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Focused Ion Beam (FIB) micromachining techniques have gained significant attention over the past few years as a promising method for the preparation of a variety of metallic and nonmetallic materials for subsequent characterization using transmission electron microscopy (TEM) The advantage of the FIB in terms of site specificity and speed for the preparation of uniform electron transparent sections has opened a wide range of potential applications in materials characterization. The ability to image the sample in the FIB can also provide important microstructural data for materials analysis. In this study, both conventionally electropolished and FIB-ed specimens were prepared in order to characterize the microstructure of a commercially-produced tube of Alloy 600 (approximately Ni-15 Cr-10 Fe- 0.05 C). The electropolished samples were prepared using a solution of 20% HClO4- 80% CH3OH at ∼-40°C. The FIB sections were obtained from a cross-section of the tube that had been mechanically thinned to ∼100 μm. The section was thinned in a Micrion 2500 FIB system with a Ga ion beam at 50 kV accelerating voltage.

Ni-Cr-Fe alloys such as Alloy 600 and Alloy 690 are single phase fee alloys with excellent resistance to corrosion. Alloy 600 is generally used in a thermally-treated condition which is characterized by the intergranular precipitation of Cr-rich M7C3and, depending upon the aging temperature, Cr-rich M23C6. These alloys are usually cast and processed by rolling or forging. These alloys can exhibit microstructures in which there are nonuniform distributions of intragranular carbides that appear as bands aligned parallel to the working direction (i.e. longitudinal) of the material. It is also possible for these materials to exhibit duplex grain sizes which can be associated with the banded carbide structure. Banded microstructures can become more prominent after further anneals. The resistance of the carbide bands to dissolution during solution annealing at temperatures up to 1150°C has prompted the further evaluation of banding in Alloy 600, specifically in terms of identifying those elements associated with the carbide bands.

Previous studies on the stress corrosion cracking (SCC) behavior of Alloy 600 forgings revealed a correlation between the SCC susceptibility and thermomechanical processing (specifically the final annealing temperature). (1) Detailed microstructural analyses using analytical electron microscopy (AEM) techniques showed that the materials containing dendritic-type intergranular M7C3carbides were also more resistant to SCC initiation and propagation. This type of analysis has been extended to the evaluation of a 0.062 wt.% C Alloy 600 plate in three conditions (as-received, and heat-treated for 1 week at 982°C and water-quenched, and annealed for 24h at 1075°C and furnace-cooled). Tests were performed in PWR environments using fatigue precracked compact tension (CT) samples. In addition, detailed microstructural analyses using light optical metallography (LOM) and AEM were performed to fully characterize each material condition. The SCC fracture morphology was evaluated using scanning electron microscopy (SEM).LOM examination of the as-received material indicated a “good” microstructure, with carbide-decorated grain boundaries as well as intragranular carbides.