Stable Intermetallics Research Articles

Hydrogenated borophene enabled synthesis of multielement intermetallic catalysts

Supported metal catalysts often suffer from rapid degradation under harsh conditions due to material failure and weak metal-support interaction. Here we propose using reductive hydrogenated borophene to in-situ synthesize Pt/B/C catalysts with small sizes (~2.5 nm), high-density dispersion (up to 80 wt%Pt), and promising stability, originating from forming Pt-B bond which are theoretically ~5× stronger than Pt-C. Based on the Pt/B/C module, a series (~18 kinds) of carbon supported binary, ternary, quaternary, and quinary Pt intermetallic compound nanocatalysts with sub-4 nm size are synthesized. Thanks to the stable intermetallics and strong metal-support interaction, annealing at 1000 °C does not cause those nanoparticles sintering. They also show much improved activity and stability in electrocatalytic oxygen reduction reaction. Therefore, by introducing the boron chemistry, the hydrogenated borophene derived multielement catalysts enable the synergy of small size, high loading, stable anchoring, and flexible compositions, thus demonstrating high versatility toward efficient and durable catalysis.

The lattice vibration, mechanical anisotropy, stress-strain behavior and electronic properties of ZrxCuySiz intermetallics: A first-principles study

In recent years, ternary metal silicides have attracted more and more interests because of their excellent physical and chemical properties. The mechanical properties, electronic structures and thermal properties of ZrCuSi, ZrCuSi2, Zr3Cu4Si2, Zr3Cu4Si4 and Zr6Cu8Si12 ternary silicide were calculated by first-principles methods. The formation enthalpies, phonon dispersions and single-crystal elastic constants reveal that ZrCuSi, ZrCuSi2, Zr3Cu4Si2, Zr3Cu4Si4 and Zr6Cu8Si12 are thermodynamically, dynamically and mechanically stable. According to the analysis of the enthalpy of formation, ZrCuSi is the most thermodynamically stable intermetallics. Besides, Push's ratio (BH/GH), Poisson's ratio v, hardness and elastic anisotropic were investigated. The results indicated that ZrxCuySiz intermetallics show brittle properties and elastic anisotropy. Cu–Si bonds can be formed in ZrxCuySiz intermetallics due to the strong hybridization between Cu-3d and Si-3s states. The analysis of the electronic properties confirms that ZrxCuySiz intermetallics show the metallic behavior. The stability and mechanical properties are further explained by Debye temperature and electronic properties.

First-principles calculations to investigate stability, mechanical and thermo-dynamic properties of AlxTMy intermetallics in aluminum alloys

The mechanical, thermal and electrical properties of 8 types of common AlxTMy (TM = Sc–Zn, Y–Cd, Hf–Hg) intermetallics in Al based alloys at various temperatures and pressures have been investigated by combining first-principles calculations with the Debye model. The results clearly show that the stability and mechanical property are closely related to the types of configurations and TM elements. The electronic structure for AlRh and AlCd alloys are used to study the characteristics of stable compounds, and it is found that more transferred electrons will make the lattice mismatch δ larger. The Cauchy pressure C12–C44 and electric local function (ELF) present underlying characteristics of bonds and physical mechanisms. Further, the Helmholtz free energies F of both Al bulk and stable AlxTMy intermetallics are decreased with increasing temperature, while the entropy S, heat capacity CV and coefficient of expansion aT show the opposite trends. And the smaller F leads to the lower aT; that is the compounds constitute of elements of IB ∼ IVB have larger F and aT, while the compounds with VB ∼ VIIB possess smaller F and aT. In short, all calculated parameters in this work show the same change with the atomic number in for three periods 3-5d, indicating that the structure of the TM valence electron layers are the main reason for the underlying mechanisms. This work is helpful for the study of high-performance of structural Al-based alloys.

Microstructural evolution and intermetallic formation in Zn-3Mg (wt%) powder mixture processed by high-pressure torsion

Severe plastic deformation (SPD) techniques have been used extensively over the past 40 years for producing strong metals and alloys. High-pressure torsion (HPT) is one of the most promising SPD techniques for achieving high strength through nanoscale grain refinement and phase transformation. In this research, a mixture of pure zinc (Zn) and magnesium (Mg) powders, Zn-3Mg (wt%), was HPT-processed under a pressure of 6 GPa for 1, 5, 10, 20, and 30 turns at room temperature to achieve a high strength biodegradable material. In order to understand the effects of pre-consolidation on the resulting microstructure and hardness, HPT processing was performed on loose powders placed in the die and also on a pre-compacted powder mixture and the resulting HPT disks were characterized by X-ray diffraction, scanning electron microscopy, atom probe tomography, and Vickers microhardness. In both cases, the HPT disk microstructures contained nanoscale grains, and stable and metastable strain-induced intermetallics, but an unusual softening appeared at large shear strains. Grain size, grain morphology, and the formation of different intermetallics were analyzed to explain the unusual hardness distribution, and it was found that an inverse Hall-Petch relationship between hardness and grain size exists. It is suggested that thermally-activated phenomena such as grain boundary sliding contributed to the strain-induced softening of this nano-structured biomaterial due to its low melting point. The current results are compared with those for HPT-processed cast alloys and hybrids of the same composition.

Post-heat treatment of laser powder bed fusion fabricated Al–La–Mg–Mn alloy: On intermetallic morphology control and strength-ductility balance

In this study, a newly developed Al–La–Mg–Mn alloy was subjected to solution heat treatment for the first time. During the solution heat treatment at 500 ℃, the continuous network Al11La3 intermetallics were transformed into a discontinuous granular form. Ultrafine intermetallics (∼0.20 µm) were achieved via multi-point spheroidization with the aid of endogenous nanoprecipitates. Of note, the intragranular intermetallics were inert to coarsening, while the intergranular intermetallics grew gradually to ∼0.58 µm within 6 h. Compared to the as-printed alloy (UTS ∼588 MPa, Elongation ∼6.4%), the solution-treated alloy at 500 ℃ for 1 h (UTS ∼477 MPa, Elongation ∼14.3%) demonstrated a remarkable improvement in ductility, with a 123% increase, despite a modest decrease of 19% in strength, thereby balancing the strength-ductility. The limited decline in strength is primarily attributed to the high-content solute Mg atoms and thermally stable intragranular intermetallics. Interestingly, the mechanical properties were independent of the holding times at 500 ℃, despite the coarsening of intergranular intermetallics. This is mainly due to the optimization of intermetallics’ scale distribution, which tailors and equalizes the stress distribution between the grain boundary and interior, as well as between the melt pool boundary and interior during deformation. These findings provide the first insight into the effect of post-heat treatment on the intermetallic evolution and mechanical properties of LPBF Al–(La, Ce) alloys.

Proposal of new stable Ru-based intermetallics, XRu3 (X = V, Nb and Ta): Prediction of their bulk properties and their surfaces adsorption behavior and sensing properties to small gases

The fundamental physical properties of VRu3, NbRu3 and TaRu3 were studied using CASTEP package. Their stabilities were confirmed by calculating the enthalpy of formation and the phonon dispersion spectra. These Ru-based intermetallics are conductors with a strong covalent bond and have demonstrated interesting thermomechanical properties: high Vickers hardness, low thermal conductivity, high fracture toughness and high melting point, making them good candidates for high-temperature application. Adsorption behavior and sensing properties of XRu3(100), XRu3(110) and XRu3(111) surfaces at H2, N2, O2, CO, NO, CO2, N2O, NO2, H2S, SO2, H2O, NH3 and CH4 have also been studied. In all cases, a physisorption interaction was found with an adsorption energy between −19 and −75 meV. The gas adsorption capacity follows the order SO2 > H2S > CH4 > CO2 > NH3 > N2O > CO > H2O > N2 > O2 > OH > H2. VRu3(111), NbR3(111) and TaRu3(110) are the most suitable adsorption surfaces. Gas recovery times have proven to be very short, on the order of a few picoseconds. Among the tested gases, CO2, H2S, SO2, NH3 and CH4 show high recovery times with VRu3(111), NbRu3(111) and TaRu3(110).

Machine learning search for stable binary Sn alloys with Na, Ca, Cu, Pd, and Ag.

We present our findings of a large-scale screening for new synthesizable materials in five M-Sn binaries, M = Na, Ca, Cu, Pd, and Ag. The focus on these systems was motivated by the known richness of M-Sn properties with potential applications in energy storage, electronics packaging, and superconductivity. For the systematic exploration of the large configuration space, we relied on our recently developed MAISE-NET framework that constructs accurate neural network interatomic potentials and utilizes them to accelerate ab initio global structure searches. The scan of over two million candidate phases at a fraction of the typical ab initio calculation cost has uncovered 29 possible intermetallics thermodynamically stable at different temperatures and pressures (1 bar and 20 GPa). Notable predictions of ambient-pressure materials include a simple hP6-NaSn2 phase, fcc-based Pd-rich alloys, tI36-PdSn2 with a new prototype, and several high-temperature Sn-rich ground states in the Na-Sn, Cu-Sn, and Ag-Sn systems. Our modeling work also involved ab initio (re)examination of previously observed M-Sn compounds that helped explain the entropy-driven stabilization of known Cu-Sn phases. The study demonstrates the benefits of guiding structure searches with machine learning potentials and significantly expands the number of predicted thermodynamically stable crystalline intermetallics achieved with this strategy so far.

B4C reinforced NiTi‐based composites: Microstructure and wear performance

AbstractIn this study, NiTi–x wt.% B4C (x = 0, 2, and 4) composites were consolidated with spark plasma sintering method, and the effects of boron carbide reinforcement addition on the microstructure and wear behavior of samples were investigated. Identification of the constituent phases of samples by the X‐ray diffraction method plus Rietveld analysis revealed that the stability of the martensite phase increased in the composite samples because of mismatch stresses between the NiTi matrix phase and the reinforcing particles, which increases the density of the dislocations and facilitates the diffusion process that subsequently leads to the formation of stable intermetallics. The results of hardness test indicated that the hardness value increased from 3.67 GPa for pure NiTi to 10.99 GPa for NiTi–4 wt.% B4C. Results of wear test revealed that boron carbide reinforced composite specimens had higher wear resistance, whereas wear rate of NiTi sample was 3.6 × 10−3 mm3/N m, and it reached to .21 × 10−3 mm3/N m for NiTi–4 wt.% B4C. Investigation of microstructure by scanning electron microscopy images and EDS analysis revealed that the wear mechanism in NiTi samples was abrasive and the addition of B4C to NiTi changed the wear mechanisms from abrasive to a combination of oxidation, adhesive, and delamination mechanisms.

Rare earth improves strength and creep resistance of additively manufactured Zn implants

Poor mechanical strength and creep resistance limit the orthopedic application of biodegradable Zinc (Zn). In present work, cerium (Ce) was alloyed with Zn using laser additive manufacturing technique. As one kind of rare earth element, Ce possessed high surface activity, which effectively interrupted the grain growth and caused the formation of stable intermetallics, thus contributing to grain refinement strengthening and precipitate strengthening. More significantly, Ce alloying activated more pyramidal slip by means of reducing the critical resolved shear stress during plastic deformation, and resultantly formed the sessile dislocations, which caused the accumulated strain hardening and improved the creep resistance. As a result, Zn-Ce alloy exhibited a considerably improved ultimate tensile strength of 247.4 ± 7.2 MPa, and a reduced creep rate of 1.68 × 10−7 s−1. Moreover, it exhibited strong antibacterial activity, as well as favorable cytocompatibility and hemocompatibility. All these results demonstrated the great potential of Zn-Ce alloy as a candidate for bone repair application.

Machine learning formation enthalpies of intermetallics

Developing fast and accurate methods to discover intermetallic compounds is relevant for alloy design. While density-functional-theory (DFT)-based methods have accelerated design of binary and ternary alloys by providing rapid access to the energy and properties of the stable intermetallics, they are not amenable for rapidly screening the vast combinatorial space of multi-principal element alloys (MPEAs). Here, a machine-learning model is presented for predicting the formation enthalpy of binary intermetallics and is used to identify new ones. The model uses easily accessible elemental properties as descriptors and has a mean absolute error of 0.025 eV/atom in predicting the formation enthalpy of stable binary intermetallics reported in the Materials Project database. The model further predicts stable intermetallics to form in 112 binary alloy systems that do not have any stable intermetallics reported in the Materials Project database. DFT calculations confirm one such stable intermetallic identified by the model, NbV2, to be on the convex hull. Furthermore, an adaptive transfer learning method is used to generalize the model to predict ternary intermetallics with a similar accuracy as DFT, which suggests that it could be extended to identify compositionally complex intermetallics that may form in MPEAs.

Effect of Mechanical Alloying on the Microstructural Evolution of Al60Cr30Si10 Alloys Processed by Spark Plasma Sintering

In this study, Al, Cr and Si fine powders were alloyed by planetary ball milling to investigate their microstructural evolution, following their mechanical alloying behavior. The formation of Al(Cr) supersaturated solid solution by the Cr grains embedded in Al-matrix, which contributed substitutional structure is discussed in relation to their structural evolution in accordance with the distortion of a crystal lattice. The compacts of Al–Cr–Si alloys were prepared by rapid sintering within a short time by dense consolidation above the density of 99.9% as the formation of intermetallics, except in the case of the distribution of single-phases induced by the Al-melting. The formation behavior of intermetallics was dominated by dependence on the structures of the milled-powder and subsequent sintering temperature. To estimate the consolidated behaviors of compacts, various approaches derived from TEM and XRD analysis were taken to obtain microstructural evidences of the inter-diffusion, following the presence of thermally stable intermetallics.

Stability, elasticity and electronic structures of Co-Zr binary intermetallic compounds

ABSTRACTThe structural properties, enthalpies of formation and entropic temperatures of five stable Co-Zr intermetallics were systematically investigated by first-principles calculations. The calculated enthalpy of formation was used to measure the phase stability of the Co-Zr intermetallic compounds, and the results reveal that CoZr has the highest phase stability among these Co-Zr compounds. Considering the change of temperature, the entropic temperature TS was used to illustrate the high-temperature stability of the Co-Zr intermetallic compounds. Additionally, the elastic constants, anisotropy indexes and Debye temperatures have been calculated. According to the calculated anisotropy indexes, CoZr2 has the highest elastic anisotropy among the Co-Zr binary compounds. Furthermore, the bonding characteristics of these compounds have also been investigated by calculating the electronic structures.

Influence of Zn Concentration on Interfacial Intermetallics During Liquid and Solid State Reaction of Hypo and Hypereutectic Sn-Zn Solder Alloys

In this study, Sn-Zn solder samples containing 2 to 12 wt.% Zn were fabricated and reflowed into a Cu substrate. The microstructure of solder samples was observed after reflow and aging for up to 1000 h at 150°C. Thermodynamically stable intermetallics (IMCs) Cu-Zn and Cu-Sn formed at the interface depending on the solder composition. Formation of different interfacial IMCs during soldering and after prolonged aging is explained by the spalling mechanism that resulted from the depletion of Zn from the solder matrix.

Phase stability, mechanical properties and electronic structures of Ti Al binary compounds by first principles calculations

Abstract The stability, mechanical properties and electronic structures of Ti3Al, TiAl, TiAl2 and TiAl3 have been systematically investigated by the first-principles calculation. The four Ti Al binary compounds are thermodynamically stable intermetallics depending on their negative formation enthalpy and cohesive energy. The bulk modulus (B), shear modulus (G), Young's modulus (E), Vicker's hardness (HV) and Poisson's ratio (ν) are calculated. With the increment of Al content, the bulk modulus and Poisson's ratio decrease while the shear modulus, Young's modulus and hardness gradually increase. The ratio of B/G decreases with Al increment, implying the weakening of ductility. In addition, the mechanical anisotropy of Ti Al compound has been revealed by the analysis of the anisotropy index, three-dimensional surface contours of the Young's modulus and the planar projections on the low index planes (including (100), (010), (100) and (110)). TiAl shows the most anisotropic structure while Ti3Al has the least anisotropy. The density of states (DOS) and electron density distribution map are discussed to analyze the electronic structures and chemical bonding characters. The Ti Al binary compounds are composed of both metallic and covalent bonds, and the covalent bonding character can be strengthened with the increase of Al content.

Structural, elastic and electronic properties of B2-type modified by ternary additions FeAl-based intermetallics: First-principles study

Abstract The structural, elastic and electronic properties of FeAl (B2) intermetallics modified by ternary additions have been investigated by first-principles calculations based on density functional theory (DFT). The calculations of the formation enthalpy indicate that Al atoms are more easily replaced by the alloying elements X (X = V, Sc, Mn, Si, Mg, Co, Zr, Cu, Zn, Nb, Cr, Mo, W, Ti, Ni). The ratio of the bulk modulus to shear modulus B/G and Poisson's ratio v show that the FeAl-Cr, FeAl-Ti, and FeAl-Co systems are ductile materials and FeAl-Co (B/G = 1.92, v = 0.28) is the most ductile phase among these alloy phases. The electronic structure calculations indicate that the doping by Cr, Ti and Co may lead to the charge redistribution, the weakening of the Al-Al interaction, and strengthening of the Fe-Fe and Fe-Al interactions. It can be concluded that Cr, Ti and Co form the most stable FeAl-based intermetallics.

Thermodynamics of Li-Si and Li-Si-H phase diagrams applied to hydrogen absorption and Li-ion batteries

A consistent thermodynamic description of the Li-Si system and its extension to the Li-Si-H system is developed based on comprehensive Calphad-type assessment of all pertinent thermodynamic and phase equilibrium data. Key experiments were performed by measuring the equilibrium pressure for the hydrogenation of the lithium silicides at 773 K, 748 K and 723 K. The Gibbs energy functions of the Li-Si compounds developed in this work are validated by direct comparison of the predicted hydrogen pressure with experimental data. The interrelation of the hydrogen absorption pressure with the calculated Li-Si-H ternary phase diagram is also elucidated. The calculated Li-Si phase diagram, both on chemical potential and composition scale, is well supported by the entity of experimental data. In addition to the stable intermetallics Li17Si4, Li16.42Si4 (Li4.11Si), Li13Si4, Li7Si3, Li12Si7, LiSi also metastable Li15Si4 and an approximate for amorphous Si(Li) is included in the thermodynamic modeling. Qualitative prediction of open-circuit voltage curves during metastable lithiation/delithiation of Si as advanced anode material for Li-ion batteries at room temperature is made.

Influence of metastable Al9Ni2 phase on the sequence of phase transformations initiated by heating of Al/Ni multilayer foils produced by EBPVD method

Al/Ni multilayer foils (MF) undergo a cascade of phase transformations at heating, initiated by diffusion interaction of Al and Ni layers. It is found that phase transformations sequence at initial stages depends on the method of producing MF: at sputtering or ion-beam deposition of elements, metastable Al9Ni2 phase forms at phase transformations initial stages, and in the case of MF produced by electron-beam physical vapour deposition (EBPVD) method or cold rolling of laminates, this is Al3Ni phase. Such a difference in phase transformations sequence is associated with the influence of the method of MF production on the possibility of intermixed zone (IZ) formation on layer interfaces. In the study it was suggested that such anomalously high diffusion mobility of atoms can be achieved in the presence of excess vacancies in MF structure. With this purpose, MF structure was produced by high-rate (up to 30 nm/s) layer-by-layer deposition of elements by EBPVD method. Phase transformations and MF were studied by the method of X-ray diffraction (XRD) and differential-thermal analysis (DTA). It is shown that irrespective of MF composition and modulation period, at their heating phase transformations start with formation of metastable Al9Ni2 phase. At further MF heating, a stable Al3Ni phase forms alongside Al9Ni2 phase. Later on, Al9Ni2 and Al3Ni phases turn into stable intermetallics characteristic of MF chemical composition.

On the initial stages of phase formation at the solid Cu/liquid Sn-based solder interface

The presence of a thin nanometric η-Cu6Sn5 phase layer at the Cu/liquid eutectic Sn-0.7 wt%Cu solder interface in absence of the ε-Cu3Sn phase is demonstrated experimentally for the first time. The principal aim of this work is to determine experimentally and unambiguously the sequence of phase formation at Cu/liquid solder interface, i.e., to determine which of the two thermodynamically stable intermetallics η-Cu6Sn5 or ε-Cu3Sn forms and grows first at this interface. For this purpose, we have performed specific experiments of ultra rapid dipping of thin Cu foils in a solder bath at 250 °C followed by air cooling of the Cu/solder couple. The contact time between the solid Cu and liquid alloy varies from about 50 ms to 65 ms. Moreover a “long time” experiment of about 1 s of dipping is performed for comparison purposes with previous results reported in the literature. The characterizations of samples by a high-resolution scanning electron microscopy and transmission electron microscopy clearly demonstrate that the only phase that forms and grows at the Cu/liquid solder interface, for contact times less than about 65 ms, is the η-Cu6Sn5 phase. The evolution of the morphology and the average thickness of the single growing η-Cu6Sn5 layer, is also studied. Finally a simple qualitative model of nucleation and growth of the η-Cu6Sn5 phase at the Cu/liquid Sn-based solder is proposed. Thus, this study provides valuable insights into the initial stages of the interactions in the Cu/liquid Sn system of very high technological interest especially in the framework of packaging challenges during miniaturization.

In-Situ Solid State NMR for Characterizing Battery Materials

The demand for inherently safe, large capacity, high-rate capability energy storage has led to the investigation of a class of metallic and semi-metallic elements that form stable intermetallics with lithium, providing Li+ storage capacities of three to ten times that of today’s lithium-ion batteries. Unfortunately, such high capacity intermetallic anodes undergo structural transformations from crystalline to amorphous states with large volume expansions upon intercalation, changing the anode's microstructure and local chemical environments, which lead to reduced capacity. Because of their amorphous nature, the atomic-level structures of the intermetallic are not amenable to study by conventional diffraction techniques. Solid-state nuclear magnetic resonance (NMR) spectroscopy is an ideal technique to probe amorphous materials and understand how kinetically driven compositional changes affect the material microstructure and the local chemical environment. We have measured the 7Li and 119Sn NMR spectra of electrochemically lithiated tin ex-situ and have identified a number of different lithium environments. Furthermore, we have measured in-situ 7Li NMR spectra as a function of lithiation and de-lithiation of different materials, including graphitic carbon, silicon, and tin. We will discuss the findings and their implications for the design of various nanostructured intermetallic anodes.

Predictions on the compositions, structures, and mechanical properties of intermediate phases in binary Mg–X (X = Sn, Y, Sc, Ag) alloys

It has been paid an increasing attention on the precipitation strengthening of Mg alloys. Toward designing high strength and toughness light-weight Mg alloys, the mechanical properties of stable or metastable intermetallics should be basically known. In this work, the compositions and corresponding structures of stable or metastable precipitations in the binary Mg–X (X=Sn, Y, Sc, Ag) alloys are predicted using ab initio evolutionary algorithm. Our predictions are well consistent with the existing phase diagrams but further yield potential phases and their crystallographic informations. Furthermore, the elastic moduli of the predicted intermetallics are studied based on first-principles calculations. Besides basic structural and mechanical data, we provide that the Mg–Ag alloy exhibits the larger bulk modulus and ductility when Ag composition increases, and the Mg–Sc alloy should be a promising candidate to meet the comprehensive strength, weight, economy, and efficiency requirements. Thus from the composition to the structure, further to the mechanical property, we present a systematically theoretical program for the alloys design.