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In recent works, it was noticed that Mg/MgH2 mixed with additives by high energy ball milling allows temperature reductions of H2 absorption/desorption without necessarily changing thermodynamic properties. Thus, the objective of this work was to investigate which additives, mixed in low fractions with MgH2 powder would act as efficient hydrogen absorption/desorption catalysts at low temperatures, mainly at room temperature (RT). MgH2 mixtures with 2 mol% additives (Fe, Nb2O5, TiAl and TiFe) were prepared by high energy reactive ball milling (RM). MgH2–TiFe mixture showed the best results, both during desorption at 330 °C and absorption at RT. The hydrogen absorption was ≈ 2.67 wt% H2 in 1 h and ≈ 4.44 wt% H2 in 16 h (40% and 67% of maximum theoretical capacity, respectively). The MgH2–TiFe superior performance was attributed to the hydrogen attraction by the created high energy interfaces and strong TiFe catalytic action facilitating the H2 flow during Mg/MgH2 reactions.

Mg is a lightweight element that can increase the gravimetric hydrogen storage capacity of high entropy alloys (HEAs). This work presents a new approach to design single-phase Mg-containing HEAs with attractive hydrogen storage properties. The design method is based on four calculated parameters (ϕ, VEC, Δ H ∞ ¯ and Δ H f o ¯ ) that allow us to find alloy compositions that form single body-centered cubic (BCC) solid solutions with high hydrogen affinity. The method was tested in the Mg–Al–Ti–Mn–Nb system and the Mg12Al11Ti33Mn11Nb33 alloy was selected among 1326 calculated compositions. This alloy was produced by high energy ball milling resulting in a homogeneous single-phase BCC alloy that absorbed 1.7 wt.% of H by forming a BCC monohydride. Despite its H uptake being H/M = 1, the gravimetric capacity of the lightweight Mg12Al11Ti33Mn11Nb33 alloy was comparable to refractory BCC-HEAs with H uptake of H/M = 2.

Recently, it has been reported that the wear resistance of different stainless steel grades can be substantially improved by addition of boron as alloying element, however, with a trade-off in the corrosion resistance. In this work, CALPHAD method was employed to design a ferritic and an austenitic stainless steel modified with 1 wt pct B with high corrosion resistance. The designed alloys were produced by conventional casting in graphite mold and characterized by X-ray diffraction (XRD), optical microscopy and scanning electron microscopy (SEM). Structural characterization revealed that the target microstructures composed of ferrite and austenite matrixes with hard borides were successfully achieved. Their corrosion resistance was evaluated by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) tests in synthetic seawater and compared to conventional 316 L stainless steel. It was shown that designing the chemical composition of the alloys’ matrixes is the key to obtain corrosion resistant boron-modified stainless steels.

The ternary MgVAl, MgVCr, MgVNi, quaternary MgVAlCr, MgVAlNi, MgVCrNi and quinary MgVAlCrNi alloys were produced by high energy ball milling (HEBM) under hydrogen pressure (3.0 MPa) as a strategy to find lightweight alloys for hydrogen storage applications. Most of the ternary and quaternary alloys presented multiphase structure, composed mainly of body-centered cubic (BCC) solid solutions and Mg-based hydrides. Only the quinary MgVAlCrNi high entropy alloy (HEA) formed a single-phase structure (BCC solid solution), which is a novel lightweight (ρ = 5.48 g/cm3) single-phase HEA. The hydrogen storage capacity of this alloy was found to be very low (approximately 0.3 wt% of H). Two non-equiatomic alloys with higher fraction of Mg and V (strong hydride former elements), namely Mg28V28Al19Cr19Ni6 and Mg26V31Al31Cr6Ni6, were then designed, aiming at higher storage capacity. Both alloys were produced by HEBM. The results show that the non-stoichiometric alloys also presented low hydrogen storage capacity. The low affinity of these alloys with hydrogen was discussed in terms of enthalpy of hydrogen solution and enthalpy of hydride formation of the single components. This study brought to light the importance of considering both enthalpy of hydrogen solution and enthalpy of hydride formation of the alloying elements for designing Mg-containing HEA for hydrogen storage. Once Mg has a positive enthalpy of hydrogen solution, the alloys composition must be balanced with alloying elements with higher hydrogen affinity, i.e., negative values of enthalpy of solution and hydride formation.

Recently, a new class of alloys, namely, high-entropy alloys (HEAs), started to be investigated for hydrogen storage as they can form metal hydrides. Considering that the properties of metal hydrides are greatly influenced by the type of phase formed, and chemical composition, HEAs (with their vastness of compositions) present a high potential for developing promising materials for this application. A crucial aspect in assessing the potential of these alloys is the effective compositional design and synthesis. Here, we evaluate the methods used for obtaining HEAs for hydrogen storage and, based on the most advanced discussions of phase formation and stability in HEAs, we expose some strategies for a better assessment of the vast compositional field. Moreover, we present and discuss the first attempts to model the hydrogenation properties of HEAs using thermodynamics and data science. The development of these kinds of predictive tools is paramount for exploring HEAs' potential for hydrogen storage. To date, the most promising HEA compositions can be classified into three classes: body-centered cubic HEAs, lightweight HEAs, and intermetallic HEAs.

The hydrogen storage properties of the novel equiatomic TiZrNbFeNi and non-equiatomic Ti20Zr20Nb5Fe40Ni15 high entropy alloys (HEAs) were studied. These alloys were designed with the aid of thermodynamic calculations using the CALPHAD method due to their tendency to form single C14 Laves phase, a phase desirable for room-temperature hydrogen storage. The alloys, which were synthesized by arc melting, showed a dominant presence of C14 Laves phases with the (Zr, Ti)1(Fe, Ni, Nb, Ti)2 constitution and small amounts of cubic phases (

Recently it was shown that the Al–Cu–Fe–Cr system can form a microstructure consisting mainly of quasicrystalline and α-Al phases by arc-melting and subsequent annealing of an appropriate composition. In the present work we demonstrate that it is possible to fabricate a large volume of this composite directly from the liquid phase by spray forming an Al85Cu6Fe3Cr6 (%at) alloy. The overspray powder and an arc-melted sample were also analyzed. Tribological properties of the Al-based composite reinforced with quasicrystals was evaluated by pin-on-disk tests, which demonstrated to be superior than an Al–Si alloy, A380, used in this work as a comparison material.

Cast Al(5.5 to 6.3 wt pct)Si-(3.1 to 3.7 wt pct)Cu alloys containing 0.6 and 1.2 wt pct Fe were processed by spray forming, rotary swaging, and homogenization/solution heat treatment, which led to a tensile ductility of 16 pct. Such outstanding ductility is ascribed to the microstructure formed: globular silicon particles, refined iron-rich intermetallics, and a supersaturated solid solution of Cu in the α-Al matrix that promoted dynamic strain aging and homogenization of the deformation process.

This study aims to better understand the Fe role in the hydrogen sorption kinetics of Mg–Fe composites. Mg-8 mol% Fe nanocomposites produced by high energy reactive milling (RM) for 10 h resulted in MgH2 mixed with free Fe and a low fraction of Mg2FeH6. Increasing milling time to 24 h allowed formation of a high fraction of Mg2FeH6 mixed with MgH2. The hydrogen absorption/desorption behavior of the nanocomposites reactive milled for 10 and 24 h was investigated by in-situ synchrotron X-ray diffraction, thermal analyses and kinetics measurements in Sieverts-type apparatus. It was found that both 10 and 24 h milled nanocomposites presents extremely fast hydrogen absorption/desorption kinetics in relatively mild conditions, i.e., 300–350 °C under 10 bar H2 for absorption and 0.13 bar H2 for desorption. Nanocomposites with MgH2, low Fe fraction and no Mg2FeH6 are suggested to be the most appropriate solution for hydrogen storage under the mild conditions studied.

Despite potential of hydride-forming alloys for hydrogen storage, there have been few alloys which can reversibly store hydrogen without heating or activation treatment. In this study, a high-entropy alloy is designed for room temperature hydrogen storage based on three criteria: total valence electron concentration (VEC) of 6.4, single-phase thermodynamic stability (examined by CALPHAD calculations) and AB2H3 hydride formation (A: hydride-forming elements, B: elements without affinity to hydrogen, H: hydrogen). The designated alloy, TiZrCrMnFeNi containing 95 wt% C14 Laves phase, absorbs and desorbs 1.7 wt% of hydrogen (hydrogen-to-metal ratio: 1) at room temperature with a fast kinetics and without activation treatment.

International audience; Ti-V-Nb-Cr alloys have been reported as potential candidates for hydrogen storage applications. Investigating the processes of absorption and desorption is paramount to understand the hydrogen storage properties of these novel alloys and to develop more efficient hydrogen storage materials. In this work, we investigated the hydrogen absorption/desorption reactions of the (TiVNb)85Cr15 BCC multicomponent alloy by laboratory and synchrotron X-ray diffraction, Pair Distribution Function (PDF) analyses, Transmission Electron Microscopy (TEM) and thermo-desorption analyses (TDS). Hydrogen absorption behavior was studied by pressure-composition-isotherm (PCI) at room temperature, which demonstrated levels of absorption around 2 H/M (H/M = hydrogen-to-metal ratio) with low equilibrium pressure. The results showed a multi-step hydrogenation process: alloy ↔ BCC solid solution ↔ BCC intermediate hydride ↔ FCC dihydride. PDF analyses showed that the partially hydrogenated alloy at different levels of H/M and the fully hydrogenated alloy can be reasonably described by models with phases presenting random atomic distribution in the metal crystallographic sites. Moreover, the partially hydrogenated sample prior to the complete formation of the intermediate hydride showed evidence of two co-existing BCC phases.

Design methods with predictive properties modelling are paramount tools to explore the vast compositional field of multicomponent alloys. The applicability of an alloy as a hydrogen storage media is governed by its pressure-composition-temperature (PCT) diagram. Therefore, the prediction of PCT diagrams for multicomponent alloys is fundamental to design alloys with optimized properties for hydrogen storage applications. In this work, a strategy to design single C14-type Laves phase multicomponent alloys for hydrogen storage assisted by computational thermodynamic is presented. Since electronic and geometrical factors play an important role in the formation and stability of multicomponent Laves phase, valence electron concentration (VEC), atomic radius ratio (r_A/r_B), and atomic size mismatch (δ) are initially considered to screen a high number of compositions and find alloy systems prone to form Laves phase structure. Then, CALPHAD method was employed to find 142 alloys of the (Ti, Zr or Nb)(Cr, Mn, Fe, Co, Ni, Cu, or Zn)2 system predicted to crystallize as single C14 Laves phase structure. In addition, we present a thermodynamic model to calculate PCT diagrams of C14 Laves phase alloys based solely on the alloy’s composition. In this work, the entropy and enthalpy of hydrogen solution in the C14 Laves phase were modelled considering that hydrogen solid solution occurs only at the A2B2-type interstitial sites of the C14 Laves phase structure. Experimental pressure-composition-isotherm (PCI) diagrams of six C14 Laves phase alloys were compared against the calculated ones resulting in a good prediction capability. Therefore, the room temperature PCI diagrams of 142 single C14 Laves phase multicomponent alloys were calculated. The results show that single C14 Laves phase multicomponent alloys within a wide range of equilibrium pressure at room temperature can be obtained, being promising candidates for different hydrogen storage applications, such as room temperature tanks, hybrid tanks and Ni-metal hydrides batteries.

The Co55Cr40Ni5 multicomponent alloy was designed to be single-phase face centered cubic (FCC), display high strength, and have the ability to undergo transformation-induced plasticity (TRIP) during plastic deformation. The alloy was produced, and phase stability was monitored in-situ by synchrotron X-ray diffraction during thermomechanical deformation to observe the formation and evolution of the hexagonal close packed (HCP) martensite phase as a function of applied stress. The alloy displays a good combination of strength and ductility, owing largely to the TRIP effect, which increases the strain hardening rate during deformation.

Spray forming is an advanced casting process that produces refined and homogenous microstructure directly from the liquid metal regardless of the alloy system. However, the microstructure evolution during spray forming is complex because the process comprises two sequential steps with very different cooling rates, i.e., atomization and deposition. It is well known that the microstructure of cast irons is highly dependent on the chemical composition and the cooling rate imposed to the liquid. In order to better understand the microstructural evolution during solidification by spray forming, this study investigated the solidification of two cast irons with different stable–metastable eutectic temperature interval (ΔTES−M), 30 K and 17 K. The microstructures of both overspray powders presented a dendritic array of austenite with cementite in the inter-dendritic spacing. Despite the high cooling rates imposed to the alloys during the atomization step, the final microstructure was defined by the cooling conditions prevailing during the final step of deposit solidification and stable eutectic was formed. This was ascribed to the dynamic process involving heating and remelting of the low melting temperature phases present in the droplets that arrives completely or partially solid in the deposition zone.

The wear resistance of boron-modified supermartensitic stainless steel coatings produced by high-velocity oxygen fuel (HVOF) was investigated through pin-on-disk measurements. It was shown that addition of boron leads to the formation of an interconnected and rigid boride network delimiting the grain boundaries of the martensitic matrix. A very refined structure was formed as result of the high cooling rates imposed to the molten alloy during the HVOF process. The specific wear rates of the HVOF coatings were about tenfold lower than the boron-free supermartensitic stainless steel, lying in the order of 10−5 mm3/N m. The refined boride skeleton along the HVOF coating was found to be effective to reduce the materials’ removal from the exposed softer martensitic matrix. While the supermartensitic stainless steel master alloy and the mild steel substrate displayed severe adhesive wear, the HVOF coatings exhibited mild delamination wear at low sliding velocities (10 and 20 cm/s) and abrasive wear at the highest tested velocity (40 cm/s). The studied boron-modified supermartensitic HVOF coatings are an interesting approach to protect the surface of inexpensive steel substrates against wear.

Iron has low solubility in solid aluminum and for this reason it can lead to the formation of Fe-rich intermetallic phases (such as β-AlFeSi in Al-Si cast alloys) severely impairing the ductility of the material. Spray forming is an advanced casting process that has been described as a potential technique to highly decrease the β-AlFeSi formation, consequently increasing the ductility. This paper aims to study the effect of pouring and tundish temperature in the microstructure of 319 aluminum alloy with high iron content processed by spray forming. Spray formed ingots and overspray particles were analyzed for two different processing conditions. In order to evaluate the mechanical properties, the spray formed deposits were mechanically conformed by hot swaging. Results showed that the decrease in the β-AlFeSi content strongly depends on the processing conditions. Spray formed deposits processed by hot swaging presented high ductility, allowing a wide range of possibilities to apply casting alloys with high impurity content to more valuable applications beyond the casting industry, therefore decreasing the needs for using primary alloys and contributing to the environment. Keywords: Spray forming, Aluminum upcycling, Solidification, AlFeSi intermetallic, 319 aluminum alloy

The openly available database provided by the US Department of Energy on hydrides for hydrogen storage were analyzed through supervised machine learning to rank features in terms of their importance for determining hydrogen storage capacity referred to as hydrogen weight percent. In this part: we employed four models, namely linear regression, neural network, Bayesian linear regression and boosted decision tree to predict the hydrogen weight percent. For each algorithm, the scored labels were compared to the actual values of hydrogen weight percent. Our investigation showed that boosted decision tree regression performed better than the other algorithms achieving a coefficient of determination of 0.83.

The openly available dataset on hydrogen storage materials provided by the US Department of Energy was used to predict the optimal materials class of metal hydrides based on the desired properties, which included hydrogen-weight percent, heat of formation and operating temperature and pressure. We performed correlation and statistical analyses to investigate the statistical characteristics of each numeric features. We employed four classification algorithms: multiclass logistic regression, multiclass decision forest, multiclass decision jungle and multiclass neural network. Feature importance analysis was carried out to investigate how each classifier utilises the information available in the dataset. In overall, multiclass neural network classifier had better classification performance obtaining an accuracy of 0.80. The results suggest that the complex material class, followed by Mg is applicable for the most wide range of operating temperatures. Positive correlation was found between hydrogen weight percent, heat of formation and temperature, suggesting that the maximum hydrogen weight percent would be achieved in the complex material class operated at a high temperature.

An in-depth analysis of the effect of boron addition on the microstructure of the equiatomic AlCoCrFeNi High Entropy Alloy (HEA) was performed. Alloys with nominal compositions AlCoCrFeNiB0.1 and AlCoCrFeNi were produced and analyzed by multi-scale characterization techniques, including X-ray diffraction, scanning and transmission electron microscopy coupled with TEM-based orientation mapping technique. CALPHAD simulation was carried out to understand the solidification sequence and the phase stability of the studied alloys. The equiatomic AlCoCrFeNi alloy forms a mixture of an Al-Ni rich B2 phase with a Fe-Cr rich body centered cubic (BCC) phase; small face centered cubic (FCC) islands were also observed. The boron addition leads to the formation of needle-like Cr-rich borides with an orthorhombic structure. Moreover, this addition changes considerably the solidification sequence in later stages of the solidification process, resulting in a more complex microstructure.

Crystal structure and hydrogen storage properties of a novel equiatomic TiZrNbCrFe high-entropy alloy (HEA) were studied. The selected alloy, which had a A3B2-type configuration (A: elements forming hydride, B: elements with low chemical affinity with hydrogen) was designed to produce a hydride with a hydrogen-to-metal atomic ratio (H/M) higher than those for the AB2- and AB-type alloys. The phase stability of alloy was investigated through thermodynamic calculations by the CALPHAD method. The alloy after arc melting showed the dominant presence of a solid solution C14 Laves phase (98.4%) with a minor proportion of a disordered BCC phase (1.6%). Hydrogen storage properties investigated at different temperatures revealed that the alloy was able to reversibly absorb and fully desorb 1.9 wt% of hydrogen at 473 K. During the hydrogenation, the initial C14 and BCC crystal structures were fully converted into the C14 and FCC hydrides, respectively. The H/M value was 1.32 which is higher than the value of 1 reported for the AB2- and AB-type HEAs. The present results show that good hydrogen storage capacity and reversibility at moderate temperatures can be attained in HEAs with new configurations such as A3B2/A3B2H7.

The applicability of an alloy as a hydrogen storage media mostly relies on its pressure-composition-temperature (PCT) diagram. Since the PCT diagram is composition-dependent, the vast compositional filed of high entropy alloys, complex concentrated alloys or multicomponent alloys can be explored to design alloys with optimized properties for each application. In this work, we present a thermodynamic model to calculate PCT diagrams of body-centered (BCC) multicomponent alloys. The entropy of the phases is described using the ideal configurational entropy for interstitial solid solutions with site blocking effect. As a first approximation, it is assumed that the H partial molar enthalpy of a phase is constant, so the enthalpy of H mixing varies linearly with the H concentration. Moreover, the H partial enthalpy of a phase for a multicomponent alloy was approximated by a simple ideal mixture law of this quantity for the alloy’s components with the same structure. Experimental data and DFT calculations were used for parametrization of the enthalpy terms of eight elements (Ti, V, Cr, Ni, Zr, Nb, Hf, and Ta), which are the components of the alloys tested in this work. Experimental PCTs of six BCC multicomponent alloys of four different systems were compared against the calculated ones and the agreement was remarkable. The model and parameters presented here can be regarded as a basis for developing powerful alloy design tools for different hydrogen storage applications.

International audience; The applicability of an alloy as a hydrogen storage media mostly relies on its pressurecomposition-temperature (PCT) diagram. Since the PCT diagram is compositiondependent, the vast compositional field of high entropy alloys, complex concentrated alloys or multicomponent alloys can be explored to design alloys with optimized properties for each application. In this work, we present a thermodynamic model to calculate PCT diagrams of body-centered (BCC) multicomponent alloys. The entropy of the phases is described using the ideal configurational entropy for interstitial solid solutions with site blocking effect. As a first approximation, it is assumed that the hydrogen partial molar enthalpy of a phase is constant, so the enthalpy of hydrogen mixing varies linearly with the hydrogen concentration. Moreover, the hydrogen partial enthalpy of a phase for a multicomponent alloy was approximated by a simple ideal mixture law of this quantity for the alloy's components with the same structure. Experimental data and DFT calculations were used for parametrization of the enthalpy terms of eight elements (Ti, V, Cr, Ni, Zr, Nb, Hf, and Ta), which are the components of the alloys tested in this work. Experimental PCTs of six BCC multicomponent alloys of four different systems were compared against the calculated ones and the agreement was remarkable. The model and parameters presented here can be regarded as a basis for developing powerful alloy design tools for different hydrogen storage applications.

International audience; A new quinary multi-principal element alloy Ti0.30V0.25Zr0.10Nb0.25Ta0.10 was prepared by high temperature melting technique and the physicochemical as well as hydrogen sorption properties have been determined. The as-cast alloy crystallizes into a single-phase bcc lattice and can very quickly absorb hydrogen at room temperature forming a fcc hydride phase with capacity of 2 H/M (2.5 wt.%). The absorption/desorption of hydrogen is reversible and occurs within one step, as proven by in situ neutron diffraction for the desorption reaction. The capacity is slightly fading during the first 10 cycles and then stabilizes at around 2.2 wt.% for the next 10 cycles.

A secondary AlSi7.5Cu3-(Fe,Mn) alloy with small additions of Mn and Fe was processed by an innovative rotational outward solidification casting process to produce a functionally graded aluminum (FGA). A cylinder was produced in a single step presenting two well-defined metallurgically bonded layers (A and B) with distinct chemical composition, structure and mechanical properties. Both layers were deeply characterized through different techniques. Layer A microstructure has more than 50%vol. of the primary α-Al12(Fe,Mn,Cr)3Si quasicrystal approximant phase embedded in an Al-FCC matrix. On the other hand, layer B presents the typical hypoeutectic microstructure of the conventional AlSi7.5Cu3 alloy. Microhardness of the primary α-Al12(Fe,Mn,Cr)3Si phase and macro-hardness profile of the FGA were measured. It was found that the hardness of layer A is approximately 1.6 times higher than layer B. This new FGA may be an alternative for producing parts that require gradient properties such as, for instance, automobile engine blocks. Keywords: AlSiCu alloy, Rotational outward solidification casting, Functionally graded aluminum, Quasicrystal approximant phase

The lightweight Fe&ndash, Mn&ndash, Al&ndash, C steels have drawn considerable attention from the literature due to their outstanding combination of high ductility and specific strength. Although the mechanical behavior of such steels has been extensively studied, the effect of Al when no C and Si are added has not been investigated in detail. For this reason, the main objective of this work was to study the microstructural evolution and mechanical behavior of carbon and silicon-free high-Mn steels with different aluminum contents. Alloys with 0, 2.5, and 5 wt. % Al were processed by spray forming to ensure high homogeneity and a fully austenitic microstructure. Cold rolling and annealing were performed to obtain a fine grain-sized material. The mechanical properties were similar regardless of the Al content, especially the work hardening rate. Deformation twinning and strain-induced phase transformation were not observed for any of the compositions. Additionally, a dislocation cell-like structure was observed for all of the alloys indicating that the Al additions did not change considerably the dislocation behavior, even though it considerably changed the estimated Stacking Fault Energy (SFE) value for all the alloys studied in this work.

The reversible reaction of hydrogen with metallic Mg to form MgH2 has been evaluated by more than 70 years, with particular attention in the last 20 to 30 years for the exceptional properties of this hydride to store energy. The main challenges to the practical use of these properties were soon identified; the high thermodynamical stability of the hydride, resulting in high temperatures for absorption / desorption of hydrogen; the low diffusion rate of hydrogen in Mg and in MgH2, which requires short diffusion distances and therefore, large surface, interface or interphase areas and the presence of catalyst to improve the absorption and desorption kinetics. Thermodynamic stability cannot be modified by the microstructure, which can, however, affect the other above-mentioned challenges. The first Mg samples to react with hydrogen were prepared by conventional metallurgical routes, but with the important addition of being crushed to be smaller than a certain particle size. Turnings of bulk Mg or Mg alloys samples were also used, resulting in the same increase in surface and interfaces area as the crushed particles. Alloys of different compositions, including eutectic alloys were designed to result in a combination of phases, with one of them in conditions to form a hydride and the other to act as catalyst, or most likely just supplying extra interphases for hydrogen diffusion. But since melting of Mg and Mg alloys is not a friendly process, mechanical alloying was considered simpler and more adequate to prepare Mg powders, resulting in the identification of the exceptional properties of nano powders to improve kinetics but also stressing the difficulties of handling nano powders and the deleterious effect of surface contamination. Severe plastic deformation was then used to prepare “bulk” nanograined material with the expectation to design microstructures with special textures and special types of grain boundaries and to avoid the problem associated with surface reactivity. Because of kinetics limitation, the SPD techniques had to be combined with other processing step, such as filing, cold rolling or milling, basically to increase the surface / interface area. Extensive plastic deformation via co-lamination had already been used but the necessity to complement SPD processing led to the use of conventional metallurgical processing such as cold rolling and cold forging to process Mg and MgH2. Such metallurgical routes prove to be simpler and cost-effective to deliver microstructures satisfying the requirement for Mg and MgH2 to act as hydrogen storage material. Other processing routes or combination of routes that could result in refined microstructure have also been studied, including melt spinning, melt spinning combined with HEBM or with cold-rolling, friction stir welding, mechanical grinding and filing and chips from lathe turnings. In this review we discuss the evolution of metallurgical processing of Mg and MgH2 based hydrogen storage material and how each processing route can lead to microstructures with the required characteristics.

In this work, boron-modified duplex stainless steels were prepared by spray forming using design guidelines provided by thermodynamic calculations. Firstly, an investigation of stable phases and phase formation sequence in duplex steels containing high levels of boron was conducted. The calculation indicated that there was an eutectic point at around 1 wt% boron with different primary phase formations upon equilibrium solidification. For hypoeutectic compositions, the primary phase was δ-Ferrite, whereas for hypereutectic a metallic boride (M2B) should form. Additionally, eutectic reactions for both compositions should lead to the formation of borides M2B and M3B2. Secondly, spray forming experiments were conducted based on the thermodynamic calculations. Sample preparation was carried out using a conventional superduplex steel (2507) as starting material. Two different compositions were selected: one hypoeutectic (0.8 wt% B) and one hypereutectic (2.5 wt% B). The microstructural investigation revealed the formation of different types of borides embedded in an austenitic-ferritic matrix. Finally, the wear resistance was evaluated with the dry sand/rubber wheel test and a significant improvement was observed for boron-containing steels in comparison with the same steel without boron. This improvement was attributed to the presence of fine and well-distributed boride particles that protected the austenitic-ferritic matrix from material removal.

Cr-Co-Ni Multi-Principal Element Alloys (MPEAs) with good combinations of strength and ductility were studied to determine their attendant corrosion performance. Three alloys – Cr45Co27.5Ni27.5, Cr33.3Co33.3Ni33.3, and Cr25Co37.5Ni37.5 – were produced in recrystallized and cold-worked states, and their electrochemical response was tested in a simulated seawater electrolyte. Increasing the Cr content improved the yield strength (σy) and ultimate tensile strength (UTS), while maintaining high (>40%) uniform elongation. Potentiodynamic polarization and electrochemical impedance spectroscopy revealed high corrosion resistance of the alloys in simulated seawater, in particular the Cr-rich alloy (Cr45Co27.5Ni27.5). Furthermore, following 40% cold work, the Cr45Co27.5Ni27.5 alloy displayed a further improvement in corrosion resistance. The Cr45Co27.5Ni27.5 alloy displays mechanical and corrosion properties that exceed those of conventional structural alloys such as Ni-superalloys, stainless steels and most 3d-transition metal MPEAs, including those without appreciable ductility. Therefore, in the present work it is shown that increasing the Cr content in Cr-Co-Ni alloys leads to a better combination of mechanical properties and corrosion resistance in saline environment, as observed especially for the Cr45Co27.5Ni27.5 alloy.

Pseudo-high entropy (PHE) amorphous alloys have emerged as novel metallic materials. Some Fe-containing PHE amorphous alloys display the unique characteristic of maintaining a clustered glassy structure in a wide temperature range, and eventually crystallize multi-principal element (MPE) phases whose composition is close to the parental alloy composition. Therefore, these PHE amorphous alloys promise to surpass the trade-off between crystallization and corrosion resistance typical of most bulk metallic glasses. This work investigates the corrosion resistance in chloride media of (Fe0.25Co0.25Ni0.25Cr0.125Mo0.125)100−xBx (x = 8, 10, 12) alloys produced by melt-spinning. The B-richest composition, (Fe0.25Co0.25Ni0.25Cr0.125Mo0.125)88B12, ensured fully amorphous ribbons, while MPE nanocrystals were formed within the amorphous (Fe0.25Co0.25Ni0.25Cr0.125Mo0.125)90B10 and (Fe0.25Co0.25Ni0.25Cr0.125Mo0.125)92B8 ribbons. All alloys were corrosion-resistant in acidic (pH 3) and alkaline (pH 10) chloride-rich electrolyte (35 g L−1 NaCl) and in a hypersaline medium (250 g L−1 NaCl, pH 3), as verified from potentiodynamic polarization and electrochemical impedance spectroscopy measurements. From x-ray photoelectron spectroscopy, the formation of a nanometric-thick film composed of Fe-, Co-, Ni-, Cr-, and Mo- compounds on the alloys' surface was observed, responsible for granting the corrosion resistance of all alloys. Although containing crystals, the B-poorest alloy, (Fe0.25Co0.25Ni0.25Cr0.125Mo0.125)92B8, presented comparable and even superior corrosion resistance than the fully amorphous B-richest alloy, (Fe0.25Co0.25Ni0.25Cr0.125Mo0.125)88B12. The maintenance of the corrosion resistance of the (Fe0.25Co0.25Ni0.25Cr0.125Mo0.125)90B10 and (Fe0.25Co0.25Ni0.25Cr0.125Mo0.125)92B8 alloys in the presence of crystals were ascribed to the reduced elemental partition upon crystallization of MPE phases. The excellent corrosion resistance nevertheless vanished if excessive crystallization occurs, forming borides. These findings open new prospects for overcoming the corrosion-wear resistance paradigm of amorphous/crystalline alloys, essential to design materials and coatings to endure harsh environments.

This paper provides insights into microstructure control of recycled Al-7Si-3Cu alloy, which is widely used in the automotive industry. Here we report a detailed study of the Mn and V addition effects on the morphological modification of the quasicrystal approximant α-phase. We demonstrate that the addition of 0.3 wt% of V and 1.0 wt% of Mn to the Al-7Si-3Cu-1Fe alloy combined with the solidification features of spray-forming leads to the formation of a microstructure containing nano-submicron size quasicrystal approximant α-phase. Such microstructure results in an improved combination of strength and ductility up to 250 °C. Rotary swaging and Si globulization treatment performed as secondary processing leads to further improvement of the alloy’s tensile properties at different temperatures. The present work provides significant contributions on recycling of alloys without addition of primary Al and a prospect for more valuable applications.

In this study, the MgZrTiFe0.5Co0.5Ni0.5 high entropy alloy was processed by high-energy ball milling under both argon and hydrogen atmospheres. The hydrogen storage behavior of the samples was evaluated by combination of thermal analyses and in-situ synchrotron powder X-ray diffraction. It is shown that this alloy forms a body-centered cubic (BCC) structure when milled under argon pressure. The BCC phase is capable to absorb up to 1.2%wt. of hydrogen and during absorption it undergoes a phase transition forming a face-centered cubic (FCC) high entropy hydride. This FCC hydride can be directly synthesized by high-energy ball milling under hydrogen pressure.

The electrochemical hydrogenation (galvanostatic charging) and desorption (potentiostatic discharging) of Nb in an alkaline solution was investigated, using Pd as a reference model system. The phases formed during hydrogenation and those present after desorption were investigated by X-ray diffraction (XRD). The cathodic charging induced the formation of NbH0.89 and NbH2 for Nb and PdH0.6 for Pd. Nb hydrides appeared to be irreversibly formed, unlikely to the PdH0.6 that spontaneously decomposed back into Pd. The irreversible character of absorption process in Nb is likely to be related to oxide/hydroxides formation on the electrode surface in the alkaline media, which might hinder the dehydrogenation reaction. The electrochemical behavior was evaluated through potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). Results were analyzed according to both thermodynamic and kinetic approaches. EIS results indicated that Pd and Nb have very similar hydrogen evolution reaction (HER) kinetic parameters. A theoretical development based on the different kinetic contributions to the overall charge-transfer resistance, including the absorption step's indirect contribution, is presented.

Mixtures of XMg–Co containing different amounts of Mg (X = 2, 3 and 7) were reactive milled under hydrogen atmosphere. 2Mg–Co only formed the Mg 2 CoH 5 complex hydride, while the mixtures 3Mg–Co and 7Mg–Co formed different contents of Mg 2 CoH 5 and MgH 2 . Their structural features and hydrogen storage properties were analyzed by different techniques. In-situ synchrotron X-ray diffraction, combined with thermal analysis techniques, (differential scanning calorimetry, thermal gravimetric analysis and quadrupole mass spectrometer) was carried out to observe the behavior of the MgH 2 –Mg 2 CoH 5 mixtures during the first H-desorption. It was found that the presence of the Mg 2 CoH 5 complex hydride has a beneficial effect on the first H-desorption of the MgH 2 . Additionally, after first desorption, conventional hydrogenation under high pressure and high temperature of 3Mg–Co and 7Mg–Co samples led to the formation of the Mg 6 Co 2 H 11 complex hydride. The presence of Mg 6 Co 2 H 11 considerably impaired the desorption properties of the nanocomposites.