In this study, three-dimensional functionally graded NiTi bulk materials were fabricated using laser powder bed fusion (LPBF) by in-situ adding Ni powder into equiatomic NiTi powder. The gradient zone exhibited a Ni composition ranging from approximately 49.6 to 52.4 at.% over a distance of about 2.75 mm. The functionalities along the compositional gradient were examined through differential scanning calorimetry analysis and spherical indentation. This unique gradient resulted in location-specific functionalities, including superelasticity characterized by wide and narrow hysteresis loops, shape memory effect, and various phase transformation temperatures. The rapid cooling rate during fabrication led to the presence of excess Ni in the solid-solute state within NiTi. This unique solid-solute compositional gradient in NiTi resulted in varying lattice parameters, influencing the compatibility between martensite and austenite and allowing for tailored hysteresis. This discovery presents new avenues for designing multifunctional materials through in-situ additive manufacturing.

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Wire arc additive manufacturing (WAAM) offers a novel approach to fabricate functionally graded components. By changing the wire consumable between layers, chemical grading can be used to obtain specific properties across a part's volume. This is an interesting approach to design large metal components that achieve unconventional performance in demanding engineering applications, such as sulphide-resistant pressure vessels or sea ballast piping with extended lifetime. However, challenges derived from dissimilar material combinations draw the need to study the effect of compositional grading on the mechanical properties. This study focuses on the deformation and fracture toughness behaviour of WAAM-fabricated high-strength low-alloy (HSLA) and austenitic stainless (AS) steel bi-material specimens, particularly examining the diluted interface layer obtained during deposition. Tensile testing results indicate that the elastic modulus at the interface matches that of un-diluted AS steel (157 ±17 GPa) along the build direction. Fracture toughness showed a lower J_IC (180 kJ/m²) when compared to the undiluted AS steel (459 ±69 kJ/m²) and HSLA steel (408 ±25 kJ/m²). Scanning electron microscopy and electron backscatter diffraction are used to establish a connection between the microstructure at the interface and the observed mechanical properties. It is concluded that deformation at the interface is in large controlled by the deformation-induced martensitic transformation of metastable austenite. These results underline the influence of chemical dilution on the deformation mechanisms and fracture behaviour of HSLA and AS steel bi-material parts, which needs to be accounted for in the design of parts composed by this bi-metal couple.

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This study concentrates on the fatigue crack propagation behaviour of a high-strength low-alloy (HSLA) steel and austenitic stainless (AS) steel bi-material part, as obtained by wire arc additive manufacturing (WAAM). Due to partial mixing in the weld pool, the first layer of AS steel laid onto the previously deposited HSLA steel results in a diluted interface layer of distinct chemical and microstructural characteristics. Average Paris parameters are obtained for the interface layer along transverse and longitudinal planes to the deposition direction (BD-LD plane: m = 2.79, log₁₀(C) = –7.83 log₁₀(da/dN)) (BD-TD plane: m = 3.47, log₁₀(C) = –8.39 log₁₀(da/dN)). However, it is observed that this interface layer manifests an intriguing crack propagation behaviour. FCGR consistently drop as the crack front transitions from undiluted AS steel to the interface. At ΔK = 20 MPa⋅m^0.5, the greatest Δ is −0.77 log₁₀ steps (R = 0.1). As cracks near the HSLA fusion line, rates re-accelerate up to + 0.75 log₁₀ steps (R = 0.5). The phenomenon is attributed to the interplay between deformation-induced martensitic transformation and pre-existing allotropic martensite. Our findings, derived from a series of fatigue tests in correlation with multiscale microstructural and fracture characterization, offer insights into the damage-tolerant behaviour of these bi-material structures.

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The effect of TiC and VC nano-precipitate size on the hydrogen embrittlement of ferritic steels was studied in this work. Steels containing two size distributions (10 nm or less and 10 - 100 nm) of TiC and VC carbides are subjected to tensile tests in-situ in an electrochemical hydrogen charging environment. Hydrogen is found to be trapped in interstitial matrix sites on the precipitate/matrix interface with activation energies of 14 - 20 kJ/mol and inside misfit dislocation cores with energies of 27 - 37 kJ/mol. All steels are embrittled by 15 to 20%, except the TiC steel with semi-coherent carbides up to 100 nm, which is embrittled by 37%. This is caused by accelerated intergranular fracture as a result of hydrogen trapped in dislocation pile-ups around grain boundary precipitates. The steel with coherent VC nano-carbides retained the highest strength and ductility during in-situ testing. This is therefore the optimal carbide configuration for use in hydrogen environments.@en

Superelastic metamaterials have attracted significant attention recently, but achieving such functionality remains challenging due to partial superelasticity and premature fracture in additively manufactured components. To address these issues, this study investigates the premature fracture in Ni-rich NiTi metamaterials fabricated by laser powder bed fusion. A comparative analysis of two structures (Gyroid network and Diamond shell) reveals that the structural stability of bending- and stretching-dominated structures is reversed compared to typical elastic-plastic response, due to the tension-compression asymmetry of base NiTi. The premature fracture and partial superelasticity of these as-fabricated samples are attributed to low deformation ability for accommodating tensile stress. Based on these findings, a heat treatment introducing Ni₄Ti₃ precipitates was employed, successfully achieving macroscopic superelasticity in the NiTi metamaterials, with consistency between model prediction and experiments.

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Microstructure features including grain morphology and texture are key factors in determining the properties of laser additively manufactured metallic components. Beyond the traditional trial-and-error approach, which is costly and time-consuming, microstructure control increasingly relies on predictions from mechanistic models. However, existing mechanistic models to predict microstructure and texture are computationally expensive. Here, we present a cellular automata solidification model, which is up to two orders-of-magnitude faster than traditional models. By analytically calculating growth length and utilizing a multi-level capture algorithm, a large time step can be employed without compromising simulation accuracy. The model is validated through simulations of 316L steel and three NiTi cases, showing good agreement with experimental results. Our findings reveal that preferential orientations are selected by the vertical and the inclined temperature gradients from multi-pass temperature profiles, leading to different microstructures and textures. Three-dimensional additive manufacturing simulations demonstrate that orientation-dependent growth patterns govern grain growth, leading to columnar, planar and spiral shaped grains. This approach offers a significant reduction in computational cost while maintaining accuracy, contributing to the practical application of microstructure control in additive manufacturing. The insights gained on grain and texture evolution pave the way for customized microstructure design through additive manufacturing.@en

Additively manufactured Nitinol (NiTi) architectured materials, designed with unit cell architectures, hold promise for customisable applications. However, the common assumption of homogeneity in modeling and additive manufacturing of these architectured materials needs further investigation because geometric-dependent melt pool behaviour results in inhomogeneous microstructure and thermomechanical properties. This study shows that property inhomogeneity at the mesoscale is one reason for pseudo-linear response and partial superelasticity of the fabricated NiTi body-centered cubic (BCC) architectured materials. We modeled using a phenomenological constitutive relation and additively manufactured NiTi architectured materials with varying relative densities. These fabricated samples showed distinct microstructural textures and compositions that affected their local recoverability. The edge effects and laser turn regions were identified as the causes underlying the observed microstructural inhomogeneity. The dimensionless Fourier number is used to describe the transition of printing modes. This study provides valuable information on rigorous experimental/computational consistency in future work.

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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

Boron doped MoSi₂ particles have been envisioned as sacrificial particles for self-healing thermal barrier coatings (TBCs) but their oxidation behaviour is yet not well understood. In this work, oxidation of MoSi₂ based particle is studied in the temperature range of 1050–1200 °C. The oxidation proceeds from a transient to a steady-state oxidation stage. The kinetics during steady-state oxidation is captured with a thermal diffusion-based model. As compared to the oxidation of pure MoSi₂ particles, the addition of boron strongly enhances the silica formation. Also, a finer dispersion of MoB_x in the MoSi₂ matrix accelerates the formation of silica. The oxide growth rate constant increases proportional with the boron content of the MoSi₂ particles. This enhanced oxidation is related to the microstructure of the oxide scale. Upon oxidation, boron yields B₂O₃, which promptly merge with SiO₂ to form amorphous borosilicate, hindering the formation of crystalline SiO₂. Consequently, the migration of oxygen in the borosilicate oxide scale is faster than in the silica oxide scale on pure MoSi₂ particles.

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The binary Fe–Ni system offers alloys with notably low linear coefficients of thermal expansion (CTE), contingent upon their Ni content. In this respect twin-wire arc additive manufacturing (T-WAAM) presents the opportunity of in-situ alloying through the simultaneous feeding of two metal wires into a weld pool to obtain desired alloy compositions. This study aims to deposit a graded wall with Ni contents targeted at 42, 46, and 52 wt% in the building direction of the wall, along with a block comprising of 46 wt% Ni, employing the T-WAAM approach. The results show effective incorporation of additional Ni into the weld pool and geometrically stable weld beads in the continuous metal transfer mode during the T-WAAM process. This mode led to the defect-free and chemically stable deposition of the graded wall and the block with average Ni contents of 42.3 ± 1.1, 45.8 ± 1.4, 52.6 ± 0.8 wt%, and 46.4 ± 0.9 wt%, respectively. The measured Curie temperatures of the as-deposited alloys and the mean CTE values of alloy 46 were found to be comparable to the commercial alloys. In summary, this study validates the feasibility of in-situ deposition of low thermal expansion alloy compositions, thereby enabling the possibility of on-demand thermal expansion properties.

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Abstract: This work studies the effect of TiC and VC precipitate sizes on hydrogen trapping and embrittlement. Two experimental ferritic HSLA steels containing either TiC or VC carbides for precipitation strengthening are annealed in nitrogen and hydrogen gas. This results in a hydrogen uptake of up to 0.91 and 0.44 wppm in the TiC and VC steels, respectively. TEM and TDS analysis indicate that semi-coherent TiC particles trap hydrogen in misfit dislocations with an activation energy of 43 kJ/mol. Coherent VC particles are suggested to trap hydrogen in interface carbon vacancies, with an energy between 53 and 72 kJ/mol. Carbon vacancies are the likely trapping site in incoherent precipitates, where SIMS imaging confirms that incoherent TiC precipitates trap preferentially near the interface, whereas incoherent VC precipitates trap throughout their bulk. Neither alloy is embrittled in SSRT tests after hydrogen absorption, which shows that these precipitates can be used as both a hydrogen sink and a strengthening mechanism in steels. Graphical abstract: (Figure presented.)

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This study examines the interface layer between a high-strength low-alloy steel and an overlaying austenitic stainless steel as deposited through wire arc additive manufacturing in a bi-metal block. By utilizing optical and electron microscopy techniques, and accompanied by phenomenological and thermodynamic modeling, the work elucidates on the nature of the distinct microstructural features at a new level of detail. Results showcase martensite in the form of a band along the fusion line of the first dissimilar layer, as well as in segregated islands. Within the same bead, yet away from the fusion line, an austenite matrix is identified alongside a large phase fraction of primary ferrite and sparse bainite. These findings enhance our understanding of the nature of the heterogeneous microstructure at the interface of a bi-metal build and establish empirical evidence for future modeling of microstructural development. Supplementary characterization reveals the impact of these microstructural heterogeneities on bulk mechanical performance. Hardness indents exhibit varied results along the interface, peaking at martensite islands with values up to 370HV_0.2, surpassing the neighboring matrix by 50%. Under quasi-static tensile loading, bi-metallic specimens display strain partitioning along the fusion boundary, as confirmed by Digital Image Correlation. When compared to the adjoining stainless steel, the diluted interface layer exhibits superior strength (σ_y: 411 MPa) and comparable ductility (24%), leading to necking and failure away from this region. These results help predict the structural performance of bi-metal parts, and build a base for further research in more intricate loading scenarios, such as crack propagation processes.

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Superelastic metallic materials possessing large recoverable strains are widely used in automotive, aerospace and energy conversion industries. Superelastic materials working at high temperatures and with a wide temperature range are increasingly required for demanding applications. Until recently, high-temperature superelasticity has only been achievable with multicomponent alloys fabricated by complex processes. In this study, a novel framework of multi-scale models enabling texture and microstructure design is proposed for high-performance NiTi fabrication via laser powder bed fusion. Based on the developed framework, a Ni-lean Ni(49.4 at.%)-Ti alloy is, for the first time, endowed with a 4% high-temperature compressive superelasticity. A 001 texture, unfavorable for plastic slip, is created to realize enhanced functionality. The unprecedented superelasticity can be maintained up to 453 K, which is comparable with but has a wider superelastic temperature range (∼110 K) than rare earth alloyed NiTi alloys, previously only realizable with grain refinement, and other complicated post-processing operations. At the same time, its shape memory stability is also improved due to existing textured 100 martensite and intergranular precipitation of Ti₂NiOx. This discovery reframes the way that we design superior performance NiTi based alloys through directly tailoring crystallographic orientations during additive manufacturing.

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In energy absorption applications, architectured metallic materials generally suffer from unrecoverable deformation as a result of local yield damage or inelastic buckling. Nitinol (NiTi) offers recoverable deformation and energy dissipation due to its unique superelasticity, which can change the way we design and additively manufacture energy-absorbing architectured materials. The interplay between microstructure, mesoscopic deformation, and macroscopic thermomechanical response of NiTi architectured materials is still not studied in depth. In this work, NiTi architectured materials featuring anisotropic superelastic response, recoverable energy absorption and damping were successfully modeled and manufactured using laser powder bed fusion (L-PBF). Extensive numerical models demonstrated that NiTi architectured materials exhibit temperature-dependent superelasticity and effective transformation stress which can be controlled by the relative density and cell architecture. An effective transformation surface was developed based on the extended Hill's model, illustrating anisotropy is temperature-independent. Stable cyclic behavior with 2.8 % of reversible strain and damping behavior was successfully achieved in cyclic compressive tests without yielding damage or plastic buckling, which further illustrates that the progressive martensitic transformation is the main deformation and energy dissipation mechanism. A comparative study between designed herein body centered cubic (BCC) and octet structures showed that local microstructures significantly affect the deformation modes. The integrated computational and experimental study enables tailoring the superelasticity by combining structural design and microstructural control. Architectured materials designed in this study are potentially applicable as reusable impact absorbers in aerospace, automotive, maritime and vibration-proof structures.

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This study investigated the in-situ gaseous (under 150 bar) hydrogen embrittlement behaviour of additively manufactured (AM) Inconel 718 produced from sustainable feedstock. Here, sustainable feedstock refers to the Inconel 718 powder produced by vacuum induction melting inert gas atomisation of failed printed parts or waste from CNC machining. All Inconel 718 samples, namely AM-as-processed, AM-heat-treated and conventional samples showed severe hydrogen embrittlement. Additionally, it was found that despite its higher yield strength (1462 ± 8 MPa) and the presence of δ phase, heat-treated AM Inconel 718 demonstrates 64% lower degree of hydrogen embrittlement compared to the wrought counterpart (Y.S. 1069 ± 4 MPa). This was linked to the anisotropic microstructure induced by the AM process, which was found to cause directional embrittlement unlike the wrought samples showing isotropic embrittlement. In conclusion, this study shows that AM Inconel 718 produced from recycled feedstock shows better hydrogen embrittlement resistance compared to the wrought sample. Furthermore, the unique anisotropic properties, seen in this study for Inconel 718 manufactured by laser powder bed fusion, could be considered further in component design to help minimise the degree of hydrogen embrittlement.@en

The pursuit of enhancing NiTi superelasticity through laser powder bed fusion (L-PBF) and [001] texture creation poses a challenge due to increased susceptibility to hot cracking in the resulting microstructure with columnar grains. This limitation restricts NiTi's application and contributes to material waste. To overcome this, we introduce a pioneering approach: utilising spark plasma sintering (SPS) to heal directional cracks in [001] textured L-PBF NiTi shape memory alloy. Diffusion bonding and oxygen utilisation for Ti₂NiO_x formation was found to successfully heal the cracks. SPS enhances mechanical properties, superelasticity at higher temperatures, and two-way shape memory strain during thermomechanical cycling. This work provides an alternative solution for healing cracks in L-PBF parts, enabling the sustainable reuse of cracked materials. By implementing SPS, this approach effectively addresses hot cracking limitations, expanding the application potential of L-PBF NiTi parts while improving their functional and mechanical properties.

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Boron containing MoSi₂ is a promising material for applications at high temperature, but the oxidation mechanism is still unclear. In this work, the high temperature (1100 °C) oxidation of B doped MoSi₂ in synthetic air has been investigated. A (boro)silicate layer is formed on the surface of the alloy, which features a mixture of amorphous SiO₂ and cristobalite. After an initial transient period, the oxidation kinetics follows a parabolic growth rate law. The growth rate constant of the oxide layer is enhanced by the boron in the alloy by 90 % per at.% B. The increase in growth rate is associated with boron mitigating the formation of cristobalite thereby promoting the formation of amorphous SiO₂.

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Mo(Al_xSi_1-x)₂ alloy with x in the range of 0.35–0.65 were prepared by a one-step spark plasma sintering. To study the exclusive formation of an α-Al₂O₃ scale, oxidation experiments were conducted in low and high oxygen partial pressure ambient at 1373 K; viz.: 10⁻¹⁴ and 0.21 atm. The oxidation kinetics follows a parabolic rate law after a transient period. A counter-diffusion process of O and Al along grain boundaries of Al₂O₃ scale is responsible for the equiaxed and columnar grain growth based on a two-layered microstructure. The formation of a dense equiaxed α-Al₂O₃ layer contributes to excellent oxidation resistance.

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Electrochemical tests and surface analysis were applied to study the corrosion behavior and passive film characteristics of three-dimensional-printed NiTi shape memory alloys fabricated by laser-powder bed fusion (L-PBF) in artificial saliva at 37 °C. The passivity of L-PBF NiTi shows to be influenced by the process parameters and resulting morphological and physicochemical surface properties. The results show that the defects at the surface of L-PBF NiTi can promote the passivation rate in the early stages of exposure but a slowly formed passive film shows the best corrosion protection. The thickness of the passive film is positively correlated with its corrosion protective performance. The L-PBF NiTi alloy prepared at a linear energy density of 0.2 J·m⁻¹ and volumetric energy density of 56 J·mm⁻³ shows the least defects and best corrosion protection. An outer Ti-rich and inner Ni-rich dense passive film could be also obtained showing higher corrosion resistance. Graphic Abstract: [Figure not available: see fulltext.]

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In this work, the hydrogen fatigue of pipeline steel X60, its girth welds and weld defects were investigated through 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 effect of hydrogen was investigated by measuring the crack initiation and growth, using a direct current potential drop (DCPD) set-up, which probes the outer surface of the specimen. The base and weld metal specimens both experienced a reduction in fatigue life in the presence of hydrogen. For the base metal, the reduction in fatigue life manifested solely in the crack growth phase; hydrogen accelerated the crack growth by a factor of 4. The crack growth rate for the weld metal accelerated by a factor of 8. However, in contrast to the base metal, the weld metal also experienced a reduction of 57% in resistance to crack initiation. Macropores (>500 µm in size) on the notch surface reduced the fatigue life by a factor of 11. Varying the pressure from 70 barg to 150 barg of hydrogen caused no difference in the hydrogen fatigue behavior of the weld metal. The fracture path of the base and weld metal transitioned from transgranular and ductile in nature to a mixed-mode transgranular and intergranular quasi-cleavage fracture. Hydrogen accelerated the crack growth by decreasing the roughness- and plasticity-induced crack closure. The worst case scenario for pipelines was found in the case of weld defects. This work therefore highlights the necessity to re-evaluate pipelines for existing defects before they can be reused for hydrogen transport.@en