Oxide dispersion strengthened (ODS) steels exhibit exceptional radiation resistance and hightemperature creep strength when compared to traditional ferritic and ferritic/martensitic (F/M) steels. Their excellent mechanical properties result from very fine nanoparticles dispersed within the matrix. In this work, we applied a high-energy synchrotron radiation X-ray to study the deformation process of a 9Cr ODS steel. The load partitioning between the ferrite/martensite and the nanoparticles was observed during sample yielding. During plastic deformation, the nanoparticles experienced a dramatic loading process, and the internal stress on the nanoparticles increased to a maximum value of 3.5 GPa, which was much higher than the maximum applied stress (~986 MPa). After necking, the loading capacity of the nanoparticles was significantly decreased due to a debonding of the particles from the matrix, as indicated by a decline in lattice strain/internal stress. Due to the load partitioning, the ferrite/martensite slightly relaxed during early yielding, and slowly strained until failure. This study develops a better understanding of loading behavior for various phases in the ODS F/M steel.

This work reports comprehensive investigations on the orientation relationship of the oxide nanoparticles in a hafnium-containing austenitic oxide dispersion-strengthened 316 stainless steel. The phases of the oxide nanoparticles were determined by a combination of scanning transmission electron microscopy-electron dispersive X-ray spectroscopy, atom probe tomography and synchrotron X-ray diffraction to be complex Y-Ti-Hf-O compounds with similar crystal structures, including bixbyite Y 2 O 3 , fluorite Y 2 O 3-HfO 2 solid solution and pyrochlore (or fluorite) Y 2 (Ti, Hf) 2−x O 7−2x. High resolution transmission electron microscopy was used to characterize the particle-matrix interfaces. Two different coherency relationships along with one axis-parallel relation between the oxide nanoparticles and the steel matrix were found. The size of the nanoparticles significantly influences the orientation relationship. The results provide insight into the relationship of these nanoparticles with the matrix, which has implications for interpreting material properties as well as responses to radiation.

a b s t r a c tAn oxide dispersion-strengthened (ODS) 316 steel was developed to simultaneously provide the advantages of ODS steels in mechanical strength and radiation tolerance as well as the excellence of austenitic steels in creep performance and corrosion resistance. The precipitate phases within the austenite matrix were identified by the combined techniques of atom probe tomography (APT), scanning transmission electron microscopy equipped with electron dispersive X-ray spectroscopy (STEM-EDS), and synchrotron wide-angle and small-angle X-ray scattering (WAXS and SAXS). Coarse TiN, hexagonal YAlO 3 and orthorhombic YAlO 3 precipitates were found along with fine Y-Ti-O nanoparticles. In situ WAXS experiments were performed at room and elevated temperatures to examine the size effect on the load partitioning phenomenon for TiN, hexagonal YAlO 3 and Y 2 Ti 2 O 7 phases. In addition, the dislocation density evolution throughout the tensile tests was analyzed by the modified Williamson-Hall method and confirmed by transmission electron microscopy (TEM) observations, revealing the difference in plasticity at various temperatures.

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Austenitic oxide dispersion-strengthened (ODS) alloys provide excellent mechanical strength and radiation tolerance along with their intrinsic advantages in corrosion resistance and high temperature creep resistance. This paper reports the in-situ synchrotron X-ray diffraction (XRD) tensile test results of ODS 304 stainless steel specimens. The oxygen-enriched nanoparticles were first characterized by both atom probe tomography (APT) and analytic scanning transmission electron microscopy (STEM). Three different types of precipitate phases were recognized, including large scale (around 100nm) TiN, intermediate scale (around 20nm) Y-Al-O, and small scale (< 5nm) Y-Ti-O. The lattice responses of different phases within the alloy to the externally applied stress indicates a prominent load partitioning phenomenon. This phenomenon was found to be highly dependent on the size of the precipitates. In addition, deformation-induced martensitic transformation was examined by the modified Williamson-Hall analyses of peak broadening, and was found to be different from that in ordinary 304 stainless steel.

Atom probe tomography (APT) was performed to study the effects of Cr concentrations, irradiation doses and irradiation temperatures on α′ phase formation in Fe-Cr model alloys (10-16 at.%) irradiated at 300 and 450°C to 0.01, 0.1 and 1 dpa. For 1 dpa specimens, α′ precipitates with an average radius of 1.0-1.3 nm were observed. The precipitate density varied significantly from 1.1x10 23 to 2.7x10 24 1/m 3 , depending on Cr concentrations and irradiation temperatures. The volume fraction of α′ phase in 1 dpa specimens qualitatively agreed with the phase diagram prediction. For 0.01 dpa and 0.1 dpa, frequency distribution analysis detected slight Cr segregation in high-Cr specimens, but not in Fe-10Cr specimens. Proximity histogram analysis showed that the radial Cr concentration was highest at the center of α′ precipitates. For most precipitates, the Cr contents were significantly lower than that predicted by the phase diagram. The Cr concentration at precipitate center increased with increasing precipitate size.

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