Nanoscopic materials response to radiation and corrosion environments

Abstract

In this thesis, computational and experimental techniques are developed to study the response of materials to radiation and corrosion environments at nanoscale, respectively. Firstly, controlled ion radiation has become a popular tool for the fabrication and modification of nanostructured materials as well as understanding materials degradation in radiation environment. Here we aim to overcome a major limitation in current 1D Monte Carlo simulation codes for ion radiation, i.e., the incapability to predict the primary radiation damage in nanoscale ion implantation experiments. A prototype code in MATLAB named "Mat-TRIM", and a more advanced code in C-language named "IM3D", are developed to accurately capture the key physics of ion-mater interaction in nano-structured materials in three-dimensions (3D). Using IM3D, we revealed the nano-beam and nano-target effect of ion radiation.

We then quantified the relative error of 1D approach in several classical examples, showing significant relative errors of more than 1000% when the beam/target- size is close to or smaller than the range of ions, indicating the necessity of full-3D simulations. We also observed a topological evolution of point defects' distributions in 3D when beam-size varies. Also, radiation is a powerful characterization tool. In particular, in-situ environmental transmission electron microscopy (E-TEM) technique, using electron radiation for imaging, enables direct observation of materials corrosion at nano/atomic resolution. Using this technique, we directly visualized the deformation of 2nm-thick surface oxide on aluminum nanotips under oxygen environment. We showed the native aluminum oxide can deform like liquid and self-heal its branches quickly at room temperature, rendering a continuous oxide layer without fracture/spallation during deformation.

We also developed a "mechanical-break-junction" method to overcome the difficulty of preparing fresh metal surface in a TEM for initial oxidation studies. A contrast experiment to aluminum oxidation is performed for zirconium alloy, a metal which is used as the cladding in water-cooled reactors. We in-situ observed the oxidation-induced crack/pore evolution at nanoscale. The crack/pores in oxide will form a percolated network, leading to the failure of oxide as a passivation layer. Our observations demonstrated that the plasticity of metal oxide is crucial for the oxidation resistance of metals.