Using local GND density to study SCC initiation

Strain and geometric necessary dislocations (GNDs) have been mapped with nm-resolution around grain boundaries affected by stress corrosion cracking (SCC) or intergranular oxidation with the aim of clarifying which local conditions that trigger SCC initiation of Alloy 600 in primary water reactor (PWR) water environment. Regions studied included the cracked and uncracked portion of the same SCC-affected grain boundaries and a comparable grain boundary in the as-received condition. High-resolution "on-axis" Transmission Kikuchi Diffraction (TKD) was used to generate strain and GND density based on the cross-correlation image processing method to probe shifts of specific zone axis in the TKD patterns from all regions. All cracked boundaries analyzed had local GND densities higher than 1 × 10¹⁶ m^-2. Similar grain boundaries, from as-received samples had GNDs of 5 × 10¹⁴ m^-2, while an intermediate level was found in the oxidized but uncracked portion of the same GB. Results, together with a discussion on the advantages and limitations of the approach, will be presented.