Radioisotope thermoelectric generators produce electricity for space exploration. These generators use thermoelectric material, usually a silicon-germanium alloy, to turn heat from radioactive decay into electricity. Recently, the thermoelectric performance (ZT) of Si80Ge20 was improved by reducing the grain size to the nanometer range, significantly lowering the thermal conductivity.
To optimize the properties of silicon-germanium we studied the effect of doping concentration and processing parameters on the microstructure and properties of boron-doped Si80Ge20 produced by arc melting, ball milling and spark plasma sintering. The thermal conductivity was estimated with a model. The electrical conductivity and Seebeck coefficient were measured.
Some conclusions from this work are that the current production process can be used to produce nanostructured Si80Ge20Bx with a crystallite size of 50-100 nm. This material reaches a maximum, but not necessarily optimal, doping concentration when x=1 due to limited solubility. The material suffers from grain growth when exposed to high temperatures for several days. The use of iron in the ball milling process significantly affected the microstructure and properties.

A multi-material structure combines diverse materials, allowing for precise tailoring of properties in specific regions. The integration of dissimilar materials leads to superior performance compared to single-material components. Bimetallic structures, a subset of multi-material systems, offer site-specific performance owing to the contrasting properties of the materials involved.

This thesis focused on fabricating a thin, functionally graded bi-metallic wall using Wire Arc Additive Manufacturing (WAAM) with Inconel 625 and HSLA steel. Functional grading of metallic materials, like Inconel 625 and HSLA steel, enables precise customisation of component properties to fulfil distinct functions within a structure. Despite its promise, this combination presents challenges, notably the formation of intermetallic compounds that act as a catalyst for forming solidification cracks in combination with varying material composition and high heat input.

Optimal process parameters, notably reduced heat input, are critical in mitigating these issues. The current study involved process parameters optimisation for Inconel 625 single bead-on-plate welds to control dilution levels at dissimilar interfaces. Extensive characterisation using Light Optical Microscopy (LOM) and Scanning Electron Microscopy (SEM) equipped with Energy Dispersive Spectroscopy (EDS) was conducted to study the microstructural evolution of the as-fabricated sandwich structure.

Results indicated that interface 1 (Inconel 625/HSLA steel) exhibited minimal dilution and zero defects, while interface 2 (HSLA Steel/Inconel 625) exhibited substantial dilution and was prone to solidification cracks. The EDS results affirmed that the variations of elemental composition and heavy dilution at interface 2 lead to inhomogeneous microstructural features, elemental segregations and the formation of brittle intermetallic phases, thereby leading to a solidification crack at this particular interface.

X-ray diffraction (XRD) measurements were conducted to identify phases at the dissimilar interfaces, corroborating the microstructural features observed using SEM and LOM. The identified phases confirm the presence of brittle intermetallics, such as the laves phase, extensively present at interface 2. Vickers hardness testing was performed to assess the mechanical behaviour at both interfaces, revealing a consistent trend of decreasing values with a sudden increase in hardness values at both interfaces due to microstructural transition.

Based on the EDS results, an optimal range of elemental compositions was speculated, focusing primarily on the significant elements Fe, Ni, and Cr. These compositions, with Ni content of approximately 20-25 wt%, Fe content of around 70-75 wt%, and Cr content of approximately 5-10 wt%, are crucial in reducing cracking susceptibility at interface 2. This study elucidates the initiation and propagation of solidification cracks at interface 2, establishing a definitive link between microstructural evolution, mechanical behaviour, and crack formation.

Ultimately, this thesis lays the foundation for future research to delve deeper into these insights, fine-tune process parameters, and fabricate compositionally graded multi-material structures with enhanced susceptibility to solidification cracking at dissimilar interfaces.

This research mainly investigates the relationship between the process condition, the microstructural evolution and the mechanical properties of the low carbon S690 steel in complex WAAM deposited structures. Besides, the optimum process parameters for improving the quality of the multi-pass multi-layer structure are also the research topic in the thesis. With a decreased heat input, achieved by a higher travel speed or a lower wire feed rate, the cooling rate became higher in the single bead. A larger amount of martensite formed in the bead due to the higher cooling rate, which led to a higher hardness. When the heat input became smaller, less metal was deposited per unit length, which decreased both the width and the height of the bead. Two-layers structures were deposited with the different process parameters, and their qualities were evaluated based on defects, surface roughness, and geometry. Step over increments were set to two-thirds of the width of each bead to improve the surface quality. Wire feed rate 4.5 m/min and travel speed 8 mm/s are the optimized parameters to fabricate the sample with the highest quality. Functional grading was achieved in a component by depositing with different process conditions. In the functionally graded component, the high heat input zone, having a lower hardness, included a large fraction of ferrite and bainite because it was fabricated with higher heat input, resulting in the lower cooling rate. The low heat input zone, having a higher hardness, included a large fraction of martensite due to higher cooling rate, caused by the lower heat input. The microstructure in the low heat input zone was not homogeneous due to the occurrence of the softer white line region, including a large amount of ferrite. The crossing structures fabricated with the interlayer temperature of 100 °C and the interlayer time of 90 sec showed the similar microstructures and hardness result. Due to the impact of a cold substrate and thermal accumulation, the varying microstructure existed from the bottom region to the top region, leading to the hardness level along the building direction became: Bottom region > Middle region ≈ Top region. The higher cooling rate at the centre was observed because there was more area for heat produced at the centre to diffuse.