Development of ferritic BCC-superalloys for high-temperature and nuclear applications

von Tiedemann, Sophia Olympia (2025). Development of ferritic BCC-superalloys for high-temperature and nuclear applications. University of Birmingham. Ph.D.

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Abstract

Advances in nuclear reactor technology, such as the ongoing development of nuclear fusion and generation-IV fission reactors, drive up operating temperatures (550°C to 1000°C) to increase thermodynamic efficiency, and require materials capable of withstanding highenergy neutron fluxes over prolonged periods of time. Particle irradiation creates an excess of point defects into the material lattice, leading to the evolvement of larger defect structures, where this microstructural evolution is highly dependent on irradiation parameters and temperature. Designing materials tolerant against the degradation under irradiation poses a major materials engineering challenge. It was the aim of this thesis to develop a series of Fe-Al-X (X = Ni, V, Ti) model alloys based on the BCC-Superalloy design strategy to address the above-mentioned challenges. Analogous to FCC Ni-based superalloys commonly used in jet engines, a ferritic disordered Fe (A2) matrix is reinforced by semi-/coherent, intermetallic, ordered (B2/L21) precipitates. A large concentration of matrix-precipitate interfaces is hypothesised to provide a high density of point defect sinks, resulting in defect annihilation/recombination, mitigating further defect evolution. The stability of the ordered phases under irradiation was also studied.

In order to study the role of sink strength and ordered phase stability, the first experimental chapter covers the design and development of a series of ferritic BCC-Superalloys: (1) Fe-Al-Ni (FAN), (2) Fe-Al-V (FAV) and (3) Fe-Al-Ti (FAT), with varying compositions, precipitate size, lattice misfit, interface sink strength and phase compositions. Each alloy exhibits a disordered A2 matrix, where FAN forms a B2 (NiAl) secondary phase, and FAV and FAT form L21 Heusler-type Fe2AlV and Fe2AlTi precipitates, respectively. Neutron activation calculations for fusion and fission cases were carried out for each alloy using FISPACT-II. Published thermodynamic data were adjusted and used to calculate isothermal sections at 700°C and theoretical phase compositions, where discrepancies in thermodynamic evaluations were discussed. To tailor precipitate size and resulting sink strength, a coarsening study was conducted for each alloy at 700°C for ageing times between 30 mins and 400 hours. Differences in precipitate morphology and coarsening rates were highlighted. After homogenisation treatments, conditions of equivalent precipitate radius across alloys were produced: (i) ‘fine’ with small precipitate size and (ii) ‘coarse’ with larger equivalent radius to create samples of varying sink strength. Microstructural characterisation was carried out using Scanning Electron Microscopy (SEM). For FAV, calculated phase compositions were compared with Atom Probe Tomography (APT) data. Precipitate-matrix lattice misfit was measured with neutron diffraction and compared with calculated values from Density Functional Theory (DFT). Differences between calculated and measured results were discussed. 4D-Scanning Electron Microscopy (4D-STEM) was carried out on all ‘fine’ samples conditions and strain maps were produced.

In the second experimental chapter, the previously developed alloys were irradiated in conditions of varying defect sink strength to test material response and to study the underlying mechanisms. For the FAN, FAV and FAT alloys, the homogenised, fine and coarse conditions were irradiated with 2 MeV Fe+ ions at 300°C to a total dose of 1.1×1015ions/cm2, to study the effectiveness of matrix-precipitate interfaces as radiation defect sinks and theeffects of interface density, precipitate size and lattice misfit on materials response, as well as ordered phase stability. Changes in nanohardness following irradiation were very different across alloy compositions and conditions and did not follow the expected trend of varying precipitate size and sink strength. In few cases, irradiation-induced hardening was observed, whereas in other alloy conditions irradiation hardening was either suppressed or softening was observed. L21-strengthened alloys FAV and FAT were observed to undergo partial disordering from A2 + L21 → A2 + B2, whereas no disordering was observed for the FAN alloy exhibiting an A2 + B2 microstructure. To unravel detailed contributions of irradiation hardening through damage accumulation, defect absorption through sink strength and radiation softening from disordering in each alloy, further study is needed.

The third experimental chapter further investigated radiation tolerance of novel Fe-AlV-Ti alloys. This was done by in-situ irradiating with 900 keV Kr3+ ions inside a TEM to observe microstructural and crystallographic changes in real time, specifically the disordering behaviour and effects of Ti-additions on the Fe-Al-V system. A series of alloys of composition Fe76Al12V(12−x)Tix was produced, where increased Ti substitution for V results in an increase in lattice misfit. Alloys from 0 to 3 at.%Ti were irradiated from 30°C up to 500°C. For the alloy containing 2 at.%Ti, dose required to disordering was found to increase non-linearly with temperature. At room temperature, a direct disordering of the precipitate phase from L21 to A2 was observed, whereas at higher temperatures (>150°C), partial disordering of L21 to B2 occurred. This was attributed to an increase with point defect mobility as well as the competition of ballistic disordering and thermal re-ordering of the thermodynamically stable ordered phase. Furthermore, at room temperature the dose to disordering of the L21 phase was found to decrease exponentially with increasing Ti-content. It is suggested that the addition of Ti lowers the energy barrier to disordering through multiple ways. Further study is needed to model and investigate the underlying mechanisms. Overall, the reversible ordering and re-ordering of this series ferritic superalloys is thought to be advantageous for nuclear applications in the suitable high temperature regime.

This PhD project resulted in new insights as to behaviour of several BCC ferritic superalloys under irradiation and important underlying mechanisms, while highlighting where further investigations are warranted. This contributed to the development of a new design strategy for novel nuclear materials, working towards the advancement of nuclear technology.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):