A major recent advance has been the ability to investigate SCC cracks and crack tips using analytical transmission electron microscopy (ATEM). ATEM enables the characterization of SCC cracks including trapped solution chemistries, corrosion product compositions and structures, composition gradients and defect microstructures along the crack walls and at the crack tip. A wide variety of methods for imaging and analyses at resolutions down to the atomic level can be used to examine the crack and corrosion film characteristics. Surface films and reaction layers have been examined by cross-sectional TEM techniques, but limited work had been conducted on environmentally induced cracks until recently [1-7]. A critical aspect of the recent work has been the development of sample preparation methods in which the crack corrosion products are protected during the ion-thinning process by embedding the cracks with a low-viscosity thermoplastic resin. This capability combined with modern ATEM techniques has enabled new insights into corrosion processes occurring at buried (limited communication with the bulk environment and surface electrochemical conditions) interfaces and is being used to identify mechanisms controlling SCC in service components. The objective of this paper is to demonstrate capabilities of cross-sectional ATEM for the characterization of buried crack tips and corrosion interfaces at high resolution. New results are highlighted focusing on SCC in Fe- base and Ni-base stainless alloys. Examples are chosen to illustrate nanometer-scale structures that can only be examined effectively by ATEM methods. CRACK-TIP SAMPLE PREPARATION AND CHARACTERIZATION APPROACH The preparation of cross-section samples with suitable electron-transparent areas for high-resolution ATEM characterization was critical for the present work. The first step involves protecting the cracks by vacuum-impregnation with a low-viscosity thermosetting resin. Following impregnation, the section containing cracks is cut out and embedded in a stainless steel tube. The composite sample is then sliced with the main crack in cross section near the disk centers and mechanically polished. Dimple grinding is used to create a bowl-shaped depression at the area of interest. Thinning is then continued from both sides by low-angle ion micromilling with final milling performed at reduced beam energy and incident angle to remove most visible ion damage. Repeated cycles of ion thinning and TEM examination are applied to obtain suitably thin areas at crack tips or other locations of interest. Crack tips represent one of the most difficult regions to effectively protect, prepare and characterize. In nearly all the high-temperature water cases examined so far, cracks are heavily branched and are filled with corrosion products. It is possible that some of these products form during cooling from service temperatures, but most of the oxide phases are expected to form in hightemperature water and will probably restrict crack closure. A detailed description of cross-section sample preparation and possible artifacts has been published elsewhere [3,4]. ATEM characterizations were performed using a 200 kV field-emission-gun TEM with a thinwindow, energy-dispersive x-ray spectrometer (EDS) and a parallel-detection electron-energy-loss spectrometer (PEELS) for microchemical analysis. High-resolution ATEM methods were used to analyze the narrow corrosion features near crack tips and along attacked grain boundaries. Besides conventional brightfield and precipitate darkfield imaging, these methods included crystal lattice imaging with Fourier-transform diffraction analysis, fine-probe (0.7-nm diameter) compositional analysis by EDS and PEELS, and fine-probe parallel-beam diffraction with electron probes as small as 5 nm in diameter. In addition, stereoscopic TEM photographs were used to observe finely porous structures along attacked grain boundaries. A Fresnel (off-focus) image contrast method was employed to reveal fine pores and other structures as small as 1 nm. EXAMPLES OF CRACK-TIP CHARACTERIZATIONS Results from 316SS and alloy 600 materials that experienced intergranular (IG) SCC in hightemperature (250-320°C), deoxygenated water environments are used to illustrate high-resolution characterizations of crack-tip, crack-wall and corrosion structures. Specific information on water environments and material conditions is given elsewhere [4-7].
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