Structural studies of stacking variants in Mg-base Friauf–Laves phases

A new ternary derivative of the Laves phases in the Mg-Co-Ga system

Crystal structures of MgCoGa, Mg0.74CoGa0.52 and Mg0.49CoGa0.15 phases from the Mg-Co-Ga system were investigated using single-crystal diffraction. These structures belong to the family of so-called Laves phases. Hexagonal MgCoGa crystallizes as a disordered phase within the MgZn2 structure type. The orthorhombic structure of Mg0.74CoGa0.52 is a distortion variant of MgZn2 and URe2 structure type, and the structural relation is demonstrated in terms of a Bärnighausen formalism group-subgroup transformation scheme. The structure of trigonal phase Mg0.49CoGa0.15 is strongly disordered, as is shown by the presence of adjacent atomic sites which cannot be occupied simultaneously. In Mg0.49CoGa0.15, two subcells (A and B) were obtained in a ratio of 9:1. Subcell A is closely related to MgZn2-type.

Elucidating the role of a unique step-like interfacial structure of η4 precipitates in Al-Zn-Mg alloy

Al-Zn-Mg alloys are widely used in the transportation industry owing to their high strength-to-weight ratio. In these alloys, the main strengthening mechanism is precipitation hardening that occurs because of the formation of nano-sized precipitates. Herein, an interfacial structure of η4 precipitates, one of the main precipitates in these alloys, is revealed using aberration-corrected scanning transmission electron microscopy and first-principles calculations. These precipitates exhibit a pseudo-periodic steps and bridges. The results of this study demonstrate that the peculiar interface structure of η4/Al relieves the strain energy of η4 precipitates thus stabilizing them. The atomistic role of this interfacial structure in the nucleation and growth of the precipitates is elucidated. This study paves the way for tailoring the mechanical properties of alloys by controlling their precipitation kinetics.

Investigating the role of phonons in the phase stability of uranium-based Laves phases

Laves phase alloys possess unique thermal and electrical conduction properties, yet the factors governing phase stability in these systems remain an open question. The influence of phonons in particular has been broadly overlooked. Here, we investigate the UCo_2x Ni_2(1-x) chemical space using density functional theory, which offers a unique opportunity to explore the factors influencing Laves phase stability as all three primary Laves phases (C14, C15, C36) can be stabilized by changing the ratio of Co to Ni. Calculations of the thermodynamic and dynamical stability of pure UCo₂ and UNi₂ in each of three primary Laves phases confirm the stability of experimentally known Laves phases for UNi₂ and UCo₂. A decrease in bonding strength is identified in UNi₂ compared to UCo₂, aligned with redshifts observed in the UNi₂ phonon density of states and a decoupling of the U and Ni vibrational modes. Phonon calculations of C14 UCo₂ reveal dynamical instabilities. Efforts to remove the unstable mode at the Γ point in UCo₂ via atomic displacements break the symmetry of the C14 phase, revealing a lower energy P2/c structure. Vibrational contributions to the free energy were calculated and did not change the thermodynamically stable Laves phase below 1000 K. The temperature-dependent free energies of single phase UCo₂ and UNi₂ were used to interpolate the relative stability of ternary UCo_2x Ni_2(1-x) in each of the three Laves phases at varying temperatures and stoichiometries. The ternary C36 phase is only predicted to be thermodynamically stable over a narrow stoichiometric range below 600 K.

Mg2MnGa3 – An orthorhombically distorted superstructure variant of the hexagonal Laves phase MgZn2

The Laves phase Mg2MnGa3 was synthesized from the elements by arc-melting and subsequent annealing in a silica ampoule at T = 670 K. The structure of Mg2MnGa3 was refined from single-crystal X-ray diffractometer data: URe2 type, Cmcm, a = 543.24(1), b = 869.59(3), c = 858.58(2) pm, wR2 = 0.0556, 273 F 2 values and 24 variables. The manganese and gallium atoms form a three-dimensional network of corner- and face-sharing MnGa3 tetrahedra that derive as a ternary ordering variant from the hexagonal Laves phase MgZn2. The structures of the distortion and coloring variants, i.e., MgZn2, URe2, Mg2Cu3Si and Mg2MnGa3 are discussed on the basis of a Bärnighausen tree. The electronic structure calculation data indicate that in addition to the metallic type of bonding an additional covalent interaction appears between the Ga–Ga and Mn–Ga atoms.

First-Principles Investigations on Structural Stability, Elastic Properties and Electronic Structure of Mg32(Al,Zn)49 Phase and MgZn2 Phase

Al–Mg–Zn alloys reinforced by T–Mg32(Al,Zn)49 phase had higher structure stability and strength than Al–Zn–Mg–(Cu) alloys reinforced by MgZn2 phase, but the reasons for these two kind of alloys was not well-known. To reveal the discrepancy between T phase and MgZn2 phase, the lattice parameters, cohesive energy, and electronic structure as well as the elastic properties were investigated based on density functional theory. Four types of T phase unit cell were employed according to symmetry of space group. The calculated lattice constants well-agreed with experimental data. Compared to MgZn2 phase, T phases obtained lower cohesive energy owing to their partial covalent bond, which may result in a higher structure stability. The elastic modulus E of T phase depended on the occupation of Al atom, and the effect of the occupation of Al atom on the structure and properties of T phase was also discussed.

Influence of Al/Ti Ratio and Ta Concentration on the As-Cast Microstructure, Phase Composition, and Phase Transformation Temperatures of Lost-Wax Ni-Based Superalloy Castings

The as-cast microstructure, alloying element segregation, solidification behavior, and thermal stability of model superalloys based on Inconel 740 with various Al/Ti ratios (0.7, 1.5, 3.4) and Ta (2.0, 3.0, 4.0 wt%) concentrations were investigated via ThermoCalc simulations, scanning and transmission electron microscopy, energy-dispersive X-ray spectroscopy, dilatometry, and differential scanning calorimetry. The solidification of the superalloys began with the formation of primary γ dendrites, followed by MC carbides. The type of subsequently formed phases depended on the superalloys' initial Al/Ti ratio and Ta concentration. The results obtained from solidification simulations were compared to the obtained microstructures. For all castings, the dendritic regions consisted of fine γ' precipitates, with their size mainly depending on the initial Al/Ti ratio, whereas in the interdendritic spaces, (Nb, Ta, Ti)C carbides and Nb-rich Laves phase precipitates were present. In high Al/Ti ratio superalloys, β-NiAl precipitates, strengthened by η and α-Cr phases, were observed. Based on dilatometric results, the dissolution of γ' precipitates was accompanied by a substantial increase in the coefficient of thermal expansion. The end of the dilatation effect took place around the γ' solvus temperature, as determined via calorimetry. Moreover, the bulk solidus temperature was preceded by the dissolution of the Laves phase, which may be accompanied by local melting.

In-situ analysis of continuous cooling precipitation in Al alloys by wide-angle X-ray scattering

The aim of this work is to investigate quench induced precipitation during continuous cooling in aluminium wrought alloys EN AW-7150 and EN AW-6082 using in situ synchrotron wide-angle X-ray scattering (WAXS). While X-ray diffraction is usually an ex situ method, a variety of diffraction patterns were recorded during the cooling process, allowing in situ analysis of the precipitation process. The high beam energy of about 100 keV allows the beam to penetrate a bulk sample with a 4 mm diameter in a quenching dilatometer. Additionally, the high intensity of a synchrotron source enables sufficiently high time resolution for fast in situ cooling experiments. Reaction peaks could be detected and compared with results from differential scanning calorimetry (DSC) by this method. A methodology is presented in this paper to evaluate WAXS data in a way that is directly comparable to DSC-experiments. The results show a high correlation between both techniques, DSC and WAXS, and can significantly improve continuous cooling precipitation diagrams.

Enhanced Formability and Accelerated Precipitation Behavior of 7075 Al Alloy Extruded Rod by High Temperature Aging

This study was conducted for high temperature aging (HTA) to simultaneously reduce current treatment time and increase the tensile ductility of 7075 aluminum alloy. Various high temperatures and different durations for artificial aging were compared. We investigated the microstructure and the tensile properties of 7075 aluminum alloy extruded rod after various HTAs, and compared them with the outcomes of full annealing (O). The total elongation (TE) of the specimen after solution heat treatment (490 °C, 1 h) and artificial aging (280 °C, 12 h) was about 25%. For full annealing, it is known as 21%. The reason for this was the formation of the η phase in the matrix, which had fewer large particles (Al-Cu phase). The hardening of HTA specimens is higher than that of O, indicating necking resistance during homogeneous plastic deformation. Thereby, HTA treatment increases the formability of 7075 aluminum alloy.

Theoretical investigation of the strengthening mechanism and precipitation evolution in high strength Al-Zn-Mg alloys

Density-functional theory calculations have been performed to systematically investigate the behaviors of solute atoms in 7000 series Al-Zn-Mg based alloys. It is found that solute atoms Mg and Zn are likely to segregate to the Σ5(210)[001] tilt Al GB. The bonding environment and interface cohesion will be affected to different degrees. Also, for GPI(100) our calculations indicate that a Zn/Mg/Zn sandwich configuration in the Al matrix (100) planes is energetically favorable. However, for GPII(111) the disordered structure turns out to be the most stable one. It mainly results from strong 3d-3s hybridization interactions between Zn and Mg atoms. Furthermore, the properties of the metastable phase η' and the equilibrium phase η have also been addressed. The present study provides valuable insight for developing Al alloys with superior performance.

Cage-like Ta@B complexes (n = 23-28, q = -1-+ 3) in 18-electron configurations with the highest coordination number of twenty-eight

Inspired by recent observations of the highest coordination numbers of CN = 10 in planar wheel-type complexes in D10h Ta@B10- and CN = 20 in double-ring tubular species in D10d Ta@B20- and theoretical prediction of the smallest endohedral metalloborospherene D2 Ta@B22- (1) with CN = 22, we present herein the possibility of larger endohedral metalloborospherenes C2 Ta@B23 (2), C2 Ta@B24+ (3), C2v Ta@B24- (4), C1 Ta@B25 (5), D2d Ta@B26+ (6), C2 Ta@B272+ (7), and C2 Ta@B283+ (8) based on extensive first-principles theory investigations. These cage-like Ta@Bqn complexes with B6 pentagonal or B7 hexagonal pyramids on their surface turn out to be the global minima of the systems with CN = 23, 24, 24, 25, 26, 27, and 28, respectively, unveiling the highest coordination number of CN = 28 in spherical environments known in chemistry. Detailed bonding analyses show that 1-8 as superatoms conform to the 18-electron configuration with a universal σ + π double delocalization bonding pattern. They are effectively stabilized via spd-π coordination interactions between the Ta center and ηn-Bn ligand which match both geometrically and electronically. Such complexes may serve as embryos of novel metal-boride nanomaterials.

Cobalt-centred boron molecular drums with the highest coordination number in the CoB16- cluster

The electron deficiency and strong bonding capacity of boron have led to a vast variety of molecular structures in chemistry and materials science. Here we report the observation of highly symmetric cobalt-centered boron drum-like structures of CoB₁₆⁻, characterized by photoelectron spectroscopy and ab initio calculations. The photoelectron spectra display a relatively simple spectral pattern, suggesting a high symmetry structure. Two nearly degenerate isomers with D_8d (I) and C_4v (II) symmetries are found computationally to compete for the global minimum. These drum-like structures consist of two B₈ rings sandwiching a cobalt atom, which has the highest coordination number known heretofore in chemistry. We show that doping of boron clusters with a transition metal atom induces an earlier two-dimensional to three-dimensional structural transition. The CoB₁₆⁻ cluster is tested as a building block in a triple-decker sandwich, suggesting a promising route for its realization in the solid state.

Boron is known to form a wide variety of molecular structures. Here, the authors observe the highly symmetric cobalt-centered boron drum-like structure of CoB₁₆⁻, characterized by photoelectron spectroscopy and ab initio calculations, in which the cobalt atom is sixteen-coordinate.

Crystallography of embedded particles in Al–Mg–Zn alloys. Symmetry analysis

Following a proper heat treatment, the alloy system Al–Mg–Zn shows a great wealth of precipitate particles forming coherent (η′ crystals) and incoherent (η crystals) boundaries with the Al matrix. Both the matrix crystal and the precipitate crystals are holohedral, as their point groups correspond to their metric symmetries (m\overline{3}m and 6/mmm). On the basis of published orientational relationships for a principal variant of every known precipitate family, the full sets of orientational variants are deduced by the concepts of intersection groups,Hβ, and variant generating sets,Vβ. The intergrowth symmetry principle has been visualized by stereographic projections. Special attention has been given to patterns of superimposed lattice nodes in reciprocal space and the implications of overlap in terms of observable reflections. It has been uncovered, by artificially reducing the point group symmetry of the coherent η′ phase, whereVβis a proper subgroup of the matrix holohedral, that twin laws for merohedry are revealed whenVβis factorized into a weak direct product.

Electron crystallography of aluminum alloy phases

A series of structure analyses during 1994-2001 by electron crystallographic techniques applied to phases in aluminum alloys are reviewed. Methods for structure solution employ electron diffraction intensity data collected by the precession technique, by selected area micro-diffraction and by the convergent-beam technique. High-resolution electron microscope images (HRTEM) are treated by different kind of processing, including exit wave reconstruction. Crystallographic calculations are performed either by direct method or Patterson and Fourier procedures, assuming kinematical scattering, or by refinement from models derived from HRTEM images. Dynamical scattering calculations can be introduced in the refinement stage or as a correction procedure applied to part of the intensity data. The phases studied include primary Al-Fe-(Si) particles, Al-Mn-Si dispersoids, Al-Co quasicrystals and two types of precipitate phases in age-hardening Al-Mg-Si and Al-Zn-Mg alloys.

Gamma-point lattice free energy estimates from O1 force calculations

We present a new method for estimating the vibrational free energy of crystal (and molecular) structures employing only a single force calculation, for a particularly displaced configuration, in addition to the calculation of the ground state configuration. This displacement vector is the sum of the phonon eigenvectors obtained from a fast-relative to, e.g., density-functional theory (DFT)-Hessian calculation using interatomic potentials. These potentials are based here on effective charges obtained from a DFT calculation of the ground state electronic charge density but could also be based on other, e.g., empiric approaches.

Laves-phase structural changes in the system CaAl2-xMgx

Compounds CaAl(2)(-)(x)Mg(x) (0 < or = x < or = 2) were synthesized and structurally characterized by X-ray diffraction experiments. With increasing Mg content x the sequence of Laves phase structures MgCu(2) --> MgNi(2) --> MgZn(2) is revealed. The homogeneity ranges of the underlying phases were determined to be 0 < or = x < 0.24(1) (MgCu(2) type), 0.66(2) < x < 1.07(3) (MgNi(2) type), and 1.51(5) < x < or = 2.0 (MgZn(2) type). Mg/Al site occupancies in CaAl(1.34)Mg(0.66) and in CaAl(0.44)Mg(1.56) were refined from neutron powder diffraction experiments and exposed a pronounced segregation of Al and Mg in MgNi(2)-type CaAl(1.34)Mg(0.66) where Al atoms preferentially occupy the positions corresponding to trigonal bipyramids. In MgZn(2)-type CaAl(0.44)Mg(1.56), however, the Mg/Al distribution was found to be nearly uniform. Structural stability in the quasi-binary system CaAl(2)(-)(x)Mg(x) was investigated by first-principles calculations in which random occupational disorder of Mg and Al was modeled with the virtual crystal approximation. The theoretical calculations reproduced the experimental compositional stability ranges of the three different Laves phase structures very well. Structural changes in the quasi-binary system CaAl(2)(-)(x)Mg(x) are induced by the electron concentration, which decreases with increasing x. The stability of the different Laves phase structures as a function of electron concentration was analyzed by the method of moments.