Xuan Zhang

This paper presents the first measurement of neutron-irradiated microstructure using far-field high-energy X-ray diffraction microscopy (FF-HEDM) in a high-temperature ultrafine-precipitate-strengthened (HT-UPS) austenitic stainless steel. Grain center of mass, grain size distribution, crystallographic orientation (texture), diffraction spot broadening and lattice constant distributions of individual grains were obtained for samples in three different conditions: non-irradiated, neutron-irradiated (3dpa/500 °C), and irradiated + annealed (3dpa/500 °C + 600 °C/1 h). It was found that irradiation caused significant increase in grain-level diffraction spot broadening, modified the texture, reduced the grain-averaged lattice constant, but had nearly no effect on the average grain size and grain size distribution, as well as the grain size-dependent lattice constant variations. Post-irradiation annealing largely reversed the irradiation effects on texture and average lattice constant, but inadequately restored the microstrain.

Microstructural changes resulted from neutron irradiation and post-irradiation annealing in a high-temperature ultra-fine precipitate strengthened (HT-UPS) stainless steel were characterized using transmission electron microscopy (TEM) and atom probe tomography (APT). Three HT-UPS samples were neutron-irradiated to 3 dpa at 500?°C, and after irradiation, two of them were annealed for 1?h?at 600?°C and 700?°C, respectively. Frank dislocation loops were the dominant defect structure in both the as-irradiated and 600?°C post-irradiation-annealed (PIAed) samples, and the loop sizes and densities were similar in these two samples. Unfaulted dislocation loops were observed in the 700?°C PIAed sample, and the loop density was greatly reduced in comparison with that in the as-irradiated sample. Nano-sized MX precipitates were observed under TEM in the 700?°C PIAed sample, but not in the 600?°C PIAed or the as-irradiated samples. The titanium-rich clusters were identified in all three samples using APT. The post-irradiation annealing (PIA) caused the growth of the Ti-rich clusters with a stronger effect at 700?°C than at 600?°C. The irradiation caused elemental segregations at the grain boundary and the grain interior, and the grain boundary segregation behavior is consistent with observations in other irradiated austenitic steels. APT results showed that PIA reduced the magnitude of irradiation induced segregations.

High performance materials that can withstand radiation, heat, multiaxial stresses, and corrosive environment are necessary for the deployment of advanced nuclear energy systems. Nondestructive in situ experimental techniques utilizing high energy x-rays from synchrotron sources can be an attractive set of tools for engineers and scientists to investigate the structure-processing-property relationship systematically at smaller length scales and help build better material models. In this study, two unique and interconnected experimental techniques, namely, simultaneous small-angle/wide-angle x-ray scattering (SAXS/WAXS) and far-field high-energy diffraction microscopy (FF-HEDM) are presented. The changes in material state as Fe-based alloys are heated to high temperatures or subject to irradiation are examined using these techniques.

In this study, we used far-field high-energy x-ray diffraction microscopy (FF-HEDM) to measure in 3D the spatial distributions of the sizes, orientations, and residual strains of grains and subgrains formed by room-temperature tensile deformation in a neutron-irradiated (450?°C, 0.01dpa) Fe-9Cr specimen and its unirradiated counterpart. We found that neutron irradiation under this condition alone had no effect on the grain size distribution. After deformation, grains fragmented into subgrains in both unirradiated and irradiated specimens: the irradiated specimen contained a few large subgrains which co-existed with many smaller subgrains, while the unirradiated specimen contained small subgrains with a relatively uniform size distribution. Prior to deformation, the irradiated specimen had higher residual strain spread compared to its unirradiated counterpart, while after deformation to the maximum uniform elongation, the strain distributions among subgrains were similar between the unirradiated and irradiated specimens. The FF-HEDM measurements provide new insight into the effects of neutron irradiation on the mechanical response of Fe-Cr ferritic alloys.

The effect of neutron irradiation on tensile deformation of a Fe-9wt%Cr alloy was investigated using in situ high-energy synchrotron X-ray diffraction during room-temperature uniaxial tension tests. New insights into the deformation mechanisms were obtained through the measurements of lattice strain evolution and the analysis of diffraction peak broadening using the modified Williamson-Hall method. Two neutron-irradiated specimens, one irradiated at 300 °C to 0.01 dpa and the other at 450 °C to 0.01 dpa, were tested along with an unirradiated specimen. The macroscopic stress–strain curves of the irradiated specimens showed increased strength, reduced ductility and reduced work-hardening exponent compared to the unirradiated specimen. The evolutions of the lattice strain, the dislocation density and the coherent scattering domain size in the deformation process revealed different roles of the submicroscopic defects in the 300 °C/0.01 dpa specimen and the nanometer-sized dislocation loops in the 450 °C/0.01 dpa specimen; the dislocation loops were more effective in dislocation pinning. While the work hardening rate of stage II was unaffected by irradiation, significant dynamic recovery in stage III in the irradiated specimens led to the early onset of necking without stage IV as observed in the unirradiated specimen.

The tensile deformation behavior of a high-temperature, ultrafine-precipitate strengthened (HT-UPS) stainless steel was characterized in-situ with high-energy X-ray diffraction at 20 and 400?°C. The HT-UPS samples were neutron irradiated to 3 dpa at 400?°C. Significant irradiation hardening and ductility loss were observed at both temperatures. Lattice strain evolutions of the irradiated samples showed a strong linear response up to near the onset of the macroscopic yield, in contrast to the unirradiated HT-UPS which showed a pronounced non-linear behavior well below the macroscopic yield. While the room-temperature diffraction elastic moduli in the longitudinal direction increased after irradiation, the 400?°C moduli were similar before and after irradiation. The evolution of the {200} lattice strain parallel to the loading axis () showed unique characteristics: in the plastic regime, the evolution of after yield is temperature-dependent in the unirradiated specimens but temperature-independent in the irradiated specimens; and the value of at the yield is an irradiation-sensitive, temperature-independent parameter. The evolution of corresponds well with the dislocation density evolution, and is an effective probe of the deformation-induced long-range internal stresses in the HT-UPS steel.

Because of its superior creep and corrosion resistance as compared with general austenitic stainless steels, NF709 has emerged as a candidate structural material for advanced nuclear reactors. To obtain fundamental information about the radiation resistance of this material, this study examined the microstructural evolution of NF709 subjected to neutron irradiation to 3 displacements per atom at 500 °C. Transmission electron microscopy, scanning electron microscopy, and high-energy x-ray diffraction were employed to characterize radiation-induced segregation, Frank loops, voids, as well as the formation and reduction of precipitates. Radiation hardening of ∼76% was estimated by nanoindentation, approximately consistent with the calculation according to the dispersed barrier-hardening model, suggesting Frank loops as the primary hardening source.