Characterization of Solution‐Annealed and Thermally‐Treated Alloy 600 Intergranular Oxidation using Advanced Analytical Electron Microscopy

Abstract

The susceptibility of Ni‐Cr‐Fe alloy (Alloy 600) to intergranular stress corrosion cracking (IGSCC) in light water reactor (LWR) primary water environment is well‐known. However, SCC initiation mechanism is still unclear and under debate, especially the key parameters responsible for the general and localised “internal” oxidation susceptibility of Alloy 600 in LWR primary environment require elucidation to develop a mechanistic understanding of the phenomena and to assess their relative importance. It is well‐known that the precipitation of intergranular M 7 C 3 carbide network improves Alloy 600 Stress Corrosion Cracking resistance in PWR primary water environment. However, the main reason for the beneficial effect of intergranular carbides is still uncertain. In this study, the effect of grain boundary carbides on the preferential intergranular oxidation susceptibility and local grain boundary (GB) migration of Alloy 600 has been studied in detail. Solution‐annealed (SA) and thermally‐treated (TT) Alloy 600 oxidation coupons were exposed for 120 hours in hydrogenated steam environment at 480°C. This oxidation system successfully simulated PWR oxide morphologies [1‐3]. A combination of field emission gun (FEG) scanning electron microscopy (SEM), focused ion beam (FIB) microscopy and analytical electron microscopy (AEM) techniques have been used to characterize in detail the type and extent of preferential oxidation associated with the development of SCC initiation sites. The surfaces of the oxidized Alloy 600SA and Alloy 600TT specimens were evaluated in an FEI Magellan XHR FEG‐SEM using both secondary electron (SE) and backscattered electron (BSE) modes. High resolution SEM characterization revealed a marked difference between the two thermal treatments especially in terms of HAGB undulations and oxide GB structure, Fig. 1. Surface oxide morphology was correlated with intergranular oxidation susceptibility and local GB migration by using FIB cross‐sections analysis and TEM examination (Fig. 2). Further AEM analyses were performed using the FEI Titan G2 80‐200 aberration‐corrected S/TEM with Super EDX. Detailed scanning transmission electron microscope (STEM) ‐ energy dispersive x‐ray (EDX) microanalysis of FIB TEM lift‐out specimens containing at least one grain boundary revealed the presence of a Cr‐Fe rich oxide at the grain boundaries and a marked microchemical redistribution in the near‐surface region for both SA and TT Alloy 600 (Fig. 3, Fig. 4). However, a noticeable difference in terms of intergranular oxide penetration and local grain boundary migration was observed when comparing the solute segregation/depletion and Intergranular oxidation between the SA and TT specimens.