The impact loading of steel is critical for structural applications, particularly in naval structures, where high strain rate loading induces significant changes in both microstructural and mechanical properties. This has driven extensive research into the behaviour of steel under ballistic and blast events, which have become more frequent due to civil wars, insurgencies, and terrorist attacks. High Strength Low Alloy (HSLA) steels are highly valued in the shipbuilding industry for their exceptional mechanical properties, making them ideal for naval vessels exposed to such extreme conditions. However, HSLA steels are prone to adiabatic shear band (ASB) formation under high strain loading, which can lead to material failure. The formation and behaviour of ASBs remain a key area of study, with no universally accepted model yet established.

This thesis focuses on the ballistic impact on thermomechanically processed (TM) and normalized (N) EH36 HSLA steels, commonly used in the hulls of ships. Past research efforts highlighted that EH36TM steel exhibits anisotropic tensile behaviour after ballistic perforation—a phenomenon not observed in EH36N steel. Understanding the anisotropic behaviour of EH36TM steel is the primary research goal of this thesis. This research aims to experimentally investigate the microstructural mechanisms responsible for the anisotropy resulting from ballistic impact. The objective is to enhance our understanding of the material's behaviour under extreme conditions and to potentially improve the performance and safety of naval structures.

The central hole tensile (CHT) tests confirmed the anisotropic behaviour of perforated EH36TM steel. However, tensile tests on as-received EH36TM and N steels demonstrated that both steels exhibit isotropic tensile behaviour along the rolling direction (RD) and transverse direction (TD), indicating that the anisotropy is attributed to the impact. To understand this behaviour, an initial analysis of the base microstructure of both TM and N steels was conducted to establish the starting point of the material's characteristics. Both steels' microstructural changes were studied after ballistic impact to identify the alterations that may contribute to the observed anisotropy of the EH36TM steel. The study of as-received steels revealed similar behaviour between EH36TM and EH36N samples, with no inherent anisotropy detected in EH36TM along the TD. The finer microstructure of TM steel was the primary difference, suggesting a slightly higher susceptibility to ASB formation, which could be a possible link for the occurrence of anisotropy after steel's perforation. Microstructural analysis showed that both perforated steels formed comparable regions, after the severe plastic deformation. However, Scanning Electron Microscopy (SEM) analysis revealed a higher susceptibility to micro-cracks and micro-voids formation in EH36TM steel, which could lead to diminished tensile behaviour and hence anisotropy under specific conditions. For both steels, adjacent to hole, ASBs were identified in the entire perimeter. Therefore, they were not correlated directly with EH36TM steel's anisotropy. Pronounced cracks parallel to the rolling direction were detected in TM steel, possibly contributing to its decreased tensile performance, in perpendicular direction, while these cracks were absent in perforated EH36N steel. Further SEM analysis across the hole's thickness revealed that ASB initiated within the thickness of the TM steel and were oriented almost parallel to the rolling direction. This could potentially weaken its tensile properties across the transverse direction. EBSD analysis indicated a high probability of (110)<111> and (112)<111> slip systems alignment across RD in TM steel, correlating with higher ductility in RD compared to TD. This behaviour of slip systems was not observed in N steel. Fractographic analysis via SEM confirmed that TM steel could sustain greater plastic deformation when loaded along rolling, as evident from an area of clear ductile fracture, while the transverse-loaded sample showed brittle and mixed modes. In EH36N samples, brittle fracture was observed adjacent the perforated hole, with mixed fracture behaviour close to it. These observations were consistent with the isotropic tensile behaviour of the perforated EH36N steel.

This thesis significantly advances the understanding of why EH36TM steel exhibits anisotropic tensile behaviour along TD after perforation. By thoroughly investigating the material before and after perforation, this research sheds light on the effects of ballistic impact on EH36TM and EH36N materials' microstructure and mechanical behaviour. The findings not only deepen our insights into the post-impact behaviour of EH36TM steel but also have broader implications for improving the design and safety of naval structures that uses this material.

To decrease vehicle emissions, automotive manufacturers aim to decrease materials usage. Dual-phase (DP) steels are used in safety critical locations of a vehicle and with stronger DP steels, less material is needed. However, DP steels are known to be affected by hydrogen embrittlement (HE), which decreases the ductility and is thus of significant concern.

This work investigates the effect of heat treatment on the microstructure and HE of DP steel. Heat treatments are developed and conducted using dilatometry, with obtained microstructures characterized using scanning electron microscopy (SEM), optical microscopy (OM), electron backscatter diffraction (EBSD) and hardness measurements. Tensile specimens are heat treated using a Gleeble 3800-GTC thermo-mechanical simulator, before evaluating the HE effect by measuring the total absorbed hydrogen content using thermal desorption spectroscopy (TDS), and determining the static mechanical properties using slow strain-rate tensile (SSRT) testing. The fracture micromechanisms are investigated with fractography using SEM and EBSD with interrupted SSRT and fractured specimens.

The critical transformation temperatures are determined at 686±1 ⁰C and 571±10 ⁰C for Ar3 and Ar1 respectively. Intercritical annealing at 590 ⁰C, 630 ⁰C, and 670 ⁰C result in a martensite content of 24.97±6.43 %, 40.41±4.21 %, and 77.63±6.97 %, respectively. Hardness values of 240±31, 300±9, and 380±20 HV1 are obtained for the respective intercritical annealing conditions and are an accurate method for comparing the martensite content and microstructure of dilatometry and Gleeble specimen. The martensite distribution changes from a discontinuous network of lath martensite with small martensite islands at 590 ⁰C, to a continuous network of lath martensite without martensite islands at 670 ⁰C.

TDS yield an increase in the total absorbed hydrogen content of 1.16±0.11 wppm, 1.29±0.13 wppm, and 1.58±0.27 wppm at increasing intercritical annealing temperature, which is attributed to an increase in the ferrite/martensite interphase.

Through SSTT the optimal combination of ultimate tensile strength (UTS) and elongation is found at 630 ⁰C, with UTS of 1025±8 MPa and elongation of 12.6±0.6 %. For all specimens significant HE is observed, with an embrittlement index of 58.9±7.1 %, 89.2±1.1 %, and 85.8±2.5 % for increasing intercritical annealing temperature. The highest embrittlement at 630 ⁰C is attributed to dislocation pile-up around martensite islands, severely increasing embrittlement.

Fractographic investigation revealed the presence of quasi-cleavage fracture bands across the specimen cross-section. This is attributed to preferred crack propagation along martensite bands in the cross-section, with preferred crack nucleation at the martensite surface layer of the specimen short edge. EBSD of the fractured specimen revealed an increase in the kernel average misorientation (KAM) surrounding the ferrite/martensite interface and martensite islands, indicating an increased HE action due to the HEDE and HELP mechanisms.

It is concluded that there is considerable embrittlement of the heat-treated DP steel, which is significantly influenced by the martensite content and distribution. Since DP steels are used in safety critical components, controlling the distribution of martensite in the microstructure is vital, and avoiding surface layer martensite and martensite bands can reduce the HE effect.

Hydrogen-assisted fatigue in pipeline steel has become a major concern in the field of energy transition. In recent decades, growing awareness of environmental issues has significantly increased interest in hydrogen as an alternative energy source. Hydrogen is affordable and can be transported using the existing natural gas infrastructure. However, the structural integrity of pipeline steels, especially under hydrogen-assisted fatigue conditions, poses significant challenges to the durability and safety of modern energy infrastructure. Since fatigue is a common cause of pipeline failure, a deeper understanding of hydrogen’s impact on the fatigue behaviour of these pipeline materials is essential. This project investigates the fatigue crack growth (FCG) behaviour of API X65 pipeline steel and its welding material in hydrogen enriched environments. To achieve this, a modular hydrogen autoclave testing device was designed and implemented to conduct in-situ hydrogen gaseous fatigue tests on eccentrically single-edge notched tension (ESE(T)) specimens. The fatigue crack growth relationship (da/dN vs. ∆K) was measured, and hydrogen-assisted fatigue crack growth was observed and compared to benchmarks in nitrogen and air. The study employs two testing frequencies to examine the influence of loading frequency under constant load and hydrogen pressure. This approach evaluates the combined effects of hydrogen and the microstructures of both base and weld metal. The investigation focuses on material responses in terms of crack propagation rates, measured using direct current potential drop, and fractographic analysis. To determine the required hydrogen pre-charging time, a hydrogen diffusivity FEA (Finite Element Analysis) modelling approach was used to estimate the hydrogen equilibrium concentration in the specimens before testing. The experimental setup involved pre-charging the specimens in a hydrogen environment at 10 MPa, followed by fatigue testing at frequencies of 0.1 Hz and 10 Hz. The findings indicate that the hydrogen environment accelerates fatigue crack growth compared to inert gases and air. The base metal (BM) at 10 Hz showed approximately two times greater susceptibility to hydrogen-assisted cracking compared to the weld metal (WM). At higher ∆K values (∆K > 50 MPa√m), the crack growth rates of base metal (BM) and weld metal (WM) were similar. However, at lower ∆K values (20-30 MPa√m)), the BM exhibited about one order of magnitude higher crack growth rates. The study emphasizes the importance of microstructural characteristics, including grain boundaries, size, and micro-hardness, in affecting fatigue crack growth. The study confirms that loading frequency significantly influences H-AFCG rates, where at loading frequency f = 0.1 Hz, FCGR increases by 2.3 times for BM and 3.9 times for WM compared to results at 10 Hz frequency, with lower frequencies allowing more hydrogen diffusion and promoting the transition from ductile to brittle cracking. Further findings indicated that finer grains in WM enhance hydrogen fatigue resistance due to increased grain boundary densities. However, this advantage diminishes at lower frequencies where hydrogen diffusion is enhanced, increasing susceptibility to hydrogen-assisted fatigue crack growth (HA-FCG). Analysis of fracture surfaces reveals a significant transition from ductile to brittle fracture modes when moving from inert environments (air or nitrogen) to hydrogen. In hydrogen environment, the presence of quasi-cleavage and intergranular facets was prevalent, compared to ductile transgranular features in air or nitrogen. Additionally, in hydrogen environment, secondary microcrack branching a characteristic of ductile fracture is notably absent, especially at lower stress intensity factors (∆K).

Wire arc additive manufacturing (WAAM) can create large parts, due to its high deposition rate. WAAM can be used to create functionally graded materials (FGMs) such as a strong core of high strength low alloy (HSLA) steel and corrosion-resistant cladding of austenitic stainless (AUS) steel. However, HSLA and AUS steel are dissimilar metals with different microstructure and mechanical properties. Joining dissimilar metals with WAAM creates a complex interface with a different set of properties compared to parent materials. The current state of the art covers the microstructure, tensile properties, and hardness of such FGM. However, there is a lack of understanding about the fatigue properties at the interface. This study investigates the fatigue crack growth behaviour at the interface of ER70S-6 (HSLA steel) and ER316L (AUS steel). Initially, a benchmark study of the HSLA and AUS steel was done to obtain Paris parameters, threshold stress intensity range (∆Kth), and hardness. HSLA steel showed 43 to 77% lower Paris slope parameters than AUS steel, which suggests a better fatigue resistance for HSLA steel. However, no significant difference in ∆Kth values was observed. The hardness of HSLA and AUS steel were 247.0±6.1 HV and 227.7±5.5 HV respectively, which suggests a correlation between hardness and fatigue resistance. Further analysis showed a power law relation between the Paris slope parameter and hardness for HSLA steel. This was attributed to the grain size. Smaller grain sizes improve fatigue resistance and increase hardness. Hence, hardness can be an indirect measure of fatigue resistance. Under constant ∆K experiments, AUS steel showed up to a 61.7% difference in fatigue crack growth rate (FCGR). The anisotropy in AUS steels is due to the large grain size in the range of 100 μm and the structure of the weld beads, resulting in heterogeneity. After the benchmark study, the graded materials were tested under similar conditions. The crack in the graded material grows from AUS steel to a 2 mm wide interface and then to HSLA steel. A variation of FCGR at the interface region was measured, notably 1) a decrease of 5.1E-05 to 6.1E-05 mm/cycle before the interface region, 2) a gradual increase of 5.5E-05 to 1.32E-04 mm/cycle within the interface region, and 3) a sudden increase of up to 3E-04 mm/cycle. To understand these behaviour microscopy, fractography, and electron backscatter diffraction (EBSD) analyses were utilised. The sudden increase of FCGR in HSLA steel was attributed to process-induced porous defects. The decrease of FCGR before the interface region was due to the increase in grain boundaries, secondary cracks, an increased crack closure effect, and martensite formation through transformation-induced plasticity (TRIP) in the interface region. The gradual acceleration of FCGR within the interface region was due to the martensite phases formed during solidification. The martensite formed during solidification had twice the hardness in comparison to HSLA steel. The martensite formed by the TRIP effect also resulted in 25-50% higher hardness near the crack flanks compared to the regions away from it. For the application of this FGM, the interface can act as a protective layer by reducing the FCGR. However, the martensite formed during solidification in the interface should be mitigated.

Micro-indentation testing has shown great promise in extracting local mechanical properties of ductile metal materials. Although the relevant contact physics has been well revealed since the 19th century, interpreting the indentation data still poses many challenges at the application level. Regarding one of the mainstream methodologies for extracting metal's representative stress-stain curve, the semi-analytical method has demonstrated remarkable performance towards the well-defined contact system involved. However, applying such a model to other indentation scenarios tends to cause some practical measuring problems. The validity of the results depends heavily on the practical experimental setup and the hardware testing calibration, which is inherently related to its accomplishment level in capturing the entire mechanical response. This thesis investigates such practical issues through a provided semi-analytical model validation. In addition, to capture a material's elastic modulus with less reliance on initial data, a novel analytical model has been proposed, with a special focus on the extensive unloading/reloading data.

However, both the analytical and semi-analytical methods are not fully applicable. For the analytical model, the first unloading segment contributes the most matched estimates of effective modulus to the tensile reference value, with an average deviation error of 6%. But there still remain relatively large discrepancies between two similar samples. Besides, the validity of its results highly depends on accurate profile radius determination, which demands a more precise profilometry system. For the semi-analytical model validation, the resulting indentation strain-stress curves appear to exhibit post-yield behavior and fail to capture the effective modulus as well as the yield strength. It reveals the model's performance that heavily relies on the initial elastic data.

Additive manufacturing (AM) refers to a series of techniques in which parts are created by successive deposition of layers. Among the different technologies, Wire Arc Additive Manufacturing (WAAM) employs a welding system in combination with a motion mechanism to deposit layers of molten metal. This technology possesses high deposition rates and an unconstrained build envelope, limited only by the reach of the motion mechanism making it suitable for fabricating large-scale parts. The use of this technique to manufacture technologically interesting materials as Invar 36, a low thermal expansion alloy, could lead to reduced material waste and components with a geometrical complexity only attainable via AM. The high heat input associated with WAAM has been proven to induce columnar grain growth causing anisotropic behavior in the materials, and in the case of Invar 36, it also promotes cracking. In this work, the addition of nucleating agents, known as inoculation, has been implemented during the deposition process to induce grain refinement and mitigate the aforementioned effects. TiC and NbC were selected as possible inoculants based on the results from the implementation of the edge-to-edge matching model as selection criterion. The inoculants, in the form of powders, were mixed with an organic carrier to create suspensions at 50 wt.% and 75 wt.% which then were applied as a coating to each layer during the deposition process. Invar 36 cuboidal specimens with dimensions of 15x15x120 mm were fabricated using GTAW-based WAAM with a heat input of 550 J mm^-1. Microstructural characterization showed that specimens with added inoculants achieved significant grain size reduction, reduced crack formation, and an increase in hardness. Defect-free depositions were attained in the specimens with inoculant-loaded suspensions at 75 wt.%.

The corrosion of bearing steel often leads to accelerated damage and loss of properties that condition their service life. This damage is further exacerbated with the absorption of hydrogen into the material, generated from corrosion reactions happening at the surface. A remanufacturing process is proposed to recover the operating capabilities of bearings after exposure to corrosive environments, during which the bearings are polished until the surface is visually clean, out of any corrosion product. The process offers the recoverability of the operating capabilities and turns the corroded bearings into a reconditioned useful part avoiding the operational effort, economic cost, and repair that the replacement of a bearing entails. However, atomic hydrogen in the bulk can be entrapped and might still condition bearings operation, often resulting in hydrogen enhanced damage in the steel.

The goal of the current research is to quantify the hydrogen uptake and critically evaluate the reapplication of such remanufactured bearings. The study analyses a bearing that experienced corrosion during its operation in the pulp and paper industry (P&P), primarily due to the cooling water used in the machines. For a better understanding of the effects of the operating environment on bearings, extreme corrosive environments were simulated in a climate corrosion chamber (CCC). The absorbed bulk hydrogen is quantified through the Melt Extraction Tester (MET), and the trapping behaviour analysed through Thermal Desorption Spectrometry (TDS). The effect of the environmental conditions is studied through a microscopical characterization of the formed corrosion product. Optical Microscopy (OM), Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Spectroscopy (EDS), and X-ray Photoelectron Spectroscopy (XPS) are the laboratory equipment used for this purpose.

It was found that the remanufacturing did not ensure meeting the acceptable hydrogen content limit in extreme environments, suggesting that the polishing process is often not sufficient, and should be adapted to specific operating conditions. However, due to material inhomogeneities such as surface pits and plastic deformation-induced traps, no clear correlation could be established between polished material and bulk hydrogen. Nonetheless, a smooth polished surface was proven to minimize the presence of microreaction sites and subsurface defects, thus reducing the absorbed hydrogen. Additionally, corrosive media also affect hydrogen behaviour in the material. In the present work the trapped hydrogen concentration is seen to be dominant, reducing the risk of hydrogen damage. However, reversibly trapped, and diffusible hydrogen is also detected, and can be desorbed or redistributed into other trapping sites during the corrosion process.

High Manganese Austenitic Steel (HMnAS) is a novel steel exhibiting excellent mechanical properties at arctic temperatures. Defense Material Organisation (DMO) is interested in the feasibility of using HMnAS in their navy's ships alongside EH36 high-strength shipbuilding steel, using 307PF stainless steel weld filler metal.
The feasibility of using HMnAS is investigated by performing mechanical tests on both the base materials and the weld. Microstructure and fractographs were analysed using optical and electron microscopy, and the chemical composition was measured using EDS, EPMA and XRD. The results of mechanical tests were compared to current standards for EH36.
This study yielded three main observations. The first important result is the finding of severe anisotropy in the HMnAS. Impact toughness in the L-T direction was threefold the amount in the T-L direction. This is thought to be because of stringer MnS inclusions, elongated in the rolling direction. The second observation was the inadequate mechanical behaviour at the EH36 fusion line and the weld, these did not meet the requirements of IACS standards. Lastly, after observing a crack path along the fusion line of the CTOD fatigue pre-cracking, microhardness tests over the fusion lines were conducted and pointed out a hard band along the EH36 fusion line. This band is found to be a martensitic band (15-108 microns in width) that hampers mechanical performance.
This investigation concludes that the current weld joint is not as desired. More research is required to improve the weld joint before HMnAS can be implemented on navy ships. Future research for DMO should mainly focus on the welding characteristics and the filler metal composition, to improve weld performance.

Austenitic stainless steels are used in waste gate systems of turbochargers for their good mechanical strength at elevated temperatures. A key component of the waste gate system is the bushing, which guides and supports a rotating shaft under dynamic conditions. In these operating conditions, wear occurs which limits the service life of a bushing. Moreover, high alloyed austenitic stainless steels are used for conventional bushings, which results in high costs. For these reasons, alternative austenitic stainless steel alloys are considered. This investigation, in collaboration with Mitsubishi Turbocharger and Engine Europe B.V, focuses on the material selection of an alternative stainless steel alloy for the bushing application based on wear and oxidation behaviour.

In this work, conventional cast austenitic alloys were compared to alternative wrought austenitic alloys. The oxidation behaviour was determined using thermogravimetric analysis (TGA) and X­ray diffraction (XRD). Wrought materials showed better resistance against oxidation and better thermal stability than cast materials, which was attributed to the formation of a uniform Cr2O3 layer. In contrast, the interdendritic carbides in cast materials acted as sites for oxidation. The wear behaviour was characterized by hardness testing, pin­-on-­disc testing and engine testing. Worn surfaces were analyzed using white light interferometry, scanning electron microscopy (SEM) and Energy Dispersive Spectroscopy (EDS). The presence of interdendritic carbides of the type MC, M7C3 and M23C6 in cast materials resulted in high hardness, which provided better resistance against adhesive wear. However, with the introduction of oxidation in the engine test, wrought materials outperformed cast materials due to the formation of stable layers consisting of oxidized wear debris. This layer acted as a solid lubricant and effectively reduced adhesive wear.

From these results, it is concluded that oxidation can have a beneficial effect on the wear behaviour of austenitic steels. Additionally, wrought alloys are suited to replace conventional alloys, which improves the service life and reduces the costs of bushings in future turbochargers.

Composite materials offer superior mechanical performance with lower weight than traditional materials. As a result, they are widely used in various industries, such as aerospace, automotive, construction, and sports. The composite industry is increasingly utilizing natural fibers and developing biodegradable composites in response to environmental sustainability concerns. Natural fibers have comparable physical and mechanical properties to glass fiber, making them suitable for use in the production of bio-composites. Despite this, natural fiber cannot fully replace glass fiber due to a variety of factors that influence the material's variable properties such as the type of fiber used, the conditions in which the fiber grows, the processing methods, and any modification of the fiber. Hence most of the research done on bio-composites has concentrated on using them in non-structural parts.

This project aims to develop and characterize a green and sustainable bio-composite system, to overcome existing application challenges. The constituent materials include a bio-resin extracted from seaweeds and Unidirectional Flax fibers as reinforcements to stiffen and strengthen for semi-structural applications. Although alginates have been studied in-depth for their biomedical applications, their potential as a bio-based matrix for biocomposites has not yet been explored.

In addition to the materials used in the composite preparation, the processing technique significantly impacts the final properties of the composite. Therefore, in the first half of the work, thorough research was conducted to study the constituent properties for their processing. The second half of the work defined a new approach to manufacturing bio-composites from natural constituents. The corresponding analysis useful for product design are thoroughly demonstrated in this thesis.

However, using natural fibers in composites and water alginate soluble matrices has three main concerns: the fiber/matrix interaction and their sensibility to moisture absorption and residual water within the composite structure. Their surfaces can be modified using physical and/or chemical methods to improve the bonding between fibers and matrix. In most studies cited in the literature, the chemical modifications employed are synthetic and toxic. It would be ideal if the chemicals used to modify natural fibers were bio-based and preserved the biodegradable nature of natural fibers.

The development of an optimized seven-step manufacturing approach is a key innovation for water removal, to enhancing fiber reinforcement and boosting overall composite performance, particularly in the use of flax fibers. This approach is notable for its novelty and challenge, as evidenced by the limited literature on the subject. It focuses on achieving sufficient impregnation by utilizing low water percentages in resin, specifically around 7\%, and applying appropriate consolidation pressure and temperatures. The most effective results were observed at 5 MPa and 95°C, which facilitated homogeneous plasticization and mouldability. Additionally, the method of wet/dry cycling, incorporating pre-soaking and heating, has proven beneficial in providing dimensional stability to flax fibers and limiting water absorption, further contributing to the technique's effectiveness.

Analytical techniques like microscopy and SEM reveal promising compatibility between components. No degradation of the bio-resin or the fibers relatable to the heat press was identified.

Finally, the research aimed to establish a meaningful relationship between the critical process variables and the properties of the bio-composite, with the ultimate goal of optimizing the production process and enhancing the quality of the final product by varying the fiber volume fraction within the range of 41-47\%.

The maximum tensile and flexural strengths achieved were 219MPa and 56Mpa, respectively. The elastic tensile and bending moduli in the composites were approximately 6.64GPa and 1.83GPa, respectively.

However, the observed properties fell below the predicted values for the biocomposite system, which were calculated using the rule-of-mixtures and Halpin-Tsai methods. Nonetheless, the experimental data confirm that these biocomposites can be used as secondary structural elements. The observed discrepancy was due to the presence of huge voids further leading to poor adhesion.

Shape Memory Alloys (SMAs) are a class of metallic multi-functional materials which possess sensing and actuation capabilities, thanks to their unique ability to couple thermal and mechanical fields. NiTi-based Shape Memory Alloys exhibit properties which guarantee their high performance in adverse environments and, with the added benefit of two functionalities, the Shape Memory Effect and Superelasticity, these materials have become ideal candidates for a variety of applications. Spatial orientation of the NiTi crystals is crucial for achieving the superelastic functional response. According to studies on NiTi single crystals, when the grains adopt a [001] orientation along the direction of compression, slip is inhibited in the austenite phase and recoverable transformation strains similar to the theoretically estimated values of 5.3% are possible, defining the benchmark for superelastic responses. Additive Manufacturing techniques, due to their inherent thermal processing conditions, have created unprecedented opportunities for fabrication of NiTi-based Shape Memory Alloys so that the thermomechanical behaviour of the material can be tailored to any application is intended for. During Laser- Powder Bed Fusion, the scanning strategy influences the thermal processing to which the alloy is subjected, affecting its solidification process as well as the heat fluxes and temperature gradients that arise during manufacturing. Meanwhile, it can play a decisive role in achieving epitaxial solidification of columnar grains extending over multiple deposition layers. In this study, five different scanning strategies were employed during Laser-Powder Bed Fusion (L-PBF) of a Ni-rich NiTi alloy and were evaluated with respect to the grain morphology and crystallographic texture that developed, as well as the superelastic response of the samples produced. It was found that a 67° interlayer rotation of the scanning vector promotes the fusion of neighbouring melt pools, resulting in columnar grains that solidify epitaxially along the build direction (BD) of the sample. Meanwhile, a strong ⟨001⟩ fibre texture emerges along the BD. The superelastic response was stabilised at 4.5% recoverable strain with 74.2% recovery ratio after 16 cycles of axial compression loading. When an island scanning pattern was incorporated into the 67° rotation scanning strategy, the crystallographic texture strengthened and the superelastic response improved, (5.0% stabilised recoverable strain and 84% recovery ratio). Furthermore, an increase in the volumetric energy density, achieved by using a flat top laser beam, produced a nearly single-crystalline microstructure, with the highest intensity of the ⟨001⟩∥BD texture. The superelasticity in this case was stabilised at 5.5% recoverable strain with 91.5% recovery ratio. The effect of the loading direction on the superelastic response was also investigated, as was the nature of the residual strain left in the samples after their superelasticity stabilised. Therefore, this study successfully demonstrated that the scanning strategy can be a vital tool in designing the crystallographic texture and the grain morphology of NiTi parts fabricated by L-PBF, and this way, effectively tailor the superelastic functional behaviour to specific requirements of potential applications.

This research aimed to optimize the process parameters of small copper parts printed with binder jetting. The optimisation was distinguished by dividing the parameters in print and post-process parameters. Binder jetting is an additive manufacturing technique that combines layers of the material in powder form with a binder. After the printing process the printed samples need curing, debinding and sintering. The binder acts as a temporary glue to hold the shape and is later removed during the debinding. The sintering fuses the powder particles together, below the melting temperature. With binder jetting it is the aim to achieve the high density parts, as there is no compression of powder during the printing process. This often leads to low density parts with binder jetting. The two problems that needed to be solved, were: how can the bleeding issues be stopped and at what temperature, time and environment can the printed copper be sintered. Multiple print sessions were done to: first test the known print parameters, second optimize the print parameters and third, print with optimized print parameters. ThermoGravimetric Analysis (TGA) was conducted to determine the correct debinding and sintering temperature of the copper samples. The bleeding issues could be solved by adjusting the saturation level. The saturation level was reduced from 104 % to 72 %. The lower percentage gave 0.5 mm dimensional accurate samples and no bleeding issues. Compression testing was done to compare the mechanical behaviour to that of conventionally manufactured copper. However the discs used for compression testing needed to be bigger in size than the cubes used for optimizing the sintering process. This lead to not fully sintered discs used for the compression testing. Meaning the achieved compressive strength and elasticity modulus were 1 % of conventionally manufactured copper. Three working sinter sessions were compared to see which gave the highest achieved density. Sintering 10 hours in argon at 1093 ℃ or in vacuum at 1080 ℃ both achieved the same high densities. Shell printed cubes performed better with an average density of 73 ± 8 % in argon and 74 ± 7 in vacuum. The samples placed on the top level inside the sintering oven achieved higher densities than in lower levels. Two shell printed cubes in argon achieved a density of 83 ± 0.5 % with a volumetric shrinkage of 40 ± 1 % and two shell printed cubes in vacuum achieved a density of 82 % with a volumetric shrinkage of 40 %. Solid cubes reached lower densities, with 67 ± 10 % in argon and 71 ± 7 % in vacuum. As with the shell printed cubes, the solid cubes have a high deviation due to the placement of the samples on different levels. The solid cubes had less volumetric shrinkage and rather high deviation due to the same reason, with 25 ± 16 % in argon and 35 ± 11 % in vacuum. The density is measured with a caliper and with Archimedes which gave quite different results. Densities measured with Archimedes were up to 22 % higher due to the porosity of the samples. All the measurements done by other copper binder jetted parts are done with Archimedes. Thus it seems the 74 % and 82 % is low when compared to 91 % density achieved in literature, but it is not known how accurate these measurements are.

Ceramics are being explored as an alternative material by ASML to replace their stainless steel 316L (SS316L) reticle masking blades. Ceramics offer distinct advantages over stainless steel in the semiconductor industry. While stainless steel exhibits good mechanical strength, ceramics excel in electrical insulation and thermal conductivity, making them essential for semiconductor manufacturing. Ceramics, with their lower thermal expansion coefficient, enhanced chemical stability, lightweight nature, and dielectric properties, are essential for improving the performance, miniaturization, and reliability of semiconductor devices. This research addresses the challenge of replacing the SS316L blades in ASML’s EUV lithography reticle masking with high-purity alumina (99.7%) through binder jetting. Concurrently, emphasis was placed on optimizing the existing REMA blade design for the binder jetting technique. This research further assesses the thermo-mechanical performance of the new blade suggesting the necessity of cooling channels in the new blades. High-purity alumina was chosen for its superior material properties compared to SS316L. The research also focuses on analyzing the impact of particle size, printing parameters, and sintering conditions on the densification and mechanical properties of binder-jetted alumina samples. Unimodal 20 μm, unimodal 10 μm, and trimodal (equal concentrations of 10, 5, and 2 μm) powder batches were printed using binder jetting. The trimodal samples with 90% binder saturation exhibited the best results in green body density (61.3%) and sintered body density (66.9%). The Young’s modulus and flexural strength achieved by the trimodal sample was also the highest with a value of 77.3 ± 4.9 GPa and flexural strength of 59.5 ± 3.2 MPa. To enhance densifications further, it is recommended to incorporate sintering additives in the powder mixture and/or utilize nanoparticle densifiers in the binder. Additionally, employing the discrete element method using smaller particles with a multimodal powder mixture is also recommended for achieving higher densification.

To mitigate global warming, society must reduce greenhouse gas emissions. The transportation sector is one of the largest energy consumers and greenhouse gas emitters. One way to mitigate the emissions of this sector is to reduce the weight of vehicles using Advanced High Strength Steels (AHSS), particularly dual phase steels (DP). However, these steels are subject to hydrogen embrittlement which hinders their full potential and application inside the automotive industry.
The effect of pre-strain level and in-situ pre-straining on hydrogen embrittlement was investigated for DP1000. Therefore, in-situ tensile test was performed at 0.5% and 2% plastic pre-strain to study hydrogen embrittlement by measuring strain at fracture. This test was done under ex-situ and in-situ pre-strain conditions. In addition, sample charging time was varied from 15 to 60 minutes for 2 % plastic pre-strain. Thermal Desorption spectroscopy was performed to measure hydrogen uptake. Fracture surface was studied by scanning electron microscopy. Results showed that pre-strain plays an important role in hydrogen embrittlement, namely 0.5% and 2 % plastic pre-strain reduced the final strain to fracture by almost 50 % for both in-situ and ex-situ charging cases. However, no significant increase in hydrogen embrittlement occurs for the increment from 0.5% to 2% pre-straining, except for the 0.1 wppm increase in hydrogen content. Differences in results for in-situ and ex-situ testing were only observed for 2% pre-strain at 60 minutes charging. Increase in charging time resulted in increased hydrogen uptake, however, no severe effects were observed in hydrogen embrittlement. For future work it is recommended to study different extra pre-strain levels, further increase the charging time and investigate the effect of various DP1000 microstructures, such as ferrite, martensite and retained austenite ratios.

Thick-section high strength steels (HSS) are widely used in engineering applications with requirements of high toughness and strength. Structural components are commonly joined by welding, known to significantly degrade the mechanical properties of steel due to changes in microstructure (e.g., grain size coarsening and martensite-austenite (M-A) constituent formation). This degradation is even more prominent at sub-zero temperatures, where steels exhibit ductile-to-brittle transition behaviour and cleavage becomes the governing fracture mechanism. The most detrimental area is the part of the base material exposed to high temperatures during welding, known as the heat affected zone (HAZ). This work investigates the microstructural features affecting cleavage fracture in the most brittle areas of a quenched and tempered (QT) S690 HSS plate of 100 mm thickness, the coarse-grained and intercritical coarsegrained HAZ (CGHAZ and ICCGHAZ). The effect of intercritical peak temperature and heat input is also examined. Results show that fracture toughness is strongly affected by the matrix properties. ICCGHAZ 750 °C zone, exhibiting the lowest hardness, preserved relatively high levels of fracture toughness, while CGHAZ with the highest hardness had a substantial fracture toughness deterioration. The comparison of the two different intercritical peak temperatures, 750 °C and 800 °C, revealed a strong influence of this parameter on fracture toughness since a significant toughness reduction of ICCGHAZ 800 °C compared to 750 °C was present. The larger area fraction of M-A constituents in ICCGHAZ 750 °C and 800 °C compared to CGHAZ does not influence fracture toughness, which is attributed to the small size of M-As. Although M-As are responsible for crack initiation, the majority have a size smaller than 1 µm which is not sufficient to cause unstable fracture. Consequently, the step of crack propagation is governed by the matrix. Regarding the heat input effect, a reduction from 2.2 to 1.1 kJ/mm resulted in an increase of fracture toughness in CGHAZ and deterioration in ICCGHAZ 750 °C, however, CGHAZ remained the most critical zone. The improvement in CGHAZ is attributed to a higher fraction of smaller M-A constituents leading to less critical crack initiation sites. The fracture profile shows that the main crack propagates transgranularly and can be diverted by PAG, with high-angle grain boundaries. The investigation of secondary cracks revealed that the slender M-A constituents do not act as crack arrestors as they can easily be cut off. On the other hand, blocky M-A constituents can act as crack arrestors when the crack width is comparable to their size. This thesis provides more knowledge on the cleavage fracture process of HAZ and guidelines on how to control cleavage fracture in welded structures and improve the design of welds for structural applications.

Hydrogen as an alternative energy source has risen in popularity due to increased environmental awareness. Existing natural gas infrastructure is considered as a means to transport hydrogen, due to practicality and financial aspects. However, hydrogen can deteriorate the fatigue behaviour and thus induce premature failure. Since fatigue is a common failure mode in pipelines, a more thorough understanding of the effects of hydrogen on fatigue behaviour is required. In this work, the hydrogen fatigue of X60 pipeline steel and its girth welds was investigated through a combined approach of modelling and in-situ fatigue testing. A novel in-situ gaseous hydrogen charging fatigue set-up was developed, which involves a sample geometry that mimics a small-scale pipeline with high internal hydrogen gas pressure. The specimen geometry involved an internal circumferential notch that induces a stress concentration factor (Kt = 3.0) related to the worst case scenario for pipelines. The effect of hydrogen was investigated by measuring the onset of crack initiation and growth using a newly designed direct current potential drop setup which probes the outer surface of the specimen. A FEA modelling approach was used to estimate the hydrogen equilibrium concentration in the specimens, as well as to determine the stress states in the material. Results showed that both materials experienced a reduction in fatigue life in the presence of hydrogen. For the base metal, the reduction in fatigue life (37%) manifested solely in the crack growth phase; hydrogen accelerated the crack growth (factor 4). In contrast, the reduction in fatigue life (68%) of the weld metal was due to accelerated crack growth (factor 8) and a decrease in resistance to crack initiation (57%). Varying the hydrogen gas pressure from 70 barg to 150 barg did not cause any differences in the fatigue behaviour. The presence of hydrogen influenced the fracture mechanisms of both materials. The fracture path of the base metal transitioned from transgranular and ductile in nature, to a mixed-mode transgranular and intergranular quasi-cleavage fracture. The weld metal exhibited a similar transition, however in the inert environment some intergranular features were observed at the prior austenite grain boundaries. The presence of hydrogen reduced the crack tortuosity. This is associated with a decrease in roughness- and plasticity-induced crack closure, thereby accelerating the crack growth. It is inferred that hydrogen-enhanced localised plasticity (HELP) and hydrogen-enhanced decohesion (HEDE) were the dominant types of hydrogen embrittlement mechanisms during fatigue of this pipeline material. It was concluded that the weld metal is more susceptible to hydrogen fatigue than the base metal in a gaseous hydrogen environment. The worst-case scenario for pipelines is in the case of weld defects. The weld defects involved in this work were macropores (0.5-1.0 mm) with a spheroid morphology. When these defects were located at the notch surface, the resistance to crack initiation decreased by 92% compared to non-porous specimens in nitrogen. The existing natural gas infrastructure could have accumulated similar flaws during service life, which would make them unreliable for safe hydrogen transport. The costs associated with the repurposing of these pipe segments could raise unexpected economic hurdles, hindering the transition to a hydrogen economy.

Additive manufacturing (AM) is quickly becoming one of the more popular methods to manufacture components made of Ti-6Al-4V in the aerospace and automobile industry due to its flexibility in producing complex geometries and reducing tooling costs. As the world of additive manufacturing is still relatively young, there is no globally accepted method for the certification of AM parts, allowing freedom to develop procedures to utilize the advantages of AM. To greatly reduce material wastage, the use of smaller specimens has been explored to characterize the mechanical properties of the material.

This study aims to analyze the influence of test specimen size on the near threshold fatigue properties of Ti-6Al-4V manufactured by conventional processes and Laser powder bed fusion (LPBF) . Two different specimen sizes were utilized, the standard size and the sub-sized specimens. The effect of two heat treatments, stress relief (SR) and annealing (AN) on the LPBF material on the fatigue properties was also studied. In addition, the effect of anisotropy on the fatigue threshold value was also touched upon in this study. A fatigue test setup for the sub-sized samples was designed and a constant Kmax test was employed to characterize the threshold properties. The direct current potential drop (DCPD) method was employed to measure the crack length during the test.

The fatigue threshold of the SR microstructure did not show any effect due to sample size. This was attributed to the partly intergranular crack propagation observed playing a prominent role in the fatigue threshold. The conventional and annealed samples witnessed a drop in threshold values of about 0.5 MPa√m when sub-sized samples were used. It was observed that the sub-sized samples had a greater crack depth at threshold leading to increased constraint at the crack tip and hence smaller degree of crack closure explaining the drop in threshold values.

The current study has thus established successfully the feasibility of using sub-sized specimens to characterize the near threshold fatigue properties of conventionally and additively manufactured Ti-6Al-4V.

Refractories are widely used as lining material in the steelmaking industry. This class of materials is known for its superior creep resistance. However, creep of refractory linings is a known phenomenon in refractory science. Therefore, knowledge on creep behaviour is critical for furnace lining life predictions and selection of materials. This work, in collaboration with Tata Steel IJmuiden, focuses on creep development in magnesia carbon (MgO-C), a refractory often found in Basic Oxygen Furnaces (BOF). In this research, the influence of binder and aluminium content on creep development in MgO-C was investigated. This was addressed, based on the microstructural changes, phase transformations and thermo-mechanical properties. Microscopy, scanning electron microscopy, X-Ray diffraction, cold crushing strength, hot modulus of rupture, dilatometry, refractoriness under load and hot compressive strength tests were conducted in order to explain creep behaviour. For creep testing, a nonstandardised set-up was designed to be able to retrieve creep data more efficiently. The experimental configuration increased the load every 5 hours (multi-stage). Resulting creep rates were validated against a single-load stage creep measurement. A small difference of 5% in creep rates was observed. This indicated that the multi- and single-stage results are within range of each other. Therefore, the proposed multi-stage creep test can be considered a viable creep testing method. A trend of decreasing creep rates with increasing binder content was found. Increasing the content from 2% to 3% showed a maximum decrease in creep rates of 80%. A further increase of binder content from 3% to 4% showed a maximum decrease in creep rates of 30%. This behaviour was mainly attributed due to volatilisation of the binder and magnesium, and densification due to carbonisation of the binder. Increasing the aluminium content from 0 to 1% resulted in a maximum decrease in creep rates of 70%. This was mainly attributed due to spinel formation. The irreversible volume expansion due to spinel formation, increased the density and the stress state within the material. As a result, micro cracks formed which strengthened the material and improved creep resistance. The outcomes of this work are an important contribution to refractory science. However, further development of refractories is still necessary, especially due to the increasing pressure on the steelmaking sector to make its processes more sustainable.

In recent decades, high strength steels in thick sections have been increasingly used in offshore structures where they are subjected to harsh service conditions such as freezing temperatures and high static/dynamic loading. At these conditions they are susceptible to a transition from ductile failure to a dangerous brittle (cleavage) predominant type of failure, which occurs well before yielding. One of the main challenges in employing thick sections is the through-thickness heterogeneous variance of microstructures as a result of the processing route owing to a gradient of cooling rates from the surface to the bulk. As the material’s mechanical and fracture behaviour strongly depend on the microstructure, the through-thickness microstructural heterogeneity leads to a significant scatter in mechanical and fracture properties, which makes it difficult to predict and control cleavage fracture.

From the body of literature establishing microstructural dependence on cleavage failure, several features contributing to this type failure can be identified. Phases, grain size, grain boundary misorientations and the presence of secondary phase constituents can play a major role in failure through cleavage. Additionally, cleavage failure is also sensitive to the crack depth to width ratio (a/W). This study investigates the microstructural features contributing to cleavage failure in a 100 mm thick S690QT high strength steel plate by performing mechanical and fracture toughness tests. In order to improve mechanical properties and cleavage fracture toughness, an isoparametric study employing rapid cyclic heating with the objective of grain refinement was performed.

The steel plate has coarser prior austenite grain (PAG) sizes, and a larger area and number fraction of inclusions in the middle section. Additionally, segregation bands as a result of solute segregation during the solidification process were observed to be dispersed throughout the middle section. The middle section was also characterized by lower hardness compared to the top section. The detrimental effects of the middle section were evidenced by inferior low temperature tensile properties and cleavage fracture toughness. This was attributed to the larger PAG sizes, larger area and number fractions of inclusions, and segregation bands in the middle section. The specimen orientation with respect to the rolling direction was found to have no effects on the tensile properties. Additionally, different a/W geometries and notch orientations with respect to the rolling direction were used to investigate the role of constraint effect and rolling orientation, respectively, in the fracture behaviour. Shallow-notched specimens representative of the defects found in offshore structures demonstrated a higher fracture toughness than the deep-notched specimens. This was attributed to lower hydrostatic stresses at the crack tip, which reduces stress triaxiality. The isoparametric study resulted in average grain size reduction by 41% and proved to improve micro-hardness, low temperature tensile properties and cleavage fracture toughness by 5%, 13% and 41% respectively. Fractographic analysis on the fracture toughness specimens revealed the presence of O, C-rich regions which are known to promote brittle behaviour.

In our modern world, where computers, mobile phones and many other applications have become indispensable, there is a growing need for high precision machines in order to be able to produce these technologies. For these complex high precision applications it is important to understand the fatigue properties of the materials used to prevent premature failure, as these materials are subjected to large numbers of stress cycles. A material that is used for high precision applications is Ti-6Al-4V, as its material properties are highly adaptable and can be fine-tuned for a wide range of applications. Material fatigue due to stress cycles knows two stages: crack initiation and crack propagation. This research focuses on the latter and looks into the influence of microstructural features on crack propagation in Ti-6Al-4V. The influence of the microstructural features is tested by applying a load shedding method to form cracks in Ti-6Al-4V samples. These cracks are analysed with Scanning Electron Microscopy (SEM) and Electron BackScatter Diffraction (EBSD) in order to relate the microstructural features to the crack path. There are two main microstructural features found to have a large influence on the fatigue crack propagation in Ti-6Al-4V. The first of these is the Schmid factor, which relates the applied stress to the slip system available. As there are only a small amount of slip systems available in Ti-6Al-4V and limited crack path propagation opportunities, large crack deflections can be the result. The second microstructural feature found to have a large influence is the misorientation angle of the grain boundaries. When the misorientation angle is large enough, a shift from transgranular cracking to intergranular cracking is observed. Intergranular cracking can cause deviations of the crack path around the grains and the formation of secondary cracks. The deviations in crack path as a result of a low Schmid factor and a high misorientation angle extend the fatigue life of the material. The Schmid factor was found to have a large influence on crack deflections observed, whereas the high misorientation angles were mostly found around sites where bifurcation occurred, especially at lower applied stress ranges. This study proposes that the influence of the microstructural features on fatigue crack propagation in Ti-6Al-4V can be expressed as two probability functions describing crack deflection and bifurcation. The probability of a crack deflecting is relatively high for a lower Schmid factor, a high misorientation angle and a low deflection angle. The probability of bifurcation is relatively high when a near-threshold stress intensity range is applied and also for a high misorientation angle and a low deflection angle.