Mg-based alloys are industrially used for structural applications, both as solid solutions alloys and as composites containing intermetallic compounds. However, a further development in terms of mechanical properties requires the investigation of underlying causalities between synthesis, processing and microstructure to adjust the mechanical and the corrosion properties, ideally down to the near atomic scale. Such fundamental scientific investigations with high resolution characterisation techniques require model materials of exceptionally high purity and strictly controlled microstructure e.g. with respect to grain size, morphology, chemical homogeneity as well as content and size of oxide inclusions. In this context, the Mg-Al-Ca system appears exceptionally challenging from a metallurgical perspective due to the high reactivity and high vapor pressures, so that conventional industrial techniques cannot be successfully deployed. Here, we demonstrate the applicability of various scientific synthesis methods from arc melting over solution growth to diffusion couples, extending to effects and parameters for thermo-mechanical processing. Suitable pathways to overcome the specific challenges of the Mg-Al-Ca system are demonstrated, as well as the persistent limitations of the current state of the art laboratory metallurgy technology.

The influence of chemical composition changes on the room temperature mechanical properties in the binary Ca33Al67 C15 CaAl2 Laves phase were investigated in two ternary alloys with off-stoichiometric compositions with 5.7 at.% Mg substitution (Ca33Al61Mg6) and 10.8 at.% Mg and 3.0 at.% Ca substitution (Ca36Al53Mg11) and compared to the stoichiometric (Ca33Al67) composition. Cubic Ca-Al-Mg Laves phases with multiple crystallographic orientations were characterised and deformed using nanoindentation. The hardness and indentation modulus were measured to be 4.1 ± 0.3 GPa and 71.3 ± 1.5 GPa for Ca36Al53Mg11, 4.6 ± 0.2 GPa and 80.4 ± 3.8 GPa for Ca33Al61Mg6 and 4.9 ± 0.3 GPa and 85.5 ± 4.0 GPa for Ca33Al67, taken from our previous study, respectively. The resulting surface traces as well as slip and crack planes, were distinguished on the indentation surfaces, revealing the activation of several different {11n} slip systems, as further confirmed by conventional transmission electron microscopic observations. Additionally, the deformation mechanisms and corresponding energy barriers of activated slip systems were evaluated by atomistic simulations.

We report a comparative study of the bulk electronic structure of two Al-based complex metallic alloys (CMAs), beta-Al3Mg2 and Al13Fe4 using hard x-ray photoemission spectroscopy (HAXPES) interpreted on the basis of density functional theory (DFT) calculations. An experimental confirmation of the role of the Hume-Rothery mechanism for the stability of the beta-Al3Mg2 phase is established by identification of a shallow pseudogap near E-F from HAXPES that is corroborated by DFT. An almost parabolic shape of the density of states (DOS), a large n(E-F), and plasmon loss features that are similar to Al metal show its nearly free-electron-like nature. In the case of Al13Fe4 the total DOS exhibits a shallow pseudogap due to Al s - Fe d hybridization, which results in the DOS at E-F [n(E-F)] being large due to Fe d states. However, the Al s states show a deep pseudogap and this is revealed in HAXPES because of the large photoemission cross section of the s states at high photon energies. The overall shape of the valence band is in excellent agreement with DFT for both the CMAs. The larger width of the Al core-level main peak and the plasmon loss peaks as well as the suppression of the intensities of the latter with respect to beta-Al3Mg2 further underline the importance of sp-d hybridization in Al13Fe4.

The (110) and (320) surfaces of the single-phase FeCrMnNiCo solid solution have been studied on two adjacent millimeter size grains using surface science and transmission electron microscopy (TEM) tech-niques. The structural and chemical evolutions of the high entropy alloy (HEA) surfaces have been determined for various sputtering conditions, annealing temperatures and durations. Up to 873 K, angle-resolved X-ray photoelectron spectroscopy measurements indicate a clear Mn and Ni surface co-segregation. We propose that the surface segregation of Mn is driven by its low surface energy. The attractive interaction between Mn and Ni promotes Ni segregation which accompanied the Mn diffu-sion to the surface. Regarding the structures investigated by low energy electron diffraction and scan-ning tunneling microscopy, the (320) surface presents a terraced morphology with an ordered structure consistent with a ( 1 & times; 1 ) termination. On the contrary, the (110) surface reveals an important degree of structural disorder and local reconstructions. Its highly anisotropic morphology resembles rows propagat -ing along the [001] direction. Above 873 K, Mn desorption occurs while the Ni content keeps increasing linearly with the temperature. TEM analysis show no evidence for HEA decomposition into metallic or intermetallic phases even after repeated annealing and sputtering cycles. The above results set the upper temperature limit above which the surface stoichiometry departs from the quinary HEA concept. It also defines the temperature range for the use of FeCrMnNiCo based coating under high vacuum conditions and for aerospace applications. (c) 2021 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

We have carried out plastic deformation experiments on single-phase samples of the body centered ZrNbTiVHf high-entropy alloy between room temperature and 1150 K. The experiments were carried out on polycrystalline samples with two different grain sizes, in compression at a true constant strain rate of 10-4 1/s. Incremental tests such as stress-relaxation tests, strain-rate changes and temperature changes were carried out in order to determine thermodynamic activation parameters of the plastic deformation process. The material displays a yield-stress anomaly in the temperature range between about 500 and 800 K, where the yield stress increases with increasing temperature. In the same temperature range, we find an extremely low strain-rate dependence of the flow stress, which is reflected in almost constant flow-stress values in stress-relaxations and strain-rate changes. At temperatures below 400 and above 800 K we find regular plastic deformation behavior with a linear temperature dependence of the yield stress of about -1.1 MPa/K, an activation volume of around 1.8 nm3 and 0.2 nm3 at low and high stresses, respectively, and an activation enthalpy of 2.7 eV in the high-temperature range.

We report on the production and characterization of a high-entropy alloy in the refractory Zr-Nb-Ti-V-Hf system. Equiatomic ingots were produced by arc and levitation melting, and were subsequently homogenized by high-temperature annealing. We obtained a coarse-grained, single-phase high-entropy alloy, with a homogeneous distribution of the constituting elements. The phase is a chemically disordered solid solution, based on a bcc lattice with a lattice parameter of 0.336(5) nm.

We report on single-crystal growth of the equiatomic FeCoCrMnAl high-entropy alloy by means of the Bridgman technique. Our crystals are about 1 cm in diameter and 6.6 cm in length. X-ray Laue images taken at various positions on their surface are sharp and mutually consistent. The material has a microstructure consisting of B2 inclusions in a body-centred cubic matrix. The inclusions are {0 0 1} platelets of about 65 nm thickness with varying lateral extensions of the order of 500 nm, predominantly containing Al and Co. The matrix is rich in Fe and Cr.

We have investigated the plastic deformation properties of single-phase Zr-Nb-Ti-Ta-Hf high-entropy alloys from room temperature (RT) up to 300 degrees C. Uniaxial deformation tests at a constant strain rate of 10(-4 )s(-1) were performed, including incremental tests such as stress relaxations, strain-rate changes, and temperature changes in order to determine the thermodynamic activation parameters of the deformation process. The microstructure of deformed samples was characterized by transmission electron microscopy. The strength of the investigated Zr-Nb-Ti-Ta-Hf phase is not as high as the values frequently reported for high-entropy alloys in other systems. At RT we measure a flow stress of about 850 degrees C. We find an activation enthalpy of about 1eV and a stress dependent activation volume between 0.5 and 2nm(3). The measurement of the activation parameters at higher temperatures is affected by structural changes evolving in the material during plastic deformation.

We report on the discovery of a high-entropy alloy with a hexagonal crystal structure. Equiatomic samples in the alloy system Ho-Dy-Y-Gd-Tb were found to solidify as homogeneous single-phase high-entropy alloys. The results of our electron diffraction investigations and high-resolution scanning transmission electron microscopy are consistent with a Mg-type hexagonal structure. The possibility of hexagonal high-entropy alloys in other alloy systems is discussed.

We have investigated the plastic deformation properties of non-equiatomic single phase Zr-Nb-Ti-Ta-Hf high-entropy alloys from room temperature up to 300 C. Uniaxial deformation tests at a constant strain rate of 10^-4 s^-1 were performed including incremental tests such as stress-relaxations, strain-rate- and temperature changes in order to determine the thermodynamic activation parameters of the deformation process. The microstructure of deformed samples was characterized by transmission electron microscopy. The strength of the investigated Zr-Nb-Ti-Ta-Hf phase is not as high as the values frequently reported for high-entropy alloys in other systems. We find an activation enthalpy of about 1 eV and a stress dependent activation volume between 0.5 and 2 nm^3. The measurement of the activation parameters at higher temperatures is affected by structural changes evolving in the material during plastic deformation.

Mechanical deformation drastically affects both microstructure and thermal stability of polycrystalline β-Al3Mg2. Upon milling, the β-Al3Mg2 phase transforms into a nanoscale supersaturated Al(Mg) solid solution with 40 at.% Mg. Upon heating, the milled powders display a complex thermal behavior characterized by four distinct exothermic events. At low temperatures, an increasing amount of Mg is rejected from the solid solution with increasing temperature. At higher temperatures, the β′-phase, a hexagonal phase with composition Al3Mg2, is formed and no traces of the solid solution can be detected, indicating that the solid solution is metastable and transforms into more stable phase(s). Finally, the subsequent exothermic events reveal the formation and growth of the β-Al3Mg2 phase.

Single crystals of Mg-32(Al,Zn)(49), a complex metallic alloy phase with 162 atoms per unit cell and a lattice parameter of 14.16 angstrom, were grown by means of the Bridgman and the Czochralski technique. We are able to produce single crystals of high structural quality with volumes of up to 3 cm(3) and a composition of about Mg36Al27Zn37. (C) 2008 Elsevier Ltd. All rights reserved.

Co-Authors: Michael Feuerbacher, Carsten Thomas, Julien P. A. Makongo, Stefan Hoffmann, Wilder Carrillo-Cabrera, Raul Cardoso, Yuri Grin, Guido Kreiner, Jean-Marc Joubert, Thomas Schenk, Joseph Gastaldi, Henri Nguyen-Thi, Nathalie Mangelinck-Noël, Bernard Billia, Patricia Donnadieu, Aleksandra Czyrska-Filemonowicz, Anna Zielinska-Lipiec, Beata Dubiel, Thomas Weber, Philippe Schaub, Günter Krauss, Volker Gramlich, Jeppe Christensen, Sven Lidin, Daniel Fredrickson, Marek Mihalkovic, Wieslawa Sikora, Janusz Malinowski, Stefan Brühne, Thomas Proffen, Wolf Assmus, Marc de Boissieu, Francoise Bley, Jean-Luis Chemin, Jürgen Schreuer Abstract. The Al−Mg phase diagram has been reinvestigated in the vicinity of the stability range of the Samson phase, β-Mg2Al3 (cF1168). For the composition Mg 38.5 Al 61.5, this cubic phase, space group Fd-3m (no 227), a = 28.242(1) Å, V = 22526(2) Å3, undergoes at 214 °C a first-order phase transition to rhombohedral β′-Mg2Al3(hR293), a = 19.968(1) Å, c = 48.9114(8) Å, V = 16889(2) Å3, (i.e. 22519 Å3 for the equivalent cubic unit cell) space group R3m (no 160), a subgroup of index four of Fd-3m. The structure of the β-phase has been redetermined at ambient temperature as well as in situ at 400 °C. It essentially agrees with Samson's model, even in most of the many partially occupied and split positions. The structure of β′-Mg2Al3is closely related to that of the β-phase. Its atomic sites can be derived from those of the β-phase by group-theoretical considerations. The main difference between the two structures is that all atomic sites are fully occupied in case of the β′-phase. The reciprocal space, Bragg as well as diffuse scattering, has been explored as function of temperature and the β- to β′-phase transition was studied in detail. The microstructures of both phases have been analyzed by electron microscopy and X-ray topography showing them highly defective. Finally, the thermal expansion coefficients and elastic parameters have been determined. Their values are somewhere in between those of Al and Mg.