Development of High Resolution 2D and 3D Electron-Based Imaging Techniques for Materials Study

Abstract

Since the establishment of electron microscopy in natural science, the two dimensional (2D) visualized information has been seen as critical evidence of phenomena to contribute in many research fields. However, specifically, in the study of corrosion of metallic materials, corrosion develops via unpredicted pathways into the materials. Thus, capturing the overall phenomena only by 2D visualization renders it difficult to disclose in detail the determination factors of the corrosion path and the relationship between the corrosion and the surrounding microstructure. Therefore, development of three dimensional (3D) visualization methods is desired in this field. The combined techniques associating ultramicrotomy and low voltage scanning electron microscopy (SEM) with selective detection of backscattered electrons have been successfully developed, which is capable of generating sequential images yielding a stack of information generated equidistantly through the volume of interest, which allows construction of 3D images. This provides structural information that is previously unobtainable with conventional 2D techniques and other 3D techniques. This innovative approach can routinely achieve significantly improved depth and lateral resolutions and atomic number contrast (Z-contrast) sensitivity. However, during the slicing and image capturing process artefacts are often introduced since the microstructure on the block face is distorted due to applying of mechanical force by the diamond knife onto the block face. This might cause misleading results or misinterpretation of the microstructure of the material. Thus, the correction of the artefact introduced by the slicing process is critically important in image processing. Further, by the optimisation of the knife angle, oscillation and cutting speed, minimum damage block faces are able to be generated. The optimized 3D imaging technique with sub-nanometre spatial resolution developed by the Author, in combination with transmission electron microscopy has been successfully employed for the study of metallic materials degradation mechanisms. The understanding of the corrosion mechanism of copper-containing aluminium alloy AA2024 has been advanced. The initiation of corrosion is found to be associated with the S phase (Al2CuMg), which changes to copper-enriched sponge like structure through selective dissolution of electrochemically more active elements magnesium and aluminium. The 3D imaging technique combined with high resolution TEM has revealed the progress of dealloying of S phase particles in details, including morphological, compositional and structural changes at all length scales. The 3D imaging technique has also been successfully employed to study the copper enrichment phenomenon during corrosion and oxidation of copper-containing aluminium alloys. It is found that the preferential oxidation of aluminium atoms takes place at the corrosion front and, consequently, copper enriched layers are formed immediately beneath the corrosion products. Further, a copper-containing binary aluminium alloy after anodizing in ammonium pentaborate is used for identifying the crystal structure of the copper-enriched layer.

Date of Award	31 Dec 2017
Original language	English
Awarding Institution	The University of Manchester