In-situ TEM Studies: Heat-treatment and Corrosion

Abstract

Transmission electron microscopy (TEM) has been well known as a powerful characterisation tool to understand the structure and composition of various materials down to the atomic level. Over the years, several TEM studies have been carried out to understand the compositional, structural and morphological changes a material undergoes as a consequence of an external stimulus (thermal, environmental, electrical as well as mechanical) post mortem. With the recent advancements in the areas of TEM and microelectronics, it is now possible to integrate the external stimuli in the TEM, making it possible to carry out in situ TEM studies. In the present study, we have used microelectromechanical system (MEMS) based devices to investigate heat-treatment and corrosion in situ in a commercial aluminium alloy, AA 2024-T3. Aluminium alloys go through several thermal and mechanical treatments before a final product is formed and during these processes, they undergo a complex compositional and structural evolution at the atomic level which in turn influences their properties like mechanical strength and resistance to corrosion. One of the common microstructural features of most of the commercial alloys is the formation of numerous nanometre sized second phase particles, known as intermetallic precipitates during heat-treatment. By investigating different samples taken at intermediate stages during the heat-treatment of bulk samples, a sequence of precipitation and its influence on the mechanical properties has been established. However, a link between all these stages by investigating the same location in one sample as a function of time and temperature is missing. In this study, using in situ heating in a high-resolution scanning transmission electron microscope (STEM), we have investigated the three-dimensional compositional and structural evolution of metal alloys during heat treatments, revealing in unparalleled detail where and how precipitates nucleate, grow or dissolve. The next part of the study is related to the influence of the nanoscale intermetallic particles on the corrosion behaviour of aluminium alloys. Due to the differences in the electrochemical potentials between the intermetallic phases and the aluminium matrix, most of the commercial aluminium alloys are highly susceptible to a localised corrosive attack. This phenomenon has been well investigated by quite a few ex situ electrochemical methods combined with analytical microscopic techniques. Here, we investigate the corrosion in situ in a gas-liquid-material system using a functional MEMS device called nanoreactor. As there have been no TEM studies investigating electrochemical corrosion of aluminium alloys in situ in a TEM, we have decided to investigate the well-studied AA2024-T3 alloy system to validate our approach. In order to determine a suitable experimental window prior to the in situ TEM studies, we have carried out ex situ and quasi in situ corrosion studies on conventional TEM specimens. Using analytical TEM studies like electron energy loss spectroscopy (EELS), energy filtered TEM (EFTEM) and energy dispersive spectroscopy of X-rays (EDX), we observe that oxygen bubbled through aqueous HCl is a suitable environment for carrying out in situ corrosion experiments in the TEM at room temperature. Using these conditions, we have investigated the initiation of localised corrosive attack in AA2024-T3. Finally, using our quasi in situ approach, we have also carried out some preliminary investigations on understanding the corrosion inhibition mechanisms of Ce-based inhibitors. The in situ TEM heat-treatment and analytical techniques used in this study are expected to accelerate investigations on new alloy compositions suggested by computational methods. Environmental TEM studies using the nanoreactor can be extended to investigate microstructural and morphological changes during chemical reactions in various gas-liquid-material systems on the nanoscale, combined with the influence of temperature. Therefore, this study expands the scope of TEM as not just a characterisation tool, but also as a laboratory to carry out many interesting in situ experiments on the nanoscale.