Highlights

Defect Evolution and Phase Transformation in Iron-chromium

Scientific Achievement

Using the IVEM-Tandem system at the EMC to conduct in-situ examinations of irradiation effects in model iron-chromium alloys, we develop a fundamental understanding of the irradiated behavior of nuclear reactor structural steels.  With binary iron-chromium (FeCr) as a model, we study the formation, evolution, and migration of defects in ferritic alloys during and after radiation exposure.  In addition, we study the irradiation-induced second phase transformation and precipitation behaviors of the alloy. 

Based on data collected at the EMC, we have established a trend of decreased defect sizes with increasing chromium content of the irradiated FeCr.  In-situ observations of irradiation using the IVEM-Tandem system have led us to conclude that chromium prevents defect growth by inhibiting defect mobility, thus keeping defects from clustering and forming extended structures.  In the pure iron system, we observed highly mobile defects that formed complex clusters and large loops under irradiation.  By contrast, in Fe14Cr and Fe19Cr, we found that defects grew very little after their initial formation and did not appear to move.  These conclusions echo those of other groups studying these effects in lower-chromium FeCr alloys (University of Oxford, Penn State). 

We have also discovered the formation of a highly-coherent second-phase precipitate under annealing and irradiation at 550°C.  These rod-like precipitates emerge from the matrix in a crosshatched pattern in TEM micrographs.  We also see evidence of second-phase formation in the appearance of extra spots and streaks in the matrix diffraction pattern.

Significance

Utilizing the data obtained at the EMC, we aim to connect the observed microstructural changes in the irradiated material to changes in its mechanical properties.  In particular, we intend to relate the development of the observed precipitate structure to an increase in the ductile-to-brittle transition temperature.  The insight gained from these investigations will extend to increasingly complex iron alloy materials that are of interest for use in advanced reactor applications.  A greater understanding of radiation effects in ferritic alloy steels will be valuable for the development and more effective optimization of new advanced nuclear materials.  Identifying the structure of FeCr model alloys under varying temperature and dose conditions will also contribute to the development of more advanced atomistic models to study steels in nuclear systems. 

Performers

C. Tomchik, J. Stubbins (U. Illinois at Urbana-Champaign – NPRE); M. Kirk (Argonne-MSD); M. Okuniewski (INL); S. Maloy (LANL, MST-8)