Application of STEM characterization for investigating radiation effects in BCC Fe-based alloys
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Alicia G. Certainb) Janelle P. Wharrya) Boise State University, Department of Materials Science & Engineering, Boise, ID 83725, USA (Received 17 September 2014; accepted 16 January 2015)
This paper provides an overview of advanced scanning transmission electron microscopy (STEM) techniques used for characterization of irradiated BCC Fe-based alloys. Advanced STEM methods provide the high-resolution imaging and chemical analysis necessary to understand the irradiation response of BCC Fe-based alloys. The use of STEM with energy dispersive x-ray spectroscopy (EDX) for measurement of radiation-induced segregation (RIS) is described, with an illustrated example of RIS in proton- and self-ion irradiated T91. Aberration-corrected STEM-EDX for nanocluster/nanoparticle imaging and chemical analysis is also discussed, and examples are provided from ion-irradiated oxide dispersion strengthened (ODS) alloys. Finally, STEM techniques for void, cavity, and dislocation loop imaging are described, with examples from various BCC Fe-based alloys.
I. INTRODUCTION
There is growing interest in advanced Fe–Cr ferritic/martensitic (F/M) alloys, oxide dispersion strengthened (ODS) alloys, and nanostructured ferritic alloys (NFA) for nuclear applications due to their enhanced radiation tolerance, strength, and low activation.1–3 These BCC alloys typically have complex microstructures containing a high density of small grains, laths and subgrains, precipitate phases, and dislocation lines.2 As a result, performing transmission electron microscopic (TEM) imaging on these materials and their micro- to nanoscale features is challenging, because of the complexity of their microstructures and the resolution required. The high density of microstructural features produces significant contrast in bright-field (BF) TEM images, making it difficult, if not impossible, to discern features. BCC alloys are currently of interest for high radiation dose applications, specifically for doses greater than about 20 displacements per atom (dpa).1–4 These service conditions induce significant microstructural changes, including defect clustering, dislocation loop formation, void and bubble formation, radiation induced or enhanced precipitation, and radiation induced segregation (RIS).4 Contributing Editor: Djamel Kaoumi a) Address all correspondence to this author. e-mail: [email protected] b) Previously at Pacific Northwest National Laboratory, Richland, Washington 99354, USA DOI: 10.1557/jmr.2015.32
These irradiation-induced changes further complicate the microstructure of BCC alloys, hindering a thorough characterization using standard TEM-based techniques. Scanning transmission electron microscopy (STEM) is a powerful and versatile tool for characterizing metals and alloys. By changing the imaging conditions such as camera length, probe size, and foil tilt, the image contrast can be readily adjusted based on several guiding principles. In addition, STEM can be used to image nanoclusters (NCs) and precipitates, while simultaneously analyzing microche
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