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DOE OSTI2012

Phase Stability of an HT-9 Duct Irradiated in FFTF

A fuel test assembly known as ACO-3 duct made out of a fully tempered ferritic/martensitic steel (HT-9) was previously irradiated in the Fast Flux Test Reactor Facility (FFTF) up to 155 dpa at a temperature range of 380-504°C. The microstructures of the samples from 5 different zones along the face of the duct were analyzed using a combination of TEM based techniques, SANS and APT. A high density of Cr rich a' precipitates together with moderate density G-phase precipitates with an average sizes of 5 and 11 nm respectively were found at 20 dpa, 380°C zone. It was found that the precipitations of the second phases are more sensitive to temperature then the dose. In general, the density of both precipitates decreases with increasing irradiation temperature. No significant change is observed in average size of a' while the average size of G-phase precipitates increases up to 27 nm at 440°C. Voids are seen at 100 (410°C) and 155 (440°C) dpa zones but none was detected at 96 dpa (466°C) zone. In contrast to what is reported in the literature, no laves or Chi phases were found in any of the zones.

36 MATERIALS SCIENCE↗

DOE OSTI2011

Reactive gas atomization processing for Fe-based ODS alloys

Gas atomization reaction synthesis was employed as a simplified method for processing oxide dispersion forming precursor Fe-based powders (e.g., Fe–Cr–Y–Hf). During this process a reactive atomization gas (i.e., Ar–O2) was used to oxidize the powder surfaces during primary break-up and rapid solidification of the molten alloy. This resulted in envelopment of the powders by an ultra-thin (t < 50 nm) metastable Cr-enriched oxide shell that was used as a vehicle to transport oxygen into the consolidated microstructure. Subsequent elevated temperature heat treatment promoted thermodynamically driven oxygen exchange reactions between trapped films of Cr-enriched oxide and internal (Y, Hf)-enriched intermetallic precipitates, resulting in highly stable nano-metric mixed oxide dispersoids (i.e., Y–Hf–O) that were identified with X-ray diffraction. Transmission electron microscopy and atom probe tomography results also revealed that the size and distribution of the dispersoids were found to depend strongly on the original rapidly solidified microstructure. To exploit this, several oxide dispersion strengthened microstructures were engineered from different powder particle size ranges, illustrating microstructural control as a function of particle solidification rate. Additionally, preliminary thermal–mechanical processing was used to develop a fine scale dislocation substructure for ultimate strengthening of the alloy.

36 MATERIALS SCIENCE↗