Powder processing of oxide dispersion strengthened steels for nuclear applications

Thesis

Ferritic ODS steels show improved high temperature strength and irradiation tolerance compared with conventional ferritic steels, and are one of the key potential materials for fusion blanket structural applications. The processing of ODS steels is critical to their subsequent performance; however knowledge of the optimum processing approaches for these alloys is not complete.

The microstructural evolution of ODS steels containing Y₂O₃ and other additions during manufacture has been investigated and the processing conditions optimised based on microstructural and mechanical investigations.

Ferritic powders with Y₂O₃ and other additions were investigated, primarily using analysis on the micro- and nano-scale, with an emphasis on identifying the requirements for homogenization of the Y within the steel matrix. The Y₂O₃ dispersion and subsequent development of the nano-precipitates during thermal treatment was investigated using in-situ neutron diffraction. The nano-precipitates were resolved at approximately 900◦C after 1hr, with coarsening and/or re-precipitation progressively increasing at higher temperatures.

A significantly increased number density of nano-precipitates (∼2x10²³m−3 to ∼7x10²³m−3) was established by hot isostatically pressing an Fe-14Cr-3W-0.2Ti0.25Y₂O₃ alloy at 950◦C compared with more traditional temperatures at 1150◦C, attributed to the increased coarsening and/or re-precipitation of the nano-precipitates at the higher temperatures.

The influence of the mechanical alloy (MA)ing conditions on bulk mechanical properties was investigated using four point bend. The highest fracture toughness of ∼55MN/m^3/2 and ultimate strength of ∼1450MPa was achieved under conditions that minimised the mechanical alloying time and increased the average final size of the powders.

An Fe-14Cr-3W-0.2Ti-0.25Y₂O₃ (wt%) ODS alloy manufactured under optimised conditions showed a bi-modal grain structure size distribution and had a comparatively high yield strength of >1200MPa at 20◦C and >330MPa at 700◦C. The grain structure and high yield strength were attributed to the random distribution of 25nm radius of gyration (R_g) Y, Ti and O rich nano-precipitates randomly dispersed throughout the alloy. Long term thermal ageing (750hr at 1000◦C) reduced the room temperature yield strength and increased the proportion of larger grains in the bi-modal distribution, but high temperature yield strength was remarkably stable.

Authors: Gorley, M

• Publication date: 2014
• DOI: 10.5287/ora-brgbrdz8n
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: Oxford University, UK
• Language: English
• Keywords: Fusion; Powder processing; Nuclear; Fission; Steels; Oxide dispersion stengthened
• Subjects: Alloys; Metals and ceramics; Atomic scale structure and properties; Processing of advanced materials; Materials Sciences; Metallurgy; High resolution microscopy; Microscopy and microanalysis; Electron image analysis; Physical metallurgy; Materials processing; Ceramics