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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.

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.

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.

International audience; In this study, the MgAlTiFeNi high entropy alloy was processed by high-energy ball milling under both argon and hydrogen atmospheres. It is shown that this alloy forms a body-centered cubic (BCC) structure when milled under an argon atmosphere (mechanical alloying-MA) and a combination of BCC, FCC, and Mg2FeH6 when milled under hydrogen pressure (reactive milling-RM). The hydrogen storage behavior of the RM-MgAlTiFeNi samples was evaluated by a combination of thermal analyses and manometric measurements in a Sieverts apparatus. The RM-MgAlTiFeNi alloy presented an initial functional hydrogen storage capacity of 0.87 wt%, which increased to 0.94 wt% after the second absorption. Also, it exhibited a high hydrogen absorption and desorption kinetics at temperatures 100 °C lower than the one for the desorption temperature of the commercial MgH2. Electrochemical discharge of RM-MgAlTiFeNi samples showed precisely the same hydrogen content as that obtained in the gas desorption. Electrochemical charging/discharging experiments were also performed in the MA-MgAlTiFeNi samples, which presented lower electrochemical storage capacity, a behavior probably resulting from the instability of the alloy in the alkaline solution with the formation of a hydroxide layer on its surface that hinders the electrochemical reactions

In this work, the corrosion behavior of amorphous and partially crystallized Fe68Cr8Mo4Nb4B16 alloys has been studied. Fully amorphous ribbons were prepared by melt spinning and then partially and fully crystallized at different heat treatment temperatures. The corrosion behavior was evaluated in chloride-rich media at different pHs, ranging from acidic to alkaline. An exceptionally high corrosion resistance was observed for the new Fe68Cr8Mo4Nb4B16 amorphous alloy in the proposed media and pHs. The amorphous phase containing corrosion-resistant alloying elements such as chromium and molybdenum led to the formation of a highly stable passivating film, which coated and protected the active exposed metal surface. Partially crystallized samples heated up to 700 °C still grant the formation of a corrosion-resistant passivating layer. The presence of such a layer was correlated to the non-percolation of crystals embedded into the corrosion-resistant amorphous matrix. The corrosion resistance of fully crystalline annealed ribbons and as-cast ingots did not present the similar superior corrosion performance. This behavior was assigned to element partitioning throughout the crystallization, particularly chromium, which led to a non-homogeneous structure that preferentially triggered and held pitting corrosion along with the percolated crystalline material. These results indicate that the Fe68Cr8Mo4Nb4B16 alloy is a candidate for corrosion resistant coating where the suppression of crystallization is unavoidable.