This study investigated the feasibility of in-situ manufacturing of a functionally graded metallic-regolith multimaterial. To fabricate the gradient, digital light processing, an additive manufacturing technique, and spark plasma sintering were selected due to their compatibility with metallic-ceramic processing in a space environment. The chosen methods were initially assessed for their ability to effectively consolidate regolith alone, before progressing to sintering regolith directly onto metallic substrates. Optimised processing conditions based on the initial powder particle size, different compositions of the lunar regolith powders and sintering temperatures were identified. Experiments have successfully proven the consolidation of lunar regolith simulants first via near-net shaping with digital light processing and then spark plasma sintering at 1050 ℃ under 80 MPa. The metallic powders were fully densified at relatively low temperatures and a pressure of 50 MPa with spark plasma sintering. Furthermore, the lunar regolith and Ti6Al4V gradient were found to be the most promising multimaterial combination. While the current study showed that it is feasible to manufacture a functionally graded metallic-regolith, further developments of a fully optimised method have the potential to produce tailored, high-performance multimaterials in an off-earth manufacturing setting for the production of aerospace, robotic, or architectural components.@en

This study investigates the feasibility of in-situ manufacturing of a functionally graded metallic-regolith. To fabricate the gradient, digital light processing, an additive manufacturing technique, and spark plasma sintering were selected due to their compatibility with metallic-ceramic processing in a space environment. The chosen methods were first assessed for their ability to effectively consolidate regolith alone, before progressing to sintering regolith directly onto metallic substrates. Optimized processing conditions based on the sintering temperature, initial powder particle size, and different compositions of the lunar regolith powders were identified. Experiments have successfully proven the consolidation of lunar regolith simulants at 1050°C under 80 MPa with digital light processing and spark plasma sintering, while the metallic powders can be fully densified at relatively low temperatures and a pressure of 50 MPa with spark plasma sintering. Furthermore, the lunar regolith and Ti ₆ Al ₄ V gradient was proven to be the most promising combination. While the current study showed that it is feasible to manufacture a functionally graded metallic-regolith, further developments of a fully optimized method have the potential to produce tailored, high-performance materials in an off-earth manufacturing setting for the production of aerospace, robotic, or architectural components.

@en

Oxide dispersion strengthened (ODS) steels are leading candidates for structural materials in nuclear fission and fusion power plants. Understanding the nature of nano-oxide particles in ODS steels is vital for a better control of the microstructure and mechanical properties to further their applications. In this study, electron microscopy and atom probe tomography (APT) have been used to investigate the nanocluster features in ODS Eurofer steel. With the addition of V and Ta in ODS Eurofer, the nanoclusters exhibit a higher number density with a decreased average diameter, indicating that V and Ta are beneficial for the formation of small clusters. Irrespective of the composition of the base material, the smaller particles have a variable stoichiometry while the larger particles are likely to have Y₂O₃ stoichiometry. The nanoclusters were found to have a core/shell structure, where Y, O and Ta are enriched in the core and Cr and V are predominant in the shell. The formation of the complex structure is possibly the result of a competing effect between Ta, Y, V and Cr binding with O. It is deduced that Ta tends to combine with O in the core (Y₂O₃) of the clusters due to a higher affinity, and pushes V and Cr to the surrounding shell during the formation of nanoclusters.

@en

Oxide dispersion-strengthened (ODS) Eurofer steel was laser welded using a short pulse duration and a designed pattern to minimise local heat accumulation. With a laser power of 2500 W and a duration of more than 3 ms, a full penetration can be obtained in a 1 mm thick plate. Material loss was observed in the fusion zone due to metal vaporisation, which can be fully compensated by the use of filler material. The solidified fusion zone consists of an elongated dual phase microstructure with a bimodal grain size distribution. Nano-oxide particles were found to be dispersed in the steel. Electron backscattered diffraction (EBSD) analysis shows that the microstructure of the heat-treated joint is recovered with substantially unaltered grain size and lower misorientations in different regions. The experimental results indicate that joints with fine grains and dispersed nano-oxide particles can be achieved via pulsed laser beam welding using filler material and post heat treatment. @en

Oxide dispersion strengthened (ODS) steels are promising candidates for use as structural materials in the next generation fission and fusion reactors. Compared to conventional ferritic or martensitic steels, ODS steels exhibit improved high-temperature creep properties and irradiation resistance. Favourable properties are mainly attributed to the fine grain features and the high number density of nanosized oxide particles in the steel matrix. These nanoparticles can act as pinning sites for dislocations and stable sinks for irradiation introduced defects, leading to significantly enhanced mechanical properties. In order to be employed in nuclear systems with large, complex structures, the fabrication and welding of ODS steels with reproducible and superior properties are inevitable and essential. However, after 10–20 years of studying since the emergence of ODS steels, these issues remain the major bottlenecks limiting further development. This thesis is concerned with ODS Eurofer steel, which is one of the representatives of ODS steels and has been the research focus in terms of promising nuclear materials within the European Union. With the aim to develop suitable and effective methods for the fabrication and welding of ODS Eurofer, the result of this study should help to extend the use of ODS steels in future nuclear applications.@en

Oxide dispersion strengthened (ODS) steels are considered to be one of the candidate structural materials for advanced nuclear applications due to their high elevated-temperature strength, corrosion resistance, and radiation tolerance. Joining of ODS steels by traditional fusion joining techniques is not applicable, because the melting process results in the coarsening of fine grains and agglomeration of nanosized oxide particles, and consequently a significant loss of strength. Spark plasma sintering (SPS) has recently been employed as a novel joining technique, which could be beneficial for joining ODS steels considering the solid state characteristic. A powder metallurgy prepared ODS Eurofer steel was successfully joined using SPS. The microstructure and mechanical properties of the joints were investigated. An almost defect-free joint was obtained at the selected processing condition. The tensile properties of the joints are comparable to the base material. Fracture analysis shows an intergranular fracture in the as-joined sample, while a ductile fracture with well-defined dimples is found in the tempered sample.

@en

The present work deals with oxide dispersion strengthened (ODS) Eurofer steel fabricated by powder metallurgy involving mechanical alloying and spark plasma sintering. A heat treatment route including normalising and tempering was applied to the as-produced steel, based on differential scanning calorimetry (DSC) measurement. The microstructure was characterised by scanning electron microscopy (SEM), electron backscattered diffraction (EBSD), electrolytic extraction, X-ray diffraction (XRD) and transmission electron microscopy (TEM). Thermodynamic calculations conducted using Thermo-Calc software were used to determine the precipitation conditions. The results show that the Vickers microhardness of the sample after the designed heat treatment is more uniform compared to the as-produced condition. A dual phase and bimodal microstructure is formed in the as-produced and tempered steels. M₂₃C₆ and M₆C carbides were found in the as-produced sample while only M₂₃C₆ carbides were observed in the tempered sample. The carbides dissolve and reprecipitate during the heat treatment, preferably at the grain boundaries. Nanosized Y₂O₃ particles were found to be homogenously distributed in the steel matrix, which is crucial for the mechanical properties. The dislocation density in the material is decreased significantly after the normalising and tempering treatment. A yield strength model was developed that includes the strengthening contributions of solid solutes, grain size, dislocation density and nanoparticles. Good agreement is obtained between the experimentally measured and theoretically calculated strength of the as-produced and tempered steels.

@en

Pulsed laser beam welding was used successfully to join the oxide dispersion-strengthened (ODS) Eurofer steel. The joining was conducted with a laser power of 2500 W and a pulsed duration of 4 ms. With the filler material being used, a minor material loss and microvoids were observed in the joint. The microstructure of the fusion zone consists of dual phase elongated structures. The heat-affected zone has a width of around 0.06 mm with finer grains. The transmission electron microscopy observation reveals that nanoprecipitates are finely distributed in the fusion zone. The tensile strength, yield strength and elongation of the joint are slightly inferior to the base material. The fractography results reveal a typical ductile fracture. The experimental results indicate a reasonable joint from the perspective of both the microstructure and mechanical behaviour.

@en

Oxide dispersion strengthened (ODS)Eurofer steel was prepared via mechanical alloying (MA)and spark plasma sintering (SPS). Different combinations of MA and SPS parameters were adopted in order to optimise the fabrication process. The experimental results show that the sample milled for 30 h, sintered at 1373 K at a pressure of 60 MPa has the highest density and microhardness among all the results obtained. As-produced ODS Eurofer shows a bimodal microstructure with homogeneously dispersed nanoscale Y ₂ O ₃ , that is beneficial for the mechanical properties. The yield and tensile strengths are higher while the elongation is lower in the top and bottom surfaces compared to the middle area of the sample. This is due to the presence of a larger number of M ₂₃ C ₆ carbides, resulting from carbon diffusion from the mould material. As-produced samples were also subjected to a heat treatment. A good balance is achieved between the strength and ductility of the heat-treated material. Yielding properties comparable to hot isostatic pressing or hot extrusion as reported in the literature, the processing route presented by this study shows potential to produce high performance ODS Eurofer.

@en

This paper studies the joining of an oxide dispersion strengthened (ODS) steel without destroying the microstructure and thereby deteriorating the mechanical performance. ODS Eurofer was successfully joined by spark plasma sintering (SPS) at 1373 K with a heating rate of 100 K/min and a pressure of 80 MPa. An almost defect-free joint was obtained with a holding time of 40 min and a powder layer between the steel disks. The fine-grained microstructure and homogeneously distributed nanoparticles in the material show no significant change after the joining process. The tensile properties of the joint material are comparable to the base material. SPS has been proven to be a promising technique to join ODS steels due to the good mechanical properties of the joints.

@en