The irradiation resistance of oxide dispersion strengthened steels
<p>Reduced activation oxide dispersion strengthened (ODS) steels are candidate alloys for use in fusion reactor systems and are fabricated by mechanically alloying yttrium oxide to a reduced activation ferritic steel powder. The product is consolidated at high temperature by hot isostatic pres...
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| Other Authors: | |
| Format: | Thesis |
| Language: | English |
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2015
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| _version_ | 1797100432854614016 |
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| author | Burrows, C |
| author2 | Roberts, S |
| author_facet | Roberts, S Burrows, C |
| author_sort | Burrows, C |
| collection | OXFORD |
| description | <p>Reduced activation oxide dispersion strengthened (ODS) steels are candidate alloys for use in fusion reactor systems and are fabricated by mechanically alloying yttrium oxide to a reduced activation ferritic steel powder. The product is consolidated at high temperature by hot isostatic pressing (HIP), producing a dispersion of nanometre sized oxide particles throughout the ferritic microstructure. These particles have been shown to both improve the high temperature mechanical properties of the alloy and provide trapping sites for helium gas. </p> <p>The use of these particles to sequester helium is of particular significance in the development of a structural ODS steel for fusion reactor systems. A fusion power reactor, based on the ITER design, is expected to produce over 2000 appm transmutant helium in any steel components exposed to the core neutron flux. At these gas concentrations, conventional steels undergo severe swelling and embrittlement, motivating the development of materials capable of managing helium accumulation. </p> <p>This thesis investigates the use of the oxide particle dispersion in sequestering helium introduced by ion implantation. An initial characterisation of a model Fe-14Cr-0.25Y<sub>2</sub>O<sub>3</sub> (wt%) system was completed using high resolution transmission electron microscopy (HRTEM) and atom probe tomography (APT). This demonstrated the efficacy of the production methods and the gas trapping capabilities of the oxide particles via argon gas, introduced during the mechanical alloying process. </p> <p>The subsequent consolidation of a full set of Fe-14Cr-3W-0.2Ti-0.25Y<sub>2</sub>O<sub>3</sub> (wt%) ODS alloys at 1150°C, 1050 °C and 950 °C produced a systematic variation in the density of the particle dispersion. The characterisation of these materials using APT provided an insight into the consistent Y<sub>2</sub>Ti<sub>3</sub>O<sub>5</sub> particle chemistry found in each consolidation, and identified a stoichiometric shift from Y<sub>2</sub>Ti<sub>3</sub>O<sub>5</sub> to YTiO2 following short term annealing periods at 1000°C. Though further work is required, this shift is thought to be consistent with a thermodynamically mediated transition of the metastable clusters to stable oxide particles. </p> <p>Following implantation with 2000 appm helium and examination under TEM, the helium bubble and particle densities were found to be closely correlated thus providing evidence for an association between the particles and the gas bubbles. Controlling the helium bubble density via the particle dispersion demonstrates the potential use of processing temperature in controlling how helium accumulates in an implanted ODS microstructure. </p> <p>The effects of both bubble and particle densities on mechanical properties were investigated further using nanoindentation methods. Significant local variation in the hardness of the ODS steels was found to result from the bimodal grain size distribution of the material. By using only those measurements taken from large grained regions of the ODS, the grain refinement and particle hardening effects could be deconvolved and used to quantify particle hardening using a dispersed barrier model. </p> <p>The significant hardening effects with helium addition observed in the reference alloys were found to be almost entirely absent from the ODS systems, though anomalous softening in the 950°C consolidation indicated a potentially unexpected interaction between the bubble and particle populations. A possible explanation for this anomaly and a proposal for further work to establish its origin is discussed.</p> |
| first_indexed | 2024-03-07T05:37:25Z |
| format | Thesis |
| id | oxford-uuid:e464cc9c-5ac0-43cb-acd2-c09706176d9a |
| institution | University of Oxford |
| language | English |
| last_indexed | 2024-03-07T05:37:25Z |
| publishDate | 2015 |
| record_format | dspace |
| spelling | oxford-uuid:e464cc9c-5ac0-43cb-acd2-c09706176d9a2022-03-27T10:16:14ZThe irradiation resistance of oxide dispersion strengthened steelsThesishttp://purl.org/coar/resource_type/c_db06uuid:e464cc9c-5ac0-43cb-acd2-c09706176d9aMaterials SciencesAtomic scale structure and propertiesHigh resolution microscopyNanostructuresMaterials processingMicroscopy and microanalysisMetals and ceramicsProcessing of advanced materialsMetallicsAlloysField Ion MicroscopyMetallurgyEnglishOxford University Research Archive - Valet2015Burrows, CRoberts, SBagot, P<p>Reduced activation oxide dispersion strengthened (ODS) steels are candidate alloys for use in fusion reactor systems and are fabricated by mechanically alloying yttrium oxide to a reduced activation ferritic steel powder. The product is consolidated at high temperature by hot isostatic pressing (HIP), producing a dispersion of nanometre sized oxide particles throughout the ferritic microstructure. These particles have been shown to both improve the high temperature mechanical properties of the alloy and provide trapping sites for helium gas. </p> <p>The use of these particles to sequester helium is of particular significance in the development of a structural ODS steel for fusion reactor systems. A fusion power reactor, based on the ITER design, is expected to produce over 2000 appm transmutant helium in any steel components exposed to the core neutron flux. At these gas concentrations, conventional steels undergo severe swelling and embrittlement, motivating the development of materials capable of managing helium accumulation. </p> <p>This thesis investigates the use of the oxide particle dispersion in sequestering helium introduced by ion implantation. An initial characterisation of a model Fe-14Cr-0.25Y<sub>2</sub>O<sub>3</sub> (wt%) system was completed using high resolution transmission electron microscopy (HRTEM) and atom probe tomography (APT). This demonstrated the efficacy of the production methods and the gas trapping capabilities of the oxide particles via argon gas, introduced during the mechanical alloying process. </p> <p>The subsequent consolidation of a full set of Fe-14Cr-3W-0.2Ti-0.25Y<sub>2</sub>O<sub>3</sub> (wt%) ODS alloys at 1150°C, 1050 °C and 950 °C produced a systematic variation in the density of the particle dispersion. The characterisation of these materials using APT provided an insight into the consistent Y<sub>2</sub>Ti<sub>3</sub>O<sub>5</sub> particle chemistry found in each consolidation, and identified a stoichiometric shift from Y<sub>2</sub>Ti<sub>3</sub>O<sub>5</sub> to YTiO2 following short term annealing periods at 1000°C. Though further work is required, this shift is thought to be consistent with a thermodynamically mediated transition of the metastable clusters to stable oxide particles. </p> <p>Following implantation with 2000 appm helium and examination under TEM, the helium bubble and particle densities were found to be closely correlated thus providing evidence for an association between the particles and the gas bubbles. Controlling the helium bubble density via the particle dispersion demonstrates the potential use of processing temperature in controlling how helium accumulates in an implanted ODS microstructure. </p> <p>The effects of both bubble and particle densities on mechanical properties were investigated further using nanoindentation methods. Significant local variation in the hardness of the ODS steels was found to result from the bimodal grain size distribution of the material. By using only those measurements taken from large grained regions of the ODS, the grain refinement and particle hardening effects could be deconvolved and used to quantify particle hardening using a dispersed barrier model. </p> <p>The significant hardening effects with helium addition observed in the reference alloys were found to be almost entirely absent from the ODS systems, though anomalous softening in the 950°C consolidation indicated a potentially unexpected interaction between the bubble and particle populations. A possible explanation for this anomaly and a proposal for further work to establish its origin is discussed.</p> |
| spellingShingle | Materials Sciences Atomic scale structure and properties High resolution microscopy Nanostructures Materials processing Microscopy and microanalysis Metals and ceramics Processing of advanced materials Metallics Alloys Field Ion Microscopy Metallurgy Burrows, C The irradiation resistance of oxide dispersion strengthened steels |
| title | The irradiation resistance of oxide dispersion strengthened steels |
| title_full | The irradiation resistance of oxide dispersion strengthened steels |
| title_fullStr | The irradiation resistance of oxide dispersion strengthened steels |
| title_full_unstemmed | The irradiation resistance of oxide dispersion strengthened steels |
| title_short | The irradiation resistance of oxide dispersion strengthened steels |
| title_sort | irradiation resistance of oxide dispersion strengthened steels |
| topic | Materials Sciences Atomic scale structure and properties High resolution microscopy Nanostructures Materials processing Microscopy and microanalysis Metals and ceramics Processing of advanced materials Metallics Alloys Field Ion Microscopy Metallurgy |
| work_keys_str_mv | AT burrowsc theirradiationresistanceofoxidedispersionstrengthenedsteels AT burrowsc irradiationresistanceofoxidedispersionstrengthenedsteels |