In situ investigation of the dealloying process using environmental transmission electron microscopy

Dealloying is a corrosion process where a constituent of a metal alloy selectively dissolves out of the host metal, leaving behind a porous network. The process is thought to proceed by either surface diffusion or bulk diffusion. While the process has been studied extensively in the past, it was primarily on model systems such as single crystal gold-based alloys which are governed by the surface diffusion mechanism. This study seeks to provide two different examples of expanding the application of dealloying investigation closer to realistic systems. These are: investigating the role of defects – in particular, grain boundaries – in the dealloying of polycrystalline materials; and applying dealloying principles to other non-typical dealloying systems that undergo the bulk diffusion mechanism. Grain boundaries of multicomponent materials frequently exhibit segregation of a single element. This change in composition at the grain boundary was found to lead to significant changes in the propagation of the dealloying reaction through the boundaries – i.e., either by selective dissolution at the grain boundaries, or by self-limiting propagation through only a portion of boundaries – which is closely related to the parting limit of the alloy. In the case of bulk diffusion, the material needs to possess significant solid-state mobility for it to be active. Lithium alloy metals, a system that has been studied extensively as a promising next generation high capacity electrode material, have this quality and as such, may be able to serve as a new model system for the investigation of the bulk diffusion mechanism of dealloying. Indeed, it has been found that the resulting structure of the delithiation process resembles that of dealloyed structures via the bulk diffusion mechanism. This opens the possibility of understanding the bulk diffusion mechanism in greater detail, while at the same time assisting in the development of methods that may help improve the use of these materials in battery systems. The investigations in this study were carried out by the new environmental transmission electron microscopy, which enables the study of materials in non-vacuum environments with nanometer scale and sub-second resolution. This allows direct visual investigation of microstructural evolutions of materials as the reaction occurs, and may be useful in many different systems. Therefore, in addition to the investigation of dealloying systems, the properties of the technique – in particular, the effects of electron beam induced ionization in gaseous and liquid environments – has also been characterized. Such investigation benefits additional understanding of the new technique, providing necessary information for additional application of the technique to other systems