Deformation Behavior of Bulk Metallic Glasses and High Entropy Alloys by Nanoindentation

Nanoindentation has become a powerful tool in the measurement of the mechanical properties of diverse materials, such as metallic materials, polymer materials, and even biomaterials.

In this thesis, three types of Zr-based bulk metallic glasses (BMGs) were investigated by nanoindentation. Our work focuses on the characterization of the hardness, the reduced modulus, and the deformation behavior under different indentation conditions. The study of the hardness and the reduced modulus is to access the effect of the indentation load on deformation behavior and to determine the inhomogeneous deformation. The morphological profiles of the residual indentation on the surface of the specimen after an indentation were observed by the atomic force microscope (AFM). Differential scanning calorimetry (DSC) measurements were performed to determine characteristic thermal properties, the glass transition temperature (T_g), and the crystallization temperature (T_x).

The serrated-flow behavior (or pop-in behavior) was investigated at different loading rates. It is concluded that the pop-in size gradually increases with the decrease in the loading rate and the increase of the indentation depth. And the research of the indentation tests on the several metallic glasses at different indentation rates indicates that a much higher critical strain rate will lead to the disappearance of flow serrations.

Another type of material of a high-entropy alloy (HEA) was also investigated in this thesis. The hardness, reduced modulus, and deformation behavior were investigated by the indentation tests. Compared to Zr-based BMGs, this type of HEA has lower hardness and higher reduced modulus. Creep behavior was observed in the indentation tests. However, serrated flow behavior disappears. The microstructure of this HEA was investigated by the X-ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM), and energy-dispersive spectroscopy (EDS). For the advanced research, the simulation of ion-implantation of HEAs was preformed, because the advanced reactor is one of the important potential applications of HEAs and advanced nuclear-energy systems, which will require materials that can withstand extreme reactor environments of high-temperature and high-doses radiation.