Understanding Oxidation Mechanism using In Situ Analytical Transmission Electron Microscopy

Environment-sensitive behaviour of materials encompasses a broad range of degradation phenomena in metals and alloys. The interaction of metallic materials with the environment is of fundamental importance in understanding a material’s performance in “real world” applications. Of particular significance is the effect of liquid and/or gaseous environments on the material of interest. The ability to visualize the localised changes associated with oxidation in gaseous environments and dissolution reactions in liquids coupled with qualitative STEM-XED spectrum imaging and analysis is now providing unprecedented opportunities for real-time observations that can lead to improved mechanistic understanding of nanoscale oxidation, and localised dissolution/corrosion.<br/>In this presentation, we will discuss the application of in situ gas microscopy to understand the oxidation behaviour of metallic materials using open cell and closed cell system. For the closed system, two 30 nm thick SiN electron transparent membranes are used to contain up to 1 bar atmospheric pressure from the vacuum of the TEM columns. We have applied the Protochips in situ platforms to examine gaseous environmental interactions in structural alloys such as Ni-base alloys (Alloy 600) and tungsten. The Protochips Atmosphere system interfaced with an FEI Titan G2 200 kV S/TEM equipped with X-FEG and Super X (4 SDDs) or a JEOL NeoARM 200kV wide gap pole piece cold FEG and Dual-EDS system has been successfully used in a variety of gaseous environments. Open cell experiments were performed on a Hitachi H-9500 TEM operating at 300 kV and fitted with an add-on gas injection system for windowless environmental conditions up to ~0.1 Pa at the sample.<br/>Critical to any in situ experiment is the preparation of representative electron-transparent samples, so as to provide a valid link with bulk behaviour. Electron-transparent specimens were prepared using the hybrid method [1]. These specimens can then be attached to an Atmosphere heating chip with Pt. A series of examples will be discussed that are related to the detailed study of bulk material behaviour including localised oxidation reactions pertinent to stress corrosion cracking in Ni-base alloys as well as the study of oxidation of tungsten alloys. The successful experiments using the gas reaction cell system in a variety of H2-containing environments at elevated temperatures can be further refined to assess variables such as H2 and O2 partial pressures, and can also be used to assess localised reactions in 1 bar gas over a range of temperatures of interest and thus provide insight at the nanoscale about diffusion-induced grain boundary migration, internal oxidation, and the role of carbides in preferential oxidation. Similarly, S/TEM imaging and Electron energy loss spectroscopy (EELS) was used for ex situ and in situ study of the oxidation of self-passivating tungsten alloys using. Mapping the elemental distribution of W, Cr, Y at the nanoscale shows that Cr-rich grains undergoes Cr/W phase separation exhibiting characteristics of spinodal decomposition, which is in good agreement with first-principles modelling predictions. Similar Cr/W phase separation was observed at grain boundaries. The in situ (S/TEM) oxidation shows that the Cr-rich grain exhibiting Cr/W phase separation are oxidized first and formed a dense Cr oxide, providing a stable oxide layer. Additionally, in the vicinity of the yttria precipitates present at grain boundaries or grain triple junctions the oxide growth is reduced in the initial stage of oxidation. Inhomogeneity in the layer oxidation could lead to localised failure of the oxide layer, such as delamination of the protective oxide layer.