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Title: Manufacturing of oxide dispersion strengthened steels for nuclear applications
Author: Connolly, Sarah C.
ISNI:       0000 0004 7653 0147
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2018
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This work concerns developing understanding of powder processing of oxide dispersion strengthened (ODS) ferritic steels for fuel cladding tubes in fission nuclear reactors. Ferritic ODS steels are candidate materials for these applications due to their good creep resistance that arises from the pinning of grain boundaries by a distribution of inert nanoparticles, as well as the pinning of radiation-induced defects such as vacancies, interstitials and He bubbles. Industrial uptake of ODS steels has been limited by their expensive and labour intensive production that typically includes a 60 h mechanical alloying (MA) step to dissolve the oxide particles into the heavily worked ferrite matrix. The key parameters of the MA process have been explored and re-optimised using a Fe-14Cr-3W-0.3Ti+0.25Y2O3 powder resulting in a 33% reduction in the time-consuming MA stage while maintaining all key microstructural features such as the acceptable dispersion of Y and O, and induced levels of mechanical work. An alternative ODS powder production method was also investigated in which a supersaturated Fe-Y solid solution was formed during the gas atomisation of Fe14Cr-3W-0.3Ti-0.4Y. The resulting powder was mixed with Fe3O4 nanoparticles in a short milling step to disperse but not dissolve the Fe3O4 and to introduce some mechanical work considered important for the subsequent fine-scale nucleation of Y-rich oxides by 'internal oxidation' of Y reacting with sacrificial Fe3O4 at elevated temperature. MA and atomised powders were sintered by hot isostatic pressing (HIP) and the field assisted sintering technique (FAST), which offered further savings in process time, from 4 h to only 5 min. Four ODS materials were studied in detail, from the two types of powder and the two sintering methods, revealing differences in both the nano-scale oxides micro-scale ferritic grains. Fine particles in the gas atomised alloy were primarily TiOx and not the Y-Ti rich oxides usually seen in MA material due to the lower oxygen afinity of Ti compared to Y. Long term thermal annealing at reactor relevant temperatures (650◦C) showed the ferritic grain structures to be stable. Following long exposure (675 h), Y-Ti rich oxides were identified. Electro-thermal mechanical testing (ETMT) was used to investigate tensile properties. MA Fe-14Cr-3W-0.3Ti+0.25Y2O3 consolidated by HIP and FAST had similar strengths to the literature, but brittle failure when tested at 350◦C or below because of the limited plasticity deformation mechanisms active in this temperature range; ductility at 650◦C was more typical. Fe-14Cr3W-0.3Ti-0.4Y+Fe3O4 showed good strength, out-performing the literature across the testing temperature range, but suffered from brittle failure with little elongation due to dislocation pinning mechanisms remaining active even at elevated temperatures. Small punch test simulations of creep testing gave comparative creep resistance between MA Fe-14Cr-3W-0.3Ti+0.25Y2O3 consolidated by HIP and FAST and Fe-14Cr-3W-0.3Ti-0.4Y+Fe3O4 consolidated by FAST. Fe-14Cr-3W-0.3Ti+0.25Y2O3 FAST resisted high loads for short amounts of time, but long term creep tests were unsuccessful, indicating crack opening resistance was being tested rather than creep resistance. The other samples behaved more as expected from literature, with Fe-14Cr-3W-0.3Ti-0.4Y+Fe3O4 FAST showing better creep resistance, but limitations of the technique mean absolute properties could not be identified. All samples failed rapidly in a brittle manner, similar to in ETMT. Overall, the alternative ODS production approach led to good strength across the temperature range of interest but ductility was limited, meaning fracture was rapid and unpredictable. Further work should reduce the density of oxide particles, or the oxygen contamination through processing, to generate a more ductile material, that is competitive with current reference austenitic stainless steels (15:15Ti).
Supervisor: Grant, Patrick S. ; Marrow, Thomas James Sponsor: National Nuclear Laboratory ; EPSRC
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available