In situ synchrotron X-ray diffraction studies of hydrogen desorption and absorption properties of Mg and Mg–Mm–Ni after reactive ball milling in hydrogen

Reactive ball milling in hydrogen was applied to synthesize nanocrystalline Mg- and Mg8Mm20Ni-based hydrides with crystallite size below 10 nm. These hydrides were studied by in situ synchrotron X-ray diffraction performed at the Swiss–Norwegian Beam Lines of the European Synchrotron Radiation Facility, Grenoble. Characterization of the phase-structural and microstructural state of the constituents during the reversible processes of synthesis and decomposition of the hydrides upon application of hydrogen pressure or vacuum at temperatures 20–350 °C and kinetics of hydrogen uptake was performed. Details of the mechanism of the phase-structural transformations were provided by high time resolution of the synchrotron X-ray diffraction data, high sensitivity in determining formation of the phase constituents, excellent accuracy in yielding the crystallographic characteristics and in probing the microstructural evolution of the constituents formed.     

Crystal structure and magnetic properties of the binary uranium–nickel alloy UNi4

The crystal structure of binary UNi₄ was studied from single crystal X-ray diffraction data. This compound crystallises with CaCu₅ structure type (space group p6/mmm), with the unit-cell dimension: a=4.8457(2) Å, c=4.0451(2) Å. This phase belongs to the limited solid solution U_1+xNi_5−x with 0.20<x<0.33, where U atoms substitute partially Ni on its two crystallographic sites. UNi₄ is paramagnetic down to 4.2 K.     

Crystal structure and stability of complex precipitate phases in Al–Cu–Mg–(Si) and Al–Zn–Mg alloys

We demonstrate how first-principles total energy calculations may be used to elucidate both the crystal structures and formation enthalpies of complex precipitates in multicomponent Al alloys. For the precipitates, S(Al–Cu–Mg), η′ (Al–Zn–Mg), and Q(Al–Cu–Mg–Si), energetics were computed for each of the models of the crystal structures available in the literature allowing a critical assessment of the validity of the models. In all three systems, energetics were also calculated for solid solution phases as well as other key phases (e.g., equilibrium phases, GP zones) in each precipitation sequence. For both the S and η′ phases, we find that recently proposed structures (based on electron microscopy) produce unreasonably high energies, and thus we suggest that these models should be re-evaluated. However, for all three precipitates, we find that structures based on X-ray diffraction refinements provide both reasonable energetics and structural parameters, and therefore the first-principles results lend support to these structural refinements. Further, we predict energy-lowering site occupations and stoichiometries of the precipitate phases, where experimental information is incomplete. This work suggests that first-principles total energy calculations can be used in the future as a complementary technique with diffraction or microscopy for studying precipitate structures and stabilities.     

Hydrogen absorption properties of Mg−Ni alloys prepared by bulk mechanical alloying

The thermodynamic properties of the hydrides of Mg_2−xNi (x=0–0.5) alloys produced by bulk mechanical alloying (BMA) were determined from pressure-composition (PC) isotherms for absorption over temperatures from 623 to 423 K. The vant Hoff plot for the plateau pressures of isotherms clearly indicated the existence of high and low temperature hydrides with different entropy and enthalpy for hydride formation. It was found that both the entropy and enthalpy values for the high temperature hydride were more negative than for the low temperature hydride. The phase transition temperature was 525 K for Mg_2.0Ni and decreased while increasing the value of x. This allotropic transformation was well confirmed by in-situ XRD observations from RT to 673 K under hydrogen atmosphere or in vacuum. This article based on a presentation made in the symposium “The 2nd KIM-JIM Joint Symposium: Hydrogen Absorbing Materials”, held at Hanyang University, Seoul, Korea, October 27–28, 2000 under the auspices of The Korean Institute of Metals and Materials and The Japan Institute of Metals.     

Multiphase and microstructure effects on the hydrogen absorption/desorption behavior of a Ti–22Al–27Nb alloy

The hydrogen absorption/desorption behavior of hydride-forming alloys depends on their microstructure. We investigated multiphase effects on the hydrogen absorption/desorption behavior at 100°C of a ternary alloy [Ti–22Al–27Nb (at.%)] whose microstructure consists of β (bcc) [or its ordered phase β₀ (B2)] and/or O (orthorhombic, based on Ti₂AlNb) phases. On exposing β₀ single-phase specimens to hydrogen, β hydride (B2) is easily formed. However, further hydriding to γ hydride (bct) does not easily occur. When the O phase exists in the β matrix phase, the β→γ hydride transformation occurs and the reversible absorption/desorption of hydrogen between β and γ hydrides is observed. Roles of the O phase in the reversible hydrogen absorption/desorption behavior are discussed on the basis of the crystal structure correlation between the β phase, O phase and γ hydride phase and the ordering of hydrogen atoms in these phases.     

Strength properties and structure of a submicrocrystalline Al–Mg–Mn alloy under shock compression

The results of studying the strength of a submicrocrystalline aluminum A5083 alloy (chemical composition was 4.4Mg–0.6Mn–0.11Si–0.23Fe–0.03Cr–0.02Cu–0.06Ti wt % and Al base) under shockwave compression are presented. The submicrocrystalline structure of the alloy was produced in the process of dynamic channel-angular pressing at a strain rate of 10⁴ s^–1. The average size of crystallites in the alloy was 180–460 nm. Hugoniot elastic limit σ_HEL, dynamic yield stress σ_y, and the spall strength σ_SP of the submicrocrystalline alloy were determined based on the free-surface velocity profiles of samples during shock compression. It has been established that upon shock compression, the σ_HEL and σ_y of the submicrocrystalline alloy are higher than those of the coarse-grained alloy and σ_sp does not depend on the grain size. The maximum value of σ_HEL reached for the submicrocrystalline alloy is 0.66 GPa, which is greater than that in the coarse-crystalline alloy by 78%. The dynamic yield stress is σ_y = 0.31 GPa, which is higher than that of the coarse-crystalline alloy by 63%. The spall strength is σ_sp = 1.49 GPa. The evolution of the submicrocrystalline structure of the alloy during shock compression was studied. It has been established that a mixed nonequilibrium grain-subgrain structure with a fragment size of about 400 nm is retained after shock compression, and the dislocation density and the hardness of the alloy are increased.     

Enhanced mechanical properties and formability of hot-rolled Mg–Zn–Mn alloy by Ca and Sm alloying

In order to broaden the application of wrought Mg alloy sheets in the automotive industry, the influence of Ca and Sm alloying on the texture evolution, mechanical properties, and formability of a hot-rolled Mg–2Zn–0.2Mn alloy was investigated by OM, XRD, SEM, EBSD, tensile tests, and Erichsen test. The results showed that the average grain size and basal texture intensity of Mg–2Zn–0.2Mn alloys were remarkably decreased after Ca and Sm additions. 0.64 wt.% Ca or 0.48 wt.% Sm addition significantly increased the tensile strength, ductility and formability. Moreover, the synergetic addition of Sm and Ca improved the ductility and formability of Mg–2Zn–0.2Mn alloy, which was due to the change of Ca distribution and further reduction of the size of Ca-containing particles by Sm addition. The results provided a possibility of replacing RE elements with Ca and Sm in Mg alloys which bring about outstanding mechanical properties and formability.     

Crystal structure of novel hydrides in a Mg–Ni–H system prepared under an ultra high pressure

The crystal structure of novel hydrides in the Mg–Ni–H system has been studied using a powder X-ray diffraction and transmission electron microscopy. A cubic-anvil-type apparatus was utilized to prepare samples. The new hydride with a chemical composition of around MgH₂–60 at% Ni was synthesized at 1073 K for 2 h under a pressure as high as 5 GPa. From TGA analysis, the new hydride was found to be Mg₂Ni₃H_3.4. Orthorhombic and monoclinic crystal systems with a primitive cell were proposed as possible symmetries of the new hydride. X-ray and electron diffraction patterns of the new hydride were indexed in an orthorhombic structure with a=0.8859(4), b=1.3740(5), c=0.4694(2) nm. Moreover, decomposition of the hydride into Mg₂Ni was observed by the transmission electron microscopy.     

Crystal structure and thermoelectric properties of chimney–ladder compounds in the Ru2Si3–Mn4Si7 pseudobinary system

Phase relationships of manganese-substituted ruthenium sesquisilicide alloys have been investigated by using X-ray powder diffraction and scanning and transmission electron microscopy. A series of chimney–ladder phases Ru_1?xMn_xSi_y (0.14 ? x ? 0.97, 1.584 ? y ? 1.741) are formed over a wide compositional range between Ru₂Si₃ and Mn₄Si₇. The compositions of these chimney–ladder compounds deviate slightly from the composition line connecting Ru₂Si₃ and Mn₄Si₇, which corresponds to the ideal composition line satisfying VEC (valence electron counting) = 14 rule. The occurrence of this compositional deviation is discussed in terms of the VEC rule and the atomic packing. The thermoelectric properties of the directionally solidified Ru_1?xMn_xSi_y alloys (0.55 ? x ? 0.90) have also been investigated as a function of the Mn content and temperature. The dimensionless figure of merit (ZT) for those alloys with a high Mn content (x ? 0.75) increased with the increase in Mn content. The ZT value for a crystal with x = 0.90 was as high as 0.76 at 874 K.     

Mechanical properties and energy absorption of extruded Mg–2.0Zn–0.3Zr alloy with Y addition

The effects of the rare earth element Y addition on mechanical properties and energy absorption of a low Zn content Mg–Zn–Zr system alloy and the deformation temperature of optimized alloy were investigated by room tensile test, optical microscopy(OM), X-ray diffraction(XRD), scanning electron microscopy(SEM), and transmission electron microscope(TEM). The results show that,after homogenization at 420 °C for 12 h for the as-cast alloys, Mg Zn phase forms, which decreases the strength of Mg–2.0Zn–0.3Zr alloy with Y content of 0.9 wt%. The tensile strength and elongation of the alloy with a Y addition of 5.8 wt% reach the max value(281 ± 2) MPa and(30.1 ± 0.7) %, respectively; the strength and elongation of Mg–2.0Zn–0.3Zr–0.9Y alloy at the optimized extrusion temperature of 330 °C reach(321 ± 1) MPa and(21.9 ± 0.7) %, respectively. The energy absorption increases with the increase of Y content, the max value reached 0.79 MJ m-3with Y content of 5.8 wt%, and the energy absorption of Mg–2.0Zn–0.3Zr–0.9Y alloy at the optimized extrusion temperature of 330 °C reaches0.75 MJ m-3.     

Effect of Mg addition on mechanical and thermoelectrical properties of Al–Al2O3 nanocomposite

The effects of Mg addition on mechanical thermo-electrical properties of Al–Mg/5%Al₂O₃ nanocomposite with different Mg contents (0, 5%, 10% and 20%) produced by mechanical alloying were studied. Scanning electron microscopy analysis (SEM), X-ray diffraction analysis (XRD) and transmission electron microscopy (TEM) were used to characterize the produced powder. The results show that addition of Mg forms a predominant phase (Al–Mg solid solution). By increasing the mass fraction of Mg, the crystallite size decreases and the lattice strain increases which results from the atomic penetration of Mg atoms into the substitutional sites of Al lattice. The microhardness of the composite increases with the increase of the Mg content. The thermal and electrical conductivities increase linearly with the temperature increase in the inspected temperature range. Moreover, the thermal conductivity increases with the increase of Mg content.     

Mechanical and corrosion properties of newly developed Mg–Mn–Ca alloys as potential biodegradable implant materials

Calcium and manganese were selected as alloying elements to develop Mg–2Mn–xCa (x?=?0·8, 1·0, 1·2 wt-%) alloys as potential biodegradable implant materials. The mechanical properties and corrosion mechanism of both as cast and solution naturally age (T4) treated Mg–2Mn–xCa alloys were investigated. The results indicated that the distribution of the second phase dominated the corrosion process. T4 treatment could transfer coarse Mg₂Ca and α-Mn phases into dispersed fine precipitated phases, which improve mechanical and corrosion properties. Mg–2Mn–1·0Ca alloy has the best integrated performance among the developed alloys. This research indicated that T4 treated Mg–2Mn–xCa alloys are a promising candidate used as biodegradable implant material.     

Hydrogen absorption properties of a Zr–Al alloy ball-milled with Ni powder

The preparation of the Zr 84 wt%–Al 16 wt% non-evaporable getter alloy by means of mechanical alloying and its hydrogen absorption characteristics were investigated. Scanning electron micorscopy and energy dispersive X-ray analysis revealed that the mechanical ball-milling with Ni was successfully employed to coated nickel particles on the surfaces of the Zr–Al getter alloy. The resulting composite particles with pure nickel on the surface of the Zr–Al getter compound show good gettering performance and fast sorption kinetics without any activation process.     

Electronic structure and transport properties of Fe–Al alloys

Electronic structure of iron-aluminides (Fe_1−xAl_x) has been calculated for a range of aluminum concentration (0⩽x⩽0.5) by using first principles density functional theory to explain the variation of electrical resistivity with increasing Al content. The Fe–Al intermetallics are modeled by a cluster of 15 atoms confined to their bulk geometry. The location of Al atoms as a function of concentration, x was determined by minimizing the total energy of the clusters. The electronic structure was determined by calculating the total as well as partial density of states around each of the Fe and Al atoms. With increasing Al concentration, the transfer of Al 3p electrons into the minority 3d orbital of Fe not only has a profound effect on the magnetic properties of these intermetallics, but affects their transport properties as well. For example, the observed anomaly in the electrical resistivity of Fe_1−xAl_x that peaks at x=0.33 is found to be a direct consequence of the filling of the Fe 3d orbital with Al valence electrons. The density of states is characterized by three distinct features: a narrow 3d band just below the Fermi energy originating from the Fe atoms, an Al s-band lying deeper in energy, and an Al p-band above the Fermi energy. The energy gap between Al 3p and Fe 3d density of states decreases with increasing Al concentration and for x=0.40, the density of states at the Fermi energy is a strongly hybridized p–d state giving Fe_1−xAl_x metallic-like properties. These features are consistent with the recent photoemission studies carried out at the synchrotron facility at Lawrence Livermore National Laboratory. An anomaly in the temperature dependence of electrical resistivity is also explained in terms of the unique electronic and magnetic structure of these intermetallics.     

Effect of Mg composition on sintering behaviors and mechanical properties of Al–Cu–Mg alloy

Al–3Cu–Mg alloy was fabricated by the powder metallurgy (P/M) processes. Air-atomized powders of each alloying element were blended with various Mg contents (0.5%, 1.5%, and 2.5%, mass fraction). The compaction pressure was selected to achieve the elastic deformation, local plastic deformation, and plastic deformation of powders, respectively, and the sintering temperatures for each composition were determined, where the liquid phase sintering of Cu is dominant. The microstructural analysis of sintered materials was performed using optical microscope (OM) and scanning electron microscope (SEM) to investigate the sintering behaviors and fracture characteristics. The transverse rupture strength (TRS) of sintered materials decreased with greater Mg content (Al–3Cu–2.5Mg). However, Al–3Cu–0.5Mg alloy exhibited moderate TRS but higher specific strength than Al–3Cu without Mg addition.     

Mechanical and dielectric properties of Ni/Al2O3 composites

Ni/Al2O3 composites were prepared by hot pressing approach. The relationship between their microstructure, mechanical, dielectric and magnetic properties with Ni particle content was studied. By increasing the amount of metal in the composite, the relative density and the bending strength decrease gradually. The possible reason is that non-wetting between Ni and alumina in the preparation results in weak adhesion of the Ni/A; interface. For the composites, the maximum fracture toughness is 6.4 MPa. m^1/2, which is about 25% higher than that of pure alumina ceramic. The increase in toughness of the Ni/Al2O3 composites is due to the deformation of nickel particles. The complex dielectric constant measurements indicate that the real part and the imaginary part increase greatly with the Ni content in the frequency range of 8.2-12.4 GHz. The real part and the imaginary part of complex permeability of the composites also increase with increasing Ni content.