‐mix map, the green areas represent the interface between particle cores, where Sc‐Sc bonding is dominating and the red areas represent the particle cores, where Fe‐Sc coordination is considerably higher.

In nanoglass systems synthesized by other method, e.g. NiTi‐Cu by magnetron sputtering [5] and NiP by electrochemical deposition, composition and structure fluctuations were also observed. The different amorphous structures constrained locally can be successfully revealed by (S)TEM spectroscopic and nanobeam diffraction methods, which is an important step towards understanding the unique structure in nanoglasses compared to the conventional glasses. The correlation between synthesis and structure of nanoglasses makes it possible to design the amorphous nanomaterials with desired functionalities.