During solidification processing, most common titanium alloys solidify as high-temperature parent phase and it subsequently transforms into the room-temperature child phase. This parent to child transformation occurred as a solid-state transformation. Even though the parent phase is not observable at room temperature, it is considerably associated with solidification defects such as porosity. Parent grain reconstruction based on Burgers orientation relationship, which shows the crystallographic relationship between two phases, has been applied to examine the high temperature and transitional parent phases of titanium alloys and steels. This contribution presents the reconstructed high temperature parent phase and the sensitivity analysis related to a series of parameters for three crucial parent phase reconstruction steps for additively manufactured titanium alloy Ti–6Al–4V. The analysis examines the reconstruction, merging of very small and neighbouring grains together, and finally cleaning of inclusive noise, which referred as inclusions during the rendering process of the parent phase. The analysis offers an instruction for threshold selection that enables optimised parent phase reconstruction.

Growth kinetics and orientation selection play a significant role in microstructure evolution during metal solidification, while gravity-induced convection adds significant complexity to the process. In-situ, time-resolved X-ray imaging of solidifying grain-refined Al-20 wt.% Cu alloy onboard the MASER-13 sounding rocket enabled the study of equiaxed dendrite growth under diffusion-controlled conditions, eliminating the influence of gravity. A machine learning-enabled analytical pipeline was developed to extract and evaluate the spatiotemporal behaviour of a large number of individual dendrites, including their growth characteristics, rotations and interactions. Post-flight synchrotron X-ray computed tomography and electron backscatter diffraction were used to reconstruct the three-dimensional dendrite structure with embedded details of crystallographic orientations. Correlated data analysis confirmed that most dendrites grew along directions parallel to the {100} plane under highly isothermal, diffusion-controlled conditions. However, growth along atypical directions was also observed, even in this simplified regime. The benchmark data revealed variation in dendrite arm evolution, influenced by local grain interactions and crystallographic orientation selection. It is shown that the equiaxed grains have random crystallographic orientations and evidence suggests that these survive from shortly after nucleation in the bulk liquid under microgravity conditions. The data processing protocols demonstrated here highlight the potential of integrating advanced experimental techniques with modern data science approaches to analyse solidification microstructure formation in metallic alloys under terrestrial and microgravity conditions.

Abstract Materials science underpins national economies and infrastructures worldwide, contributing significantly to the delivery of key services, as well as supporting multiple industrial sectors, including space. The space sector, represents a cornerstone of technological progress, driving both direct and indirect innovation across many terrestrial fields. Taken together, materials science and the space sector represent a transformative frontier that remains only partially exploited, offering opportunities for scientific, economic, and societal advancement. In this context, this roadmap presents a special focus on where such interplay can yield fruitful outcomes over the next two decades, exploring specific intersections between these fields. The rationale behind this is routed in the current demand and forward view for specialized materials to address both terrestrial challenges and extraterrestrial ambitions, with the global space economy projected to reach $1.8 trillion by 2035, requiring a coordinated interdisciplinary effort. Microgravity provides a unique platform to achieve this, enabling critical insights, more precise control over material formation and the creation of advanced materials with enhanced properties for diverse applications. These breakthroughs are already informing applications across a range of industries on Earth, from semiconductors to pharmaceuticals, while laying the groundwork for advanced manufacturing in space. Emerging sectors such as the ‘In-Orbit Economy’ and in-situ resource utilization (ISRU) further underscore this critical opportunity, emphasizing the need for resilient materials that can withstand the harsh conditions of space and utilize extraterrestrial resources sustainably. This roadmap brings together a diverse array of contributions covering relevant subjects across these areas—from space exploration and ISRU to the production of new inorganic, organic, and even ‘living’ materials on microgravity platforms—while paying special attention to concrete examples, i.e. cases with a technological readiness level of 4 or higher, which warrant continued attention and effort.

The role that gravity plays on the solidification behaviour of equiaxed dendrites in metallic alloys has long been an area of research interest. Experimental data has previously been successfully obtained in pursuit of an answer to this question, most notably via the acquisition of in-situ X-ray solidification videos under different gravitational conditions, including in microgravity through the use of sounding rockets. However, analysis of these videos has, until now, proven to be a manually-intensive, time-consuming and costly exercise, with the image characteristics of these videos demanding pixel-by-pixel and frame-by-frame measurement. This has made attempting to gain insights from already completed experiments very expensive. To rectify this situation, an automated analysis system has been developed, using the result of the MASER-13 sounding rocket flight for primary experimental inputs. Through the use of machine learning and a number of bespoke image analysis and dendritic behavioural estimation techniques, two in-situ, X-ray videos of Al-Cu were successfully analysed: one under terrestrial gravitational conditions and the other under microgravity. The obtained and discussed dendritic measurements highlight that terrestrial dendrites exhibit greater out-of-plane orientations and an increased prominence of secondary arms, despite solidifying at an identical rate to their microgravity equivalents. These results demonstrate the effectiveness of the developed solution while the 72 hour timescale utilised provides a significant reduction in the time and resources required to obtain dendritic measurements, when compared to the manual alternative.

With a high enough cooling rate from above the liquidus, certain Mg-Zn-Ca alloys form Bulk Metallic Glasses. Here, we study the cooling rate-driven amorphous/crystalline phase transformations in Mg-Zn-Ca alloys with limited glass-forming ability. We investigated the microstructural evolution of bioresorbable Mg-Zn-Ca alloys initially melted using induction heating in an inert atmosphere. Rods of Mg66Zn30Ca4 (at. %) of different lengths and diameters were fabricated using suction casting into a cylindrical water-cooled copper mould, in which the cooling rate varies along the length. In parallel, a computer simulation was conducted using finite element-based software to estimate the cooling rate in the alloy, based on the melt pouring and mould temperatures measured during the experiment. Microstructural and compositional characterisations were carried out to reveal the effects of phase transformation kinetics on the degree of crystallinity in the resulting rods. These analyses were conducted using X-ray diffraction and scanning electron microscopy with energy-dispersive X-ray spectroscopy. The phi 3 mm rod can be cast to create an amorphous structure at all axial locations, indicating the critical cooling rate has been achieved throughout, but the microstructure of phi 5 mm rod is highly dependent on the cooling rate, characterised by location in the axial direction. Melt flow discontinuity and compositional inconsistencies are also possible factors contributing to the structural inhomogeneity. This finding will be useful in determining the required setup for producing Mg66Zn30Ca4 bulk metallic glass and composites for biomedical applications.

Processing (3-titanium alloys via metal additive manufacturing (AM) is a rapidly expanding area, hence understanding their solidification behaviour under rapid cooling is critical. The solidification of Ti-Nb-Ta alloys was investigated under various cooling rates (c. 2000-20,000 K/s) using suction casting. An inverted partitioning effect was determined for Ti-20Nb-10Ta (wt.%) alloy, where solute is preferentially retained in the dendrites under very rapid solidification. In relatively slower-cooled samples, the microsegregation followed Scheil solidification theory. Moreover, columnar morphology was replaced with equiaxed structures with increasing cooling rate. These effects were not observed in the thermodynamically similar binary alloys, Ti-25Nb and Ti-35Ta.

Titanium alloys, mainly Ti-6Al-4V, are renowned for their impressive strength-to-weight ratio and stand as some of the most widely used metallic materials for bioimplants. Additive manufacturing introduces a paradigm shift in the short turnaround times for the availability of such implants. The biological performance of these implants is critical to ensure their success and is understood to be affected by a variety of factors, including surface characteristics and phase composition of the material, often determined by the manufacturing approach. The experimental investigation of the difference in biological performance caused by surface roughness and phase compositions resulting from manufacturing methods that involve laser powder bed fusion (LPBF) and hot isostatic pressing (HIP) has been conducted. Surface roughness was found to be the prevailing effect over the reported phase composition difference, with a relatively rougher surface seeming to be better for biological performance in this contribution. Meanwhile, HIP-ed Ti-6Al-4V samples exhibit better cell viability compared to that of the as-built LPBF-ed Ti-6Al-4V samples.

The equiatomic face-centred cubic (FCC) CoNiCrFeMn alloy, known as the Cantor alloy, is renowned for its high ductility under extreme conditions, such as cryogenic temperatures. Despite this, it suffers from low hardness and yield strength (YS) and includes elements with significant supply concerns. This study introduces novel non-equiatomic CoNiCrFeMn alloys, designed using machine learning (ML)-assisted-high-throughput atomistic simulations to enhance sustainability and mechanical properties such as hardness and YS while maintaining the alloy's single-phase FCC structure. We incorporated two critical sustainability indicators, end-of-life recycling rate (EOL - RR) and lifetime (tau), to guide the alloy design process. Mean-flow stress (MFS), measured from the yield point to 15% strain during tensile tests, was used to assess and predict the mechanical properties. Two alloys, with cobalt contents of 12.5% and 21.5%, were developed and analysed. The Co12.5 alloy showed better sustainability (44% improvement in tau with almost the same mechanical properties), while the Co21.5 alloy exhibited better mechanical properties (20% improvement in MFS with almost the same sustainability) as the equiatomic system. Their stable FCC microstructures were confirmed through CALPHAD modelling and X-ray diffraction (XRD) analysis of vacuum arc melted, as-cast samples. The results highlight the potential of integrating sustainability metrics into high-performance alloy design.

X-ray radioscopy enables the in-situ monitoring of metal alloy processes and then gives access to crucial information on the dynamics of the underlying phenomena. In the last decade, the utilisation of this powerful imaging technique has been adapted to microgravity platforms such as sounding rockets and parabolic flights. The combination of microgravity experimentation with X-ray radioscopy has resulted in a leap in the understanding of fundamental science and has opened new paths in the fields of materials science. The present review focuses on the short history of this research, which includes facility developments, microgravity experiments and results obtained by partners of the XRMON (In-situ X-Ray MONitoring of advanced metallurgical processes under microgravity and terrestrial conditions) research project in the framework of the MAP (Microgravity Application Promotion) programme of the European Space Agency. Three illustrative research topics that were advanced significantly through the use of X-ray radioscopy will be detailed: solidification of metal alloys, metallic foam formation and diffusion in melts.

A computational framework is developed to understand the transient behavior of isothermal and non-isothermal transformation between liquid and solid phases in a binary alloy using a phase-field method. The non-isothermal condition was achieved by applying a thermal gradient along the computational domain. The bulk solid and liquid phases were treated as regular solutions, along with introducing an order parameter (phase field) as a function of space and time to describe the interfacial region between the two phases. An antitrapping flux term was integrated into the present phase-field model to mitigate the amount of solute trapping, which is characterized by the non-equilibrium partitioning of the solute. The governing equations for the phase field and the solute composition were solved by the cell-centered finite volume method using the open-source computational tool OpenFOAM. Simulations were carried out for the evolution of equiaxed dendrites inside an undercooled melt of a binary alloy, considering the effect of various computational parameters such as interface thickness, strength of crystal anisotropy, stochastic noise amplitude, and initial orientation. The simulated results show that the solidification morphology is sensitive to the magnitude of anisotropy as well as the amplitude of noise. A strong influence of interface thickness on the growth morphology and solute redistribution during solidification was observed. Incorporating antitrapping flux resulted in the solute partitioning close to the equilibrium value. Simulations show that the grain shape is unaffected by changes to crystallographic orientation with respect to the Cartesian computational grid. Thermal gradients exerted discernible effects on the solute distribution and the dendritic growth pattern. Starting with multiple nucleation events the model predicted realistic polycrystalline solidification and as-solidified microstructure.

In-situ compression of Cu38Zr54Al8 metallic glass micropillars in a scanning electron microscope was performed together with continuous acoustic emission recording on samples in the as-cast state and after pre-deformation by high pressure torsion. A size effect was detected in both shear band operation and acoustic emission signal, irrespective of the preliminary deformation. This phenomenon was explained by the size dependent compliance of the pillar/indenter system. Differences between acoustic emission signals from the as-cast and pre-deformed states indicated changes in the shear band formation mechanism due to high pressure torsion. These differences were clear in the elastic regime of the in-situ compression and supported the role of rejuvenation in stabilization of unstable shear bands.

This study investigated the print homogeneity of Ti-6Al-4 V alloy parts, when printed over a large build area of 250 $$\times$$ 250 $$\times$$ 170 mm3, using a production scale laser powder bed additive manufacturing system. The effect of part location across this large build area was investigated based on printed part porosity, microstructure, hardness, and tensile properties. In addition, a Hot Isostatic Pressing (HIP) treatment was carried out on the as-built parts, to evaluate its impact on the material properties. A small increase in part porosity from 0.01 to 0.09%, was observed with increasing distance from the argon gas flow inlet, which was located on one side of the build plate, during printing. This effect, which was found to be independent of height from the build plate, is likely to be associated with enhanced levels of condensate or spatter residue, being deposited at distances, further from the gas flow. Despite small differences in porosity, no significant differences were obtained for microstructural features such as prior β grain, $$\alpha$$ lath thickness, and phase fraction, over the entire build area. Due to this, mechanical performances such as hardness and tensile strengths were also found to be homogenous across the build area. Additionally, it was also observed based on the lattice constants that partial in-situ decomposition of $${\alpha }^{^{\prime}}\to \alpha +\beta$$ phases occurred during printing. Post HIP treatment result showed a decrease of 7 and 6%, in the yield strength (YS) and ultimate tensile strength (UTS), respectively, which was associated with a coarsening of $$\alpha$$ lath widths. The potential of the laser powder bed system for large area printing was successfully demonstrated based on the homogenous microstructure and mechanical properties of the Ti-6Al-4 V alloy parts.

Additive manufacturing has transformed the way we think about component fabrication. Generating a geometry in a layer-by-layer fashion presents many advantages over traditional subtractive methods, but also presents many challenges pertaining to the highly localised and energetic nature of the heat source. Since the material passes through multiple heating and cooling cycles throughout the build, some of which completely melt and erase the microstructure, a dynamic simulation is necessary to determine the grain structure that emerges. Grains are generally, but not exclusively, highly textured with columnar grains commonly spanning multiple layers. Fast, efficient and parallelised envelope cellular automata based models are used to simulate the nucleation and growth of the individual crystals that comprise the grain structure, with trade-offs being made between intra-grain detail and computational efficiency so that meso-scale simulations are possible. Simplified, but physically sound thermal models are used to predict the thermal conditions at the melt pool periphery, which are weakly coupled to the grain growth model. Dendrite tip kinetics models are used to determine alloy specific growth laws as a function of local undercooling. The effect of various processing parameters on as-solidified grain size, morphology and texture are investigated for aluminium alloys 3D printed by laser powder bed fusion.

John Hunt was a scholar of the solidification of metallic alloys, and published seminal works that are of influence today in the design of alloy casting, welding and additive manufacturing processes. John was raised in the West Country of England, and following school did National Service in the Royal Air Force. He entered the University of Cambridge in 1957, being awarded an honours BA in metallurgy in 1960, and a PhD for studies of solidification of eutectic alloys in 1963. John met Ann Carroll during his Cambridge studies; they married in 1961 and later had three children. From 1963 to 1965 John carried out research with Ken Jackson at Bell Laboratories, New Jersey, USA. They published groundbreaking papers on solidification and pioneered the use of transparent organic analogues. John returned to England and, following a year working at the UK Atomic Energy Authority at Harwell, he joined the faculty at the University of Oxford in 1966. During a 36-year academic career at Oxford, John made significant contributions to the scientific understanding of solidification on many topics, including dendritic array growth, eutectic and peritectic solidification, the columnar-to-equiaxed transition and twin roll casting. As well as being a talented experimentalist, he published many groundbreaking theoretical analyses and also pioneered the use of numerical methods for the simulation of solidification phenomena. He was an accomplished educator of undergraduate and postgraduate students, and a fellow of St Edmund Hall, Oxford. John was a family man, and kept geese and pigs, grew apples and made legendary cider at his old farmhouse home north of Oxford.

Laser powder bed fusion (LPBF) offers unique opportunities to produce metallic components without conventional design and manufacturing constraints. During additive manufacturing process, titanium alloys like Ti-6Al-4V undergo solid-state transformation that conceals initial solidification microstructure from room-temperature observations. Revealing the as-solidified microstructure can be critical to understanding the early stages of solidification. Using orientation relationships between parent (α) and child (β) phases, the as-solidified microstructures across the LPBF build volume has been reconstructed. Based on the as-solidified parent phase information, variations of the thermal and solidification conditions that occur during the LPBF of Ti-6Al-4V are revealed. The results show that how high cooling rates in the initially solidified lower layers contributed to orientation distribution during parent phase solidification, compared to upper layers in the build volume. Furthermore, the approach demonstrates the potential to further explore solidification microstructure and defect formation in titanium alloys during additive manufacturing.

Bulk metallic glass of Cu 38 Zr 54 Al 8 nominal composition was synthesized by copper mold casting into 6 mm diameter rods. Disks of the as-cast glass were subjected to severe plastic deformation by high-pressure torsion for different number of revolutions. The microstructure and the thermal behavior of the as-cast, isothermally annealed and deformed glass have been investigated by X-ray diffraction and differential scanning calorimetry, respectively. Continuous heating experiments revealed a two-stage devitrification event with excellent glass forming parameters, such as glass transition ( T g = 671 K), supercooled liquid region (∆ T x = 80 K), reduced glass transition ( T r = 0.57) and gamma parameter ( γ = 0.41). Power law crystal growth during diffusion-controlled homogeneous nucleation was observed for isothermal annealings. Glassy state was preserved almost in the entire sample volume of the as-cast alloy during the high-pressure torsion process, corresponding to the extreme stability of the Cu 38 Zr 54 Al 8 alloy against deformation-induced devitrification. This is in accordance with the transition of the reversible specific heat from the glassy to supercooled liquid state measured by modulated calorimetry. It was also concluded that glassy structure is more ordered in the severely deformed state.

Extraterrestrial environments expose engineering components to severe operating conditions for extended durations. Challenges of extreme temperature changes and high vacuum are being met by careful selection of available materials or by development of novel high-performance materials and processes. Bulk Metallic Glasses (BMGs) are amorphous alloys that exhibit very high strengths, hardness, strain energy storage, corrosion resistance, and the capacity for thermoplastic formability, and are of interest in space engineering design. Using criteria driven by the requirements of specific space-based mechanisms, BMG alloys were selected from the literature for processing and performance evaluation. Alloys particularly suited for gearing applications and flexure-based compliant mechanisms were identified and standard test specimens produced. Four Zr-based BMG alloys were selected for analysis: Zr53Al16Co23.25Ag7.75, Cu47Zr46Al5Y2, Zr49Ti1.96Cu37.24Al9.8Y2, and Zr60Ti2Nb2Al7.5Ni10Cu18.5, with compositions in at.%. Presented in this work are the results of compression testing, fatigue testing, and differential scanning calorimetry; previous results of wear and hardness testing have been presented elsewhere. A maximum space service temperature of 400 °C is considered. The alloys exhibited significant differences in processability and thermomechanical performance. The alloy Zr49Ti1.96Cu37.24Al9.8Y2 exhibited the best overall performance for gearing applications; and Zr60Ti2Nb2Al7.5Ni10Cu18.5 was the best alloy for compliant flexures.

Developing computational tools appropriate for modelling physical phenomena, such as alloy solidification, requires making optimisation decisions on the time and spatial scales that can be resolved with finite computational resources. Here we report on development of a model of grain nucleation and growth, based on a Cellular Automata (CA) approach, that can directly simulate columnar and equiaxed solidification, and their competition to trigger a columnar-equiaxed transition (CET), at the scale of a casting. Previous work has been to develop a Front Tracking (FT) model of columnar solidification and an equiaxed index capable of predicting the relative likelihood of CET formation in a casting case study. This FT model has been validated by extensive experimental studies. Here we apply the new CA model and compare the predictions to those of the FT model. The close agreement between the two models serves to verify the new CA model. The study also presents insights into the thermophysical phenomena affecting grain structure evolution in alloy solidification and confirms the validity and utility of the equiaxed index.

Hydropower production is one of the greatest threats to fluvial ecosystems and freshwater biodiversity. Now that we have entered the Anthropocene, there is an opportunity to reflect on what might constitute a ‘sustainable’ Anthropocene in the context of hydropower and riverine fish populations. Considering elements of existing practices that promote favorable social-ecological outcomes (i.e., ‘bright spots’) is timely given that there are plans to expand hydropower capacity in previously undammed rivers, intensify dam development in some of the world's largest river systems, and re-license existing facilities. We approach this from a pragmatic perspective: for the foreseeable future, hydropower will likely remain an important source of renewable electricity. To offer support for moving toward a more ‘sustainable’ Anthropocene, we provide syntheses of best practices during the siting, design, construction, operation, and compensation phases of hydropower development to minimize impacts on inland fish. For each phase, we offer positive examples (or what might be considered ‘bright spots’) pertaining to some of the approaches described within our syntheses, acknowledging that these projects may not be viewed as without ecological and (or) societal detriment by all stakeholders. Our findings underscore the importance of protecting critical habitat and free-flowing river reaches through careful site selection and basin-scale planning, infrastructure designs that minimize reservoir effects and facilitate safe passage of fish, construction of hydropower plants using best practices that minimize long-term damage, operating guidelines that mimic natural flow conditions, and compensation that is lasting, effective, inclusive, and locally relevant. Learning from these ‘bright spots’ may require engagement of diverse stakeholders, professionals, and governments at scales that extend well beyond a given site, river, or even basin. Indeed, environmental planning that integrates hydropower development into broader discussions of conserving regional biodiversity and ecosystem services will be of utmost importance.

Columnar and equiaxed structures, which occur during solidification of metallic alloys, influence the texture and properties of castings, welded joints and additively manufactured components. During transient solidification, where grain refiner particles provide the predominant nucleation mechanism, a Columnar to Equiaxed Transition (CET) occurs when conditions that had originally favoured directional columnar growth change to those favouring equiaxed. Constitutional undercooling ahead of the columnar front can permit equiaxed nucleation and growth. By carrying out experiments in microgravity conditions, liquid flows due to thermal and solutal buoyancy effects are suppressed. In these diffusion-controlled conditions, we have observed examples of both sharp (clear) and progressive (gradual) CET. The experimental outcomes, especially the observation of a progressive CET, has highlighted the need for a continuum model that allows for competitive columnar and equiaxed structure development; hence, the Concurrent Columnar to Equiaxed Transition (C2ET) model is proposed. The C2ET is thermally transient and relies on the well-known concept of extended growth for impingement mechanics; thereby, greatly reducing numerical complexity. Importantly, the proposed approach removes the need for a specific equiaxed-blocking criterion, which is often proposed as an essential requirement in other CET models. The C2ET model is validated by four experimental solidification scenarios: two velocity jumps and two thermal-gradient decreases. The velocity jumps induced sharp CETs; whereas, thermal-gradient decreases gave progressive CETs. The C2ET model gave good agreement for the columnar and equiaxed transition zones for both sharp and progressive CET. Results are compared with the classic Hunt model. Unlike Hunt's model, the C2ET model predicted all macrostructure transitions faithfully using a single (or consistent) set of nucleation input parameters across all four scenarios. Since, the same level of grain refinement was used in each experiment, a consistent set of nucleation parameters was expected. The validated approach can enable effective simulation at lower computational cost for industrial processes that rely on a solidification processing step.