Hexagonal Alloys Research Articles

A comprehensive theoretical scheme for understanding the core structure and the mechanical behavior of basal dislocations with solute atmosphere in Mg alloys

The basal dislocations play a key role in the deformation of hexagonal Mg alloys. However, its core structure and mechanical behavior have thus far not yet been understood adequately, largely owing to the fact that the basal dislocation, often decorated with segregated solute atmosphere and dissociated into two partials, cannot be modeled accurately. Significant discrepancies exist between theoretically calculated and experimentally observed structures for these dissociated dislocations. Here we present a comprehensive theoretical scheme to model them in the Mg alloys with alloying elements of Y, Zn, Al and Li, respectively. Firstly, solute–dislocation interactions were computed by appropriate first-principles calculations. Secondly, the statistic solute concentration at a dislocation is estimated by a Fermi–Dirac distribution. Finally, all these effects on dislocations are integrated into an improved two-dimensional Peierls–Nabarro model to predict their core structures and mechanical behaviors in Mg alloys. It is shown that although Zn, Al and Li atoms have little effect on basal dislocations, Y atoms can largely increase their dissociated width. Furthermore, when it moves from its ground state configuration, the dissociated dislocation can further be widened, such that its dissociated widths in a Mg–0.8Y (at%) alloy at 300 K can reach 12–36 nm, in rather good agreement with the experimentally observed values of 20–30 nm. Besides, the Peierls and yield stresses of Mg alloys shall increase with increasing added solute concentration, while they are decreased drastically with increasing the temperature from 300 to 800 K, but with the Mg-Y alloy as somewhat exception, which is again consistent with experimental observations.

Analysis of structural and thermomagnetic properties in hexagonal Fe50Mn35Sn15 alloy

Analysis of structural and thermomagnetic properties in hexagonal Fe50Mn35Sn15 alloy

KEARNS TEXTURE PARAMETERS AND PROPERTIES OF HEXAGONAL MONO- AND POLYCRYSTALS

Problem statement. Polycrystals' physical and mechanical properties are determined by the corresponding properties of single crystals (crystallites) that make up the polycrystal, and their distribution by orientation in the polycrystal (texture). In metals with a hexagonal structure, the use of Kearns texture parameters, which show the degree of coincidence of the hexagonal axis of crystallites with a given direction in a polycrystalline sample, allows determining the property of a polycrystal in this direction, if the properties of a single crystal in the direction of its hexagonal axis and the perpendicular direction are known. It is also possible to solve the inverse problem: determining the properties of a single crystal in the direction of its hexagonal axis and the perpendicular direction based on the properties of the polycrystal and the determined of the Kearns texture parameters. Materials and methods. Elastic and mechanical characteristics of hexagonal alloys based on titanium (Grade 1 and VT1-0) and magnesium (Mg − 10 % Li and ZE10) were studied after different types of deformation − rolling, alternating bending, and twist extrusion. The Kearns texture parameters were determined by the X-ray method based on the data of the construction of inverse pole figures (IPF) in the normal direction (ND) to the sheets plane and the rolling direction (RD). The results of the experiment. It is shown that the specified method allows to calculate the elastic and mechanical properties of the specified polycrystals after they undergo various types of deformation with an error of no more than 5−10 %, as well as to solve the inverse problem of calculating the properties of single crystals with an error of no more than 5 %. Conclusions. Using the parameters of the Cairns texture and the characteristics of single crystals of magnesium alloys ZE10, Mg 5 % Li, titanium Grade1 and VT1-0 made it possible to calculate the corresponding properties of polycrystals and their anisotropy. The use of Cairns texture parameters, experimental values of the modulus of elasticity, tensile strength and yield strength of polycrystalline sheets of the studied magnesium and titanium alloys made it possible to evaluate the characteristics. There are strong correlations between the values of the modulus of elasticity, the mechanical characteristics of the studied sheets of magnesium and titanium alloys, on the one hand, and the corresponding parameters of the Cairns texture, on the other hand.

Experimental investigation of intrinsic mechanism dominated anomalous Hall effect in hexagonal alloys Fe3-xMnxGe (x = 0.05, 0.1, 0.5)

Iron alloys are among the most fundamental and extensively studied alloys, particularly in recent years with the advancements in spintronics. The anomalous Hall effect based on intrinsic Berry curvature has sparked a significant research interest in magnetic materials. In this experimental study, we quantitatively elucidated the AHE mechanism in Fe3-xMnxGe (x = 0.05, 0.1, 0.5) alloy using the Tian-Ye-Jin model. It is found that the intrinsic mechanism associated with the band structure dominates the anomalous Hall effect of alloys, akin to the parent compound Fe3Ge. In addition, the total and separated intrinsic anomalous Hall conductivity decrease with the increase of doping level. The percentage of anomalous Hall conductivity in total anomalous Hall conductivity increases with increasing temperature and decreasing doping level. The percentage of intrinsic contribution was significantly higher than extrinsic contribution, confirming the intrinsic mechanism dominated anomalous Hall effect. This study provides a reference for tailoring transverse transport properties through doping.

Ultra-refractory metal assemblages in calcium-aluminum-rich inclusions: Probes of the inner solar protoplanetary disk

Calcium-aluminum-rich inclusions (CAIs) are the first formed solids in our solar system. Information regarding their formation and alteration is imprinted within their crystal structures, and so analysis of CAIs can provide insight into the early stages of solar system formation. Here we report on micrometer-sized metal grains that occur inside of fluffy type A (FTA) CAIs in the NWA 8323 and Leoville CV3 chondrites. Transmission electron microscopy (TEM) shows that the ultra-refractory metal assemblages contain subhedral grains of crystalline alloys of Pt, Os, Ir, Ru, Fe, Ni, and Mo, with minor amounts of oxides and silicates inclusions. These assemblages occur in melilite and are surrounded by or adjacent to spinel and perovskite. TEM analysis shows that the majority of the alloys present in the assemblages are significantly enriched in Pt-group elements, with compositions of 75 wt % Pt in some Fe-Ni-Pt grains, and >90 wt % Pt-group elements in Os-Ir-Ru grains. Electron diffraction shows that the alloys occur predominantly in a hexagonal (HCP) structure, with a minority of the grains exhibiting cubic (FCC) and tetragonal lattices. To support these findings, we present a thermodynamic model for the formation of hexagonal (HCP) and cubic (BCC and FCC) ultra-refractory alloys. We use an Fe-Os-Ir ternary system to approximate the various compositions and crystal structures observed in the metal grains. Modeling results indicate a condensation temperature for the alloys as high as 1831 K (HCP, 10−4 bar), placing them well above those predicted for the major CAI phases that surround them. Based on the spatial relationships of the refractory metal grains to their host CAIs, our thermodynamic predictions, and prevailing astrophysical models of the solar protoplanetary disk, the data imply that the grains could have formed inward of the regions where CAI materials condensed. We hypothesize that the refractory metal grains were transported radially outward to the part of the disk where CAIs formed and provided a nucleation site for the condensation of CAI phases such as melilite, hibonite, perovskite, and spinel.

Accurate simulation of texture evolution and mechanical response of cubic and hexagonal structural alloys using self-consistent polycrystal plastic method

Simulating the evolution of microstructures and the mechanical response of alloys with cubic and hcp structures is crucial for the industrial manufacturing of key alloy components. In this work, a modified Voce (ME-Voce) model is proposed, based on the extended Voce hardening model, to depict the variations of hardening rate with cumulative shear strain. Also, an orientation-dependent weight that describes the neighboring grain-induced ‘drag’ effect is evaluated. These improvements are numerically implemented in a standard viscoplastic self-consistent (VPSC) model. The ME-Voce parameters are determined through multi-parameter optimization using Matlab software. The influences of grain shape and neighboring grains on the hot deformation behavior of alloys are investigated. The proposed ME-Voce model is validated through hot compression experiments of alloys with cubic and hcp structures. The results show the VPSC embedded in the ME-Voce can reasonably predict the mechanical response under different deformation conditions. This enhances the predictive capability of the VPSC model and provides more options for crystal plasticity simulation of alloys with different crystal structures.

Compressed Ru skin on atomic-ordered hexagonal Ru-Ni enabling rapid Volmer-Tafel kinetics for efficient alkaline hydrogen evolution

Ru-based materials are promising electrocatalysts for hydrogen evolution reaction (HER) owing to their low-cost (one twenty-fifth of Pt) and similar Gibbs free energy of H* adsorption (ΔGH*) to that of Pt. However, the inadequate ability for the water dissociation of Ru catalysts is significantly impeding the water splitting efficiency. In this work, we demonstrate an atomically ordered hexagonal Ru-Ni alloy with compressive-strained Ru skin as a high-performance HER catalyst for significantly boosting the water dissociation. The Ordered Ru-Ni achieves an optimized catalytic activity for alkaline HER with a low overpotential of 23 mV at 10 mA cm−2, Tafel slope of 25.9 mV dec−1 and superior mass activity of 4.83 A mg−1Ru, which is 15.1 times higher than Ru/C. According to DFT calculations, the ordered RuNi core imposes homogeneous compressive strain on the Ru skin, resulting in an optimum binding energy towards H*/OH* intermediates on Ru active sites for rapid Tafel step kinetics. Moreover, the decreased H2O dissociation energy barrier of Ordered Ru-Ni suggests a promoted Volmer-Tafel kinetics is achieved, significantly boosting the overall HER efficiency. This work provides a practical avenue for surface strain engineering of Ru-based catalysts to promote the HER activity.

A Novel Hexagonal Close-Packed High-Entropy Alloy with Outstanding Strength-Ductility Synergy

Refractory high-entropy alloys (HEAs) are new potential candidates in high temperature applications. However, most present refractory HEAs are single-phase body-centered cubic (BCC) structure, which is brittle at room temperature. Then strategies to ductile the refractory HEAs and maintain their good high temperature strength at the same time should be under consideration. In the present study, a novel WReOsIr HEA with hexagonal close-packed (HCP) structure was developed. This alloy not only has excellent high-temperature strength (416.7 MPa at 1473 K), but also exhibits good ductility (30.7%) at room temperature. The better room temperature plasticity is found to originate from the deformation twins formed inside the grains.

Anisotropic growth of nano‐precipitates governed by preferred orientation and residual stress in an Al‐Zn‐Mg‐Cu alloy

Through an understanding of diffusion, precise control of the size distribution of nano-precipitates can be essential to developing superior properties in precipitation-strengthened alloys. Although a significant influence of crystallographic orientation on the diffusion process is known to exist in low-symmetry hexagonal close-packed alloys, such anisotropic diffusion is still unidentified in high-symmetry cubic alloys. In this work, we reveal the diffusion-controlled coarsening induced anisotropic growth process of nano-precipitates in an Al-Zn-Mg-Cu alloy. Our experimental and theoretical studies demonstrate that with an increase in the residual stress, the diffusion-controlled coarsening rate is slow along the 〈112〉 fiber texture in the alloy matrix with smaller grain sizes. As such, we find that the diffusion activation energy will be increased along the preferred orientation with largest residual stress, which leads to a reduced diffusion-controlled coarsening rate. Specifically, we demonstrate that the increase in the volume fraction of nano-precipitates originates from the rapid grain-boundary controlled coarsening of the grain-boundary precipitates. Based on these results, an underlying microstructural design strategy is proposed, involving the crystallographic orientation, the residual stress and the grain boundaries to manipulate the precipitate size distribution in this class of alloys.

First-principles investigation on twin-related lattice reorientation in hexagonal metals and alloys

High-throughput ab initio calculations were employed to study the lattice reorientation process accompanying twinning under plastic deformation in hexagonal metals, as well as in alloyed Mg and Ti. The results indicate that, in unalloyed hexagonal metals, reorientation consumes the highest and lowest energies in Be and Mg, respectively, with intermediate energies in Sc, Co, Ti, Zr and Dy. The shuffle component always contributes a significant part of the reorientation energy in Mg, whereas in Ti with sufficient shear strain, subsequent reorientation process becomes energy downhill. In alloyed Mg and Ti, appropriate alloying significantly reduces the reorientation energy, and especially in alloyed Ti, there is significant negative correlation between the reorientation energy and certain properties of the alloying elements. The results are consistent with the fact that reorientation has been experimentally observed in compressed Mg nanopillars, but only in atomic scale simulations under high strain rate in Ti.

Inelastic mechanical behaviour of an additively manufactured titanium alloy: a statistical continuum mechanics theory perspective

Statistical continuum mechanics theory was used to simulate the inelastic stress of polycrystalline materials using two-point statistics. For the experimental part, the Electron beam melting (EBM) technique (Arcam EBM Q10 additive machine) was used to fabricate cylindrical rods of Ti-6Al-4V both in horizontal and vertical directions. Electron backscatter diffraction (EBSD) technique was employed to achieve statistically reliable orientation maps of vertically and horizontally printed samples. In this study, high strain rate compression tests at six different strain rates were performed, and the stress–strain curves were generated. This work is amongst the first attempts to model the microstructure of additively manufactured hexagonal alloys under compressive loadings using the statistical continuum mechanics theory. The model is capable of simulating reasonably large microstructures (statistically representative) with a practical computational cost and accuracy, unlike numerical models that require a high computational cost. It should be noted that in additive manufacturing, due to large grains and high anisotropy, microstructures used in the simulations should be large enough to include sufficient information from the material’s structure. Therefore, using finite element models would be very challenging here. On the other hand, the statistical continuum mechanics theory uses the statistical representation of the material’s characteristics for solving the governing equations with Green’s function that enables this methodology to use more microstructure characteristic information without having a noticeable change to the computational cost. The proposed model in this study uses different microstructure characteristics such as crystal grain orientation, total slip systems, active slip systems, gain morphology, and chemical phases that are obtained from EBSD images for simulating the inelastic mechanical behavior of polycrystalline materials. Although this model simulates polycrystalline materials by considering various crystal and grain information, unlike numerical methods, it doesn’t simulate the grain interactions well and we cannot study local deformation and crack nucleation sites. This model works very well for simulating the overall behavior of material instead of each individual grain and failure analysis. This model has shown a good combination of computational cost and accuracy in which the error between the simulated and experimental strength for vertical and horizontal samples was 6.21% and 8.07%, respectively.

The influence of refractory metals on the elastic modulus, elastic anisotropy and thermodynamic properties of ZrAl2 high-temperature alloy

To improve the mechanical and thermodynamic properties of ZrAl2 high-temperature alloy, the influence of refractory metals on the structural stability, elastic modulus, elastic anisotropy and thermodynamic properties of the hexagonal ZrAl2 high-temperature alloy is studied by the first-principles calculations. The calculated results show that three refractory metal-doped ZrAl2 alloys are stability at the ground state. Importantly, it is found that these refractory metals enhance the bulk modulus of ZrAl2 because the introduction of refractory metal improves the localized hybridization between Zr atom and Al atom. The calculated bulk modulus follows the sequence of W-doping>Mo-doping>Hf-doping>ZrAl2. In addition, the parent ZrAl2 shows smallest shear anisotropy along the (001) plane. However, the W-doped and Mo-doped ZrAl2 alloys show the isotropic. Furthermore, it is found that these refractory metals enhance the melting point and Debye temperature of ZrAl2. Here, the calculated melting point and Debye temperature of the W-doped ZrAl2 are much higher than the parent ZrAl2. Therefore, we believe that the W element is an effective metal to improve the overall properties (such as elastic modulus, elastic anisotropic and thermodynamic properties) of ZrAl2 high-temperature alloy.

Determination of crystallographic texture in polycrystalline materials from wavelength-resolved neutron transmission experiments: application to high-symmetry crystals

Wavelength-resolved neutron transmission experiments are useful for characterizing the microstructure of macroscopic specimens with 2D spatial resolution perpendicular to the beam direction. The crystallographic texture can affect the neutron transmission in the thermal neutron energy range, which manifests as changes in the shape and height of Bragg edges as a function of neutron wavelength. Models have been proposed to predict the transmission of textured polycrystalline materials from knowledge of the material texture and have proved to accurately predict the observed transmission data. In recent work, a novel method was described and tested for obtaining texture integral parameters from the combined analysis of transmission data measured along several directions of a specimen in a hexagonal crystal Zr alloy. However, this procedure has limitations when dealing with high-symmetry crystal structures. In this work, a generalization of such a method based on the expansion of the orientation distribution function (ODF) in symmetric generalized spherical harmonics that is applicable to all crystal and sample symmetries is presented. Using this method, the low-order Fourier coefficients of the ODF can be estimated by analyzing transmission data obtained for a reduced set of beam directions. This method was verified using a cubic Cu sample, for which transmission data were available along five different directions. Two sample symmetries were assumed to reduce the number of Fourier coefficients of the ODF. In the case of cylindrical symmetry (fiber-type texture), the results were good; but in the case of orthorhombic symmetry, some bias was observed which was attributed to the reduced number of beam directions used to perform the evaluation.

Topological Hall effect and the transition of the anomalous Hall effect mechanism in hexagonal alloys Mn3.1− xFexGe0.9 (x = 1.6, 1.8, 2.0)

Hexagonal Mn3Ge, with both kagome lattice and triangular antiferromagnetism, has gained significant attention due to its large anomalous Hall effect (AHE), resulting from the non-vanishing Berry phase. In this study, we present the magnetic and anomalous transport properties of a series hexagonal D019 type Fe-doped Mn3Ge alloys with the composition of Mn3.1−xFexGe0.9 (x = 1.6, 1.8, 2.0). The ferromagnetic interactions gradually increase with increasing Fe content. The longitudinal resistivity of all alloys exhibits a typical metallic behavior, increasing with temperature from 5 to 390 K. The residual resistivity decreases from 120.4 to 67.8 μΩ·cm as x increases from 1.6 to 2.0. A temperature-driven Lifshitz transition and a spin reorientation have been observed in the x = 1.6 alloy. Topological Hall effect accompanied by the spin reorientation is demonstrated. The maximum value of the topological Hall resistivity ρxyT is approximately 0.16 μΩ·cm. The relationship of ρxyA∝ ρxx in x = 1.6 alloy suggests that the extrinsic skew scattering predominantly contributes to the AHE mechanism. In the case of x = 1.8 and 2.0, both intrinsic and extrinsic factors contribute to the AHE. The anomalous Hall conductivity of our polycrystalline samples at room temperature is comparable to that of single crystal Mn3Ge, which is advantageous for practical applications. This study reveals the effectiveness of chemical engineering in tailoring nontrivial spin textures and the AHE.

Superconducting properties of new hexagonal and noncentrosymmetric cubic high entropy alloys

Superconducting high-entropy alloys (HEAs) are a newly burgeoning field of unconventional superconductors and raise intriguing questions about the presence of superconductivity in highly disordered systems, which lack regular phonon modes. In our study, we have synthesized and investigated the superconducting characteristics of two new transition elements based HEAs Re0.35Os0.35Mo0.10W0.10Zr0.10 crystallizing in noncentrosymmetric α-Mn structure, and Ru0.35Os0.35Mo0.10W0.10Zr0.10 crystallizing in hexagonal closed-packed structure (hcp). Due to its high hardness, transition-metal-based hexagonal hcp HEA is rare and highly desirable for practical applications. Bulk magnetization, resistivity, and specific heat measurements confirmed bulk type-II superconductivity in both alloys. Specific heat analysis up to the measured low-temperature range suffices for a Bardeen-Cooper-Schrieffer explanation.

First principles investigations of the electronic, elastic, mechanical, anisotropic, optical, and thermoelectric performance of monoclinic SiGe semiconductors

Since crystal structure dictates the resultant physical properties of materials, titled physical features of the P21 monoclinic SiGe semiconductors, which are still unclear, have been revealed by density functional theory (DFT). The electronic band gap value with 0.49[Formula: see text]eV was found to be in the order of previously published cubic and hexagonal closed-packed SiGe semiconductors. Surprisingly, P21 monoclinic SiGe has a room temperature Seebeck coefficient of 1500[Formula: see text][Formula: see text]V/K which is higher than the reported data of both cubic and hexagonal SiGe alloys. Further, the mechanically stable P21 monoclinic phase of the SiGe displays a brittle mechanical character with clear elastic anisotropy has also been deduced. The P21 monoclinic phase of the SiGe can be also considered a good high-dielectric material and beneficial for practical applications of IR or UV goals due to its high refractive index.

Characterization approaches affect asymmetric load predictions of hexagonal close-packed alloy

The anisotropic plasticity constants of the CPB06 criterion for Ti64, previously identified with experiments performed in all three dimensions, are applied here to evaluate the effect of the direct and inverse calibration strategies of the tensile-compression asymmetry parameter k on the predictive behavior of large deformations of the hexagonal close-packed (hcp) alloy. The direct calibration strategy is based on model fitting with experimental strain hardening data up to the onset of plastic instability. The inverse calibration strategy reduces prediction errors of the load-displacement curves of both the cylindrical bar tensile and the elliptical cylinder compression tests. The results provide interesting insights into the identification of the laws modeling large deformations of hcp materials.

First-principles calculations to investigate Band structure and effective masses of direct bandgap hexagonal GeSn alloys

It is well known that hexagonal Ge has a direct bandgap and exhibits excellent light emission. The structure and electrical characteristics of hex-GeSn alloys are investigated using an ab initio computation and the PBEsol-meta-GGA-mBJ function. In all alloys, the direct bandgap is observed. Due to the high radius of the Sn atom, when Sn content increases, the bandgap decreases and lattice parameters expand. Their spin–orbit, crystal field splitting and effective mass of electron and hole were also determined, indicating that the direct bandgap of hex-GeSn alloys has significant application in optoelectronic.

Self-Consistent Crystal Plasticity Modeling of Slip-Twin Interactions in Mg Alloys

Parsing the effect of slip-twin interactions on the strain rate and thermal sensitivities of Magnesium (Mg) alloys has been a challenging endeavor for scientists preoccupied with the mechanical behavior of hexagonal close-packed alloys, especially those with great latent economic potential such as Mg. One of the main barriers is the travail entailed in fitting the various stress−strain behaviors at different temperatures, strain rates, loading directions applied to different starting textures. Taking on this task for two different Mg alloys presenting different textures and as such various levels of slip-twin interactions were modeled using visco-plastic self-consistent (VPSC) code. A recently developed routine that captures dislocation transmutation by twinning interfaces on strain hardening within the twin lamellae was employed. While the strong texture was exemplified by traditional rolled AZ31 Mg alloys, the weak texture was represented by ZEK100 Mg alloy sheets. The transmutation model incorporated within a dislocation density based hardening model showed enhanced flexibility in predicting the complex strain rate and thermal sensitive behavior of Mg textures’ response to various mechanical loading schemes.

Solute-solute interactions and their impacts on solute co-segregation and interfacial cohesion of {101¯2} twin boundary in zinc

• Zn-based biodegradable alloys are receiving increasing attention in recent years. Alloying is an effective approach to increase mechanical properties of such alloys for load bearing biodegradable applications, but it is unclear how solute atoms of the added alloying elements interact with each other and how these solute-solute interactions affect the mechanical properties of these alloys. In this work, the interactions between solutes (Li, Mg, Mn, Cu, and Ag) commonly used in biodegradable Zn alloys are investigated using first-principles calculations. The segregation ability of these solutes and the effects of the segregations of these solutes on { 10 1 ¯ 2 } twin boundary cohesion are also examined. This work uncovers the effect of solute-solute interactions on solute co-segregation and interfacial cohesion of twin boundaries along which fracture occurs. The findings of the present work are extendable to other hexagonal alloys such as Mg that often contain multiple alloying elements. Interactions of solute atoms in biodegradable zinc alloys and their effect on alloy mechanical properties have been less investigated. In this work, the interactions between the common solutes (Li, Mg, Mn, Cu, and Ag) used in the biodegradable Zn alloys, including a solute-solute pair with the same element or with two different elements, are investigated based on first-principles calculations. It is found that the energetically favorable configuration is the third nearest-neighboring for most solute-solute pairs in the bulk lattice because of the relatively strong electronic interaction between solute and Zn atoms or the relatively small local elastic deformation associated with the configuration. Considering that interfacial cleavage is a key fracture mode of zinc, the segregation ability of these solutes and their effect on the { 10 1 ¯ 2 } twin boundary cohesion are also examined. The result shows that Li tends to fully occupy its preferred site in the twin boundary, while Mg, Mn, Cu, or Ag has a concentration limitation in the twin boundary. The twin boundary cohesion can be significantly enhanced by the segregation of Mn, followed by Cu and Ag, because of the contribution of their d states close to the Fermi level. Furthermore, the co-segregation ability of two solute atoms in the twin boundary increases with increasing the binding tendency of these two solute atoms in the boundary. Mn and Li or Mg show a relatively strong co-segregation ability in the twin boundary. Adding Mn to Zn-Li or Zn-Mg alloys can significantly enhance the resistance to fracture of twin boundaries.