Abstract
Melt spinning of an Fe-5Y and Fe-1Y-1Ti (wt pct) alloy produced a relatively uniform spatial distribution of Y and Ti in solid solution and ribbons with consistent yield (> 60 pct by weight), fast processing time (< 10 seconds), good scalability (up to > 100 g feedstock material), and repeatability. Heat treatment in the presence of Fe2O3 as an oxygen source (Rhines pack method) at 973 K validated the potential of forming < 20 nm Y-rich oxides in the Fe-5Y ribbons. Pulverized Fe-1Y-1Y ribbons were consolidated to bulk using the field-assisted sintering technique (FAST) incorporating nano-sized Fe3O4 powder as the oxygen source. After FAST at 1273 K, 50 MPa, and 30 minutes, a comparatively high number density of sub-micron Y and/or Ti-rich oxides were developed. Further formation of fine-scale oxides took place during post-FAST annealing, resulting in an approximate 20 pct increase in hardness at temperatures below 573 K, but with a reduced hardening effect above 673 K due to a small fraction of persistent porosity and mechanically weak prior ribbon boundaries that were decorated with Ti-rich oxides.
Highlights
The first step was to validate the melt spinning process for a model Fe-Y alloy with a non-typical high concentration of 5 wt pct chosen to (i) account for evaporative/reactive loss of Y that is well-known in the foundry industry, and (ii) to ensure any resulting Y-rich oxides could be readily resolved without the need to use time-consuming, very high resolution microscopy techniques
These results provided encouragement that a relatively uniform Y distribution in solid solution with Fe, without excessive microsegregation at the micron-scale, could be achieved by melt spinning, with a tolerable loss of Y
Since the recovery and recrystallization of oxide dispersion-strengthened (ODS) steels is determined by the balance between the driving force dominated by stored energy of cold work and the retarding force exerted by the Y- and/or Ti-enriched oxides,[39,41,42] another possible factor for the relatively homogeneous grain size distribution in the annealed alloy may be the lower amount of work hardening for the pulverized melt-spun ribbons compared with conventional mechanical alloying (MA) powder, which reduced the driving force for recrystallization and grain growth at elevated temperature during consolidation
Summary
Steels that contain a high number density but low volume fraction of evenly distributed Y-, Ti-, and O-enriched nano-sized precipitates have emerged as one of the most promising structural material candidates for Generation IV fission and future fusion power plant concepts.[1,2,3,4,5] The widely practiced processing route for these ODS steels is essentially a two-step powder metallurgy process, consisting of mechanical alloying (MA) of 10- to 90-lm-diameter pre-alloyed Fe-based alloy powder together with a normally nano-sized (20 to 50 nm) Y2O3 powder until fine-scale mixing/alloying is achieved, followed by consolidation of the powder into a bulk form typically by hot isostatic pressing (HIP) or related technique.[5,6,7,8,9,10] This MA approach is well optimized and convenient for laboratory-based studies, providing good quality material sufficient for detailed. While key properties of ODS alloys, such as creep and irradiation resistance, are attractive, it is worthwhile to continue to explore alternative fabrication routes to replace/circumvent the MA process that might potentially achieve a higher throughput more suited for industrial production, probably with some, but acceptable, compromises in microstructure and mechanical properties Recent efforts in this direction include in situ oxidation of a Y-containing melt during gas atomization, oxidation of a gas atomized Y-containing powder, and spray forming of a Y-containing melt.[17] While providing some encouragement, so far none of these approaches have developed to the point that they may be considered likely as replacements for MA-based processing. Two alloys were studied: Fe-5Y (wt pct, MS01) with a comparatively high Y concentration to facilitate easy identification and study of any internal oxidation response, and Fe-1Y-1Ti (MS02) to confirm the scalability of melt spinning, to explore a more dilute composition closer to widely explored MA ODS steels, and to investigate the feasibility of internal oxidation during consolidation
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