SOLUTION ANNEALED AND THERMALLY TREATED ALLOY 600 PREFERENTIAL INTERGRANULAR OXIDATION: A COMPARISON

摘要

It is well-known that a semi-continuous intergranular carbides network improves Alloy 600 Stress Corrosion Cracking resistance in PWR primary water environment. However, the main reason responsible for the beneficial effect of intergranular carbides is still unclear and under debate. In this study the carbides effect on the preferential intergranular oxidation susceptibility and local grain boundaries (GB) migration of Alloy 600 was studied in details. Solution-annealed (SA) and thermally treated (TT) Alloy 600 oxidation coupons were exposed for 120 hours in hydrogenated steam environment at 480°C and analyzed afterwards with several characterization techniques. Secondary electrons (SE) and backscattered electrons (BSE) scanning electron microscopy (SEM) analysis of the surface and of Focused Ion Beam (FIB) cross-sectioned GB revealed a marked difference between SA and TT Alloy 600 intergranular oxidation susceptibility. High resolution scanning transmission electron microscope (STEM) - energy dispersive x-ray (EDX) microanalysis of on in-situ FIB lift-out specimens containing at least one GB revealed the presence of a Cr-Fe rich oxide at the GBs as well as a marked micro-chemical elemental redistribution in the near surface region for both SA and TT Alloy 600. However, comparing SA and TT GBs oxidation and elemental redistribution it was observed a noticeable difference in terms of intergranular oxide penetration and local GB migration. These results are discussed in this paper in terms of carbides consumption, GBs pinning and local carbon content in solid solution.