Atomic scale characterisation of radiation damage and radiation induced precipitation in tungsten rhenium alloys

Tungsten is considered the prime candidate material for plasma facing components within fusion reactors. However, exposure of tungsten to neutron flux brings about transmutation of tungsten into by-products: Re, Os and Ta. Under increasing levels of radiation damage, irradiation induced clustering/precipitation takes place that embrittles and thus reduces lifetime of such tungsten components. This thesis was undertaken to explore this subject on a deeper level.
There are three components to this study. The first part considers the effect of Re content on irradiation induced clustering. Lab-made plate W-xRe (x: 2, 5, 10 and 25at.%) alloys were exposed to 1.2, 3.9 and 33dpa, self-ion irradiated at 773K. Analysis of cluster number density and volume fraction found they increase with damage level and bulk Re content. Based on these trends and existing literature data, a hypothesis was proposed suggesting clusters originate from vacancy clusters. Also, at 33dpa, rod shaped clusters form in W-25Re alloys while spherical clusters are present in other alloys. The clusters show close correspondence with irradiation induced precipitates and appear to be precursor phase.
In the second part of this thesis, the effect of Os and Ta on cluster formation and alloy mechanical properties is examined. Lab-made plate W-1Re-1Os and W-2Re-1Ta alloys were irradiated at 33dpa at 573 and 773K and compared against control W-2Re alloy. At 33dpa and 573K, the Os and Ta presence suppresses cluster formation. Both ternary alloys contain smaller cluster diameter, composition, number density and volume fraction than the W-2Re alloy. However, at 33dpa and 773K, Os and Ta have opposing effects on cluster behaviour. Os increases the cluster nucleation rate and raises irradiation hardening (compared to W-2Re). Meanwhile, Ta presence decreased cluster number density and reduced the irradiation hardening (compared to W-2Re alloy). As well, Ta showed no evidence of clustering, only Re clusters form in the W-2Re-1Ta alloy.
The final aspect of the thesis analyzes the effect of material microstructure and external variables on cluster formation in W-Re alloys. Commercial wire form W-25Re alloy was irradiated at 1.2dpa at 573 and 773K as atom probe needles and bulk sample. The larger free surface on atom probe needle samples appears to act as a sink for self-interstitials and vacancies at both temperatures.
The effect of grain size and dislocation density was examined by irradiating commercial W-5Re wire (0.5-1μm diameter) and plate (1-3mm diameter) samples (annealed and unannealed) to 33dpa and 573K. It was found grain boundaries and dislocations act as defect sinks at 573K. Additionally, radiation enhanced Re grain boundary enrichment was observed for first time. The effect of grain size on cluster behaviour at 773K was also analysed. Commercial wire and lab-made plate W-3Re, W-5Re and W-25Re alloys were irradiated to 33dpa at 773K. The larger grain boundary area in wire samples is suspected of acting as a sink for self-interstitials leaving more vacancies for promoting cluster formation compared to lab-made samples.
The discoveries made in this thesis broaden our current understanding of irradiation induced phase formation in tungsten. Their implications on plasma facing component design are discussed as well as recommendations for improvements. Further, areas requiring further research in this field are also highlighted.

Advisor: Roberts, S
Bagot, P