Size effects on deformation behavior of thin film metallic glasses

Amorphous metallic alloys which are now commonly called metallic glasses (MGs) possess a unique combination of mechanical, chemical and physical properties in comparison to conventional crystalline metals. Due to the absence of crystal lattice and lattice defects the MGs are stronger and more elastic, they are stable against corrosion and wear damage and they can have good biocompatibility. The main weak point of MGs is that they fracture in brittle catastrophic manner if applied mechanical stress exceeds the critical value. To overcome this limitation one should design MGs with ability to deform plastically as, for example, copper, alluminium or steel. Plastic deformation (or ductility) of MGs was demonstrated in micromechanical experiments where mechanical force is applied to very small test samples with the dimensions less than a micrometer. However it is still under discussion whether it is possible to make MGs more ductile by decreasing the sample size or the observed effect is caused by sample preparation procedure or other external factors. In this project the size-dependent mechanical properties will be investigated by an approach which is totally different to micromechanical testing. Thin film metallic glasses with thicknesses between 5 nm and 1000 nm will be deposited on polymer substrates. Then, macroscopic samples (width of about 4 mm and length of about 40 mm) will be prepared and mechanical force will be applied to polymer-supported films. The changes in the morphology of MG films (such as cracks or shear bands) will be detected by recording their electric resistance during mechanical test. Our short preliminary study showed that that brittle-like fracture of amorphous PdSi films changes to ductile-like behavior for the film thicknesses below 15 nm. During the project a systematic study of monotonic and fatigue behavior of thin MG films involving four different MG compositions (PdSi, CuZr, AuSi, WNiB) and seven different thicknesses will be performed. By using electronic microscopy techniques the thickness-dependent mechanisms of mechanical damage formation will be experimentally revealed. Based on the experimental results a model explaining the observed phenomena will be developed using molecular dynamics simulations. In contrast to the micromechanical testing, the MGs considered in this project will be prepared by an industrial deposition technique (magnetron sputtering), using commercially available substrates and the tests will be performed at ambient conditions. Thus, the results of the project will have great potential for such applications as mechanically robust and reliable hard coatings, functional layers in microelectronics or ultrathin and half-transparent conductors for flexible electronics.

Amorphous metallic alloys, also known as metallic glasses, possess unique combination of mechanical properties. For example, metallic glasses can be stronger than steels and, at the same time, as elastic as polymers. However, when the critical mechanical load is reached, metallic glasses fracture in a brittle manner due to the specific and poorly understood phenomenon of shear banding. The aim of the current project was to investigate shear banding in detail using thin film metallic glasses deposited on polymer substrates. The particular focus was lying on size effects, i.e. potential change in deformation behavior as a function of film thickness. During the project novel amorphous alloys (e.g. the noble AgAuSi metallic glass family) were discovered through successful synthesis and systematic characterization of the correlations between alloy composition and atomistic structure on the one hand and their electrical and mechanical properties on the other. Another amorphous alloy (WNiB) was shown to have very high hardness (23 GPa) and high elasticity (>2%), a combination which is inaccessible for crystalline metallic alloys. The use of polymer-supported films instead of free-standing metallic glass samples enabled direct observation of size effects, since exactly the same mechanical testing procedure was applied to the wide range of film thicknesses - from 7 nm to 1000 nm. The changes in the morphology of cracks and shear bands as well as the significant extension of crack-free deformation of ultrathin films were well documented and explained by the developed models. Although metallic glasses are poorer electrical conductors of electric current than crystalline metals, it was demonstrated that the resistivity of metallic glasses does not grow with decreasing films thickness. Therefore, for very thin films (below 10 nm) the resistivity of metallic glasses becomes comparable with the resistivity of ultrathin crystalline films. The most significant contribution of the project lies in the fundamental understanding of the shear banding phenomenon. Based on the series of specifically developed in-situ experiments a new theory of shear banding was developed. The theory not only explains the experimental observations obtained during the project, but also allows to resolve some long-standing controversies in interpretation of the observed mechanical behavior of metallic glasses. In summary, the scientific progress achieved by the research work carried out during the project can be divided into two areas: (i) the synthesis of new amorphous alloys and the demonstration of their properties evolution due to size effects, and (ii) the formulation of the new concept of the shear banding phenomenon suggesting a paradigm shift in the interpretation of the deformation behavior of metallic glasses.