PhD Studentship: Sub-room temperature performance of austenitic stainless steels for near-magnet fusion plant components

The construction of demonstration-class fusion tokamak reactors requires a wide range of structural materials, each with the mechanical performance and integrity required for the expected local service parameters [1]. While Reduced Activation Ferritic-Martensitic (RAFM) steels have attracted most research attention in fusion, the importance of Austenitic Stainless Steels (ASSs) has largely been overlooked. Austenitic steels, though high in nickel content and not designed to be ‘reduced-activation’, will be necessary for structural components in low-temperature regions of the breeder blanket, such as water-cooled sections, and especially in ultra-cool sections under mechanical stresses around the superconducting magnets [2-4]. Plasma vertical displacement events can cause significant deformations of the tokamak components and supporting structures associated with severe stress states [5]. The appeal of ASSs is due to being able to maintain a reasonable fracture toughness and ductility at low temperature, unlike ferritic-martensitic steels [6]. However, a largely overlooked concern with austenitic stainless steels is their structural stability and mechanical performance at cryogenic temperatures.

The aim of the project is to assess the thermo-mechanical stability of austenitic structures to transform into martensite at sub-room temperatures in conventional and additive-manufactured 316L stainless steels irradiated to close-to-magnet particle doses. The stability of a given austenite grain is governed by local microstructural parameters such as the local chemistry, grain size, orientation, the surrounding microstructure or local plasticity effects. The austenite transformation into martensite, induced either thermally or mechanically, brings along a 3% volume expansion of the crystal lattice. Such a transformation could thus lead to rapid and potentially catastrophic failure of the structural component of the tokamak’s superconducting magnets, yet a full understanding of the likelihood of this occurring remains largely unknown.

You will carry out in-situ cooling/deformation experiments of as-manufactured and irradiated steel specimens using High-Energy Synchrotron X-ray Diffraction (HE-SXRD) in transmission mode. This allows to avoid surface effects in austenite stability and simultaneously probe sufficient austenite grains in Bragg condition for acceptable grain statistics [7, 8]. The HE-SXRD work will be supported by analytical electron microscopy in-house, together with a fractography analysis of the post-mortem specimens. Furthermore, steel irradiations will be performed using the high-energy proton beam generated by the Birmingham MC40 cyclotron facility. This experimental campaign and the new data/insights will allow you to validate structural integrity models of ASSs linking meso- and macro-scales.

During this PhD project, you will acquire a unique and valuable set of transferrable skills ranging from designing complex sample environments and experimental protocols, to programming and data mining, effective communication skills and project/time management. You will also gain in-depth knowledge about physical metallurgy of steels, radiation effects and experimental mechanics focusing on a material critical for fusion technology.

Funding notes:

A 3.5-year PhD studentship is available in the fusion materials group of Prof. Enrique Jimenez-Melero within the School of Metallurgy and Materials at the University of Birmingham, with a stipend of at least £20,780 per year. This project is in collaboration with Frazer-Nash Consultancy.

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