SUMMARY OF THE SPECIAL SESSIONS ON EBSD AND EAC OF SS IN PWRs- Chaired by Mychailo Toloczko and Armin Roth

The intent of this year’s part 1 of the special session was to provide examples of how electron backscatter diffraction (EBSD) can be used in SCC research. Four presentations were made. The first three presentations showed distinctly different uses of EBSD in SCC research, namely grain-averaged strain, nanometer-level strain, and grain orientation analysis, while the fourth presentation focused on using grain-average strain.

The first presentation by Alberto Sáez-Maderuelo (CIEMAT, Spain) provided an overview of how EBSD is able to provide an image showing spatial strain distributions down to the micrometer level along with how an average strain across multiple grains can be measured by various approaches such as integrated average local misorientation. Alberto also explained that EBSD is only able to detect geometrically necessary dislocations which are net dislocations that contribute to strain in a material. He then showed examples of how integrated average local misorientation has been used at CIEMAT for SCC research including for the purpose of measuring wide strains in weld mockups and in deformed surface layers. The final slides of his presentation described a new technique estimate stresses within a grain. He explained that this is a relatively new technique and provided a qualitative explanation of how it works.

The second talk given by Sergio Lozano-Perez (University of Oxford, UK) focused on a technique called transmission Kikuchi diffraction (TKD) which is capable of measuring strains within a grain down to the nanometer level. This is also a relatively new technique. He showed examples of how TKD can be implemented in an SEM and then followed with a qualitative explanation of how nanometer-scale dislocation density is obtained by this method. Sergio then showed how he is using it to measure strains in the vicinity of grain boundaries and correlating this behavior to SCC growth. He showed that strains around grain boundary precipitates in cold-worked materials are significantly higher than along grain boundaries away from grain boundary precipitates. In another example of its use, he showed how he has used it to characterize nanocrystalline grains in zircaloy fuel cladding.

The third presentation given by Catherine Guerre (CEA, France) showed how she has used EBSD to measure grain orientation in Alloy 82 and how this information has been used to study SCC behavior of that material. EBSD along with bicrystal finite element modelling revealed that SCC growth occurred through regions of fine grain size with varied crystallographic orientation, suggesting that it was more likely on high angle grain boundaries, especially along grain boundaries where the stress normal to the grain boundary is highest.

In the final talk given by Mychailo Toloczko of (PNNL, USA) EBSD was used to provide a relative measure of grain averaged strain and assess how this strain relates to SCC growth rate (SCCGR) of Alloy 690. The functional relationship of SCCGR to hardness (which is a measure of bulk YS) and material cold-work level were also made.  He first showed that EBSD grain average strain exhibits a power law relationship to hardness. Comparisons of SCCGR to material EBSD grain averaged strain, hardness, and applied cold-work level revealed that Alloy 690 SCCGR exhibits a power law strain hardening relationship with grain averaged strain. Importantly, it was found that SCCGR did not exhibit a universal correlation to grain average strain, with GB carbide-free Alloy 690 exhibiting a distinctly different correlation than heats with grain boundary carbides. It was hypothesized that this is due to differences in local strain at grain boundaries between grain boundaries with and without precipitates as shown in Sergio’s presentation.

The second part of this year’s special session was dedicated to EAC of stanless steels in PWRs (in the light of the recent EAC events in some French plants). (The summary of part 2 of the special session will be added as soon as it will be made available by the corresponding session chair.)

Peter Andresen (Honorary Member, USA), “Stress corrosion crack growth of stainless steels in pressurized water reactor primary water”.