In-situ synchrotron X-ray investigation of microstructure evolution during creep in Grade 91, Grade 92, and 14YWT oxide dispersion strengthened steel

Abstract

Knowledge of the effects of creep on the microstructure of G91, G92, and 14YWT ODS steel is essential for their use in the high temperature and pressure environment of Generation IV reactors. Previous research on creep deformation in these three advanced reactor structural materials has focused on transmission electron microscopy (TEM) techniques to characterize the fracture properties and changes in the microstructure from creep deformation. TEM is inherently limited in that one can only sample a small portion of the specimen. Synchrotron X-ray scattering techniques allow for real-time in-situ thermo-mechanical characterization of a material, resulting in X-ray data that represents the grain averaged bulk behavior of the specimen. From analysis and interpretation of the X-ray scattering data, the evolution of the average dislocation density and coherently scattering domain size (e.g., subgrain size) was measured with increasing strain.

This dissertation presents the results of several in-situ synchrotron X-ray high-temperature creep measurements for tensile specimens of G91, G92, and 14YWT ODS steel. The high-temperature creep behavior and effects of creep on the microstructure of these three advanced materials were examined via several in-situ thermo-mechanical loading experiments with simultaneous X-ray diffraction (XRD) and small-angle X-ray scattering (SAXS) measurements at the Advanced Photon Source, beamline 1-ID. In addition to XRD and SAXS profile analysis, post-mortem scanning electron microscopy (SEM) images were taken of the tensile specimens to allow for direct observation of deformation induced voids, as observed in SAXS profile analysis.

The present work provides new insight into how the dislocation density and coherent scattering domain size change during high-temperature creep deformation in G91, G92 and 14YWT ODS steels at 650°C and 750°C via synchrotron X-ray experiments. The results presented here will contribute to the understanding of high-temperature creep deformation effects in these three advanced candidate reactor structural materials, and provide experimental data for future modeling endeavors of dislocation behavior during creep in advanced steels.

Due to the increase in the demand for energy, advanced nuclear power plants (i.e., Generation IV nuclear reactors) are encouraged to increase their operating temperature, to increase fuel burn-up, thereby increasing the energy efficiency and energy capacity of the reactor. The harsher environment of advanced reactor systems means that candidate structural material and fuel cladding material must be able to operate in an environment with elevated temperatures, pressures, radiation damage, and corrosion compared to current commercial light water reactor systems. The present work investigated the effects of high-temperature creep on the microstructure of three promising candidate structural materials for advanced reactor systems, namely Grade 91 (G91), Grade 92 (G92) and 14YWT-SM12 (14YWT) oxide dispersion strengthened (ODS) steels.

Phase-specific lattice strain was calculated from XRD data, for all three steel specimens, with increasing deformation. Phase-specific lattice strain for G91 and G92 steel specimens crept at 650°C, reflected a loss of load carrying capability of second phase precipitates, suggesting that the precipitates (i.e., M23C6 and Nb(C, N)) did not strengthen the matrix significantly during creep deformation. Modified Williamson-Hall (MWH) peak broadening analysis of several α-Fe diffraction peaks for G91, G92, and 14YWT ODS steels, illustrated a multi-phase dislocation density activity during high-temperature creep deformation. A pattern of hardening and recovery was observed for the first time in G91, and 14YWT steel specimens crept at 650°C and 750°C. Coherent scattering domain (CSD) size increased with increasing dislocation density, indicating the formation of subgrains, whereas CSD size decreased with recovery, due to a reduction in dislocation boundaries from reorganization and annihilation of dislocations. Microstructural characterization via SEM images and quantitative analysis of SAXS profiles, demonstrated that the 14YWT ODS steel specimen crept at 650°C developed a higher volume of deformation induced voids compared to G92 steel crept at the same temperature under similarly applied load(s), providing insight into the lower creep ductility of 14YWT ODS steel.