Stoichiometric Intermetallic Compound Research Articles

Fundamental Thermodynamic, Kinetic, and Mechanical Properties of Lithium and Its Alloys.

Lithium alloying reactions are beneficial in promoting uniform plating and stripping of lithium metal in all-solid-state batteries. First-principles calculations are performed to predict thermodynamic, kinetic, and mechanical properties of lithium and several important Li-M alloys (M = Mg, Ag, Zn, Al, Ga, In, Sn, Sb, and Bi). While the Li-Mg binary system forms a solid solution, most other lithium-metal alloys prefer stoichiometric intermetallic compounds with common local motifs that enable fast Li diffusion. Lithium and Li-rich alloys exhibit an unusually flat energy landscape along paths that connect BCC to close-packed structures like FCC and HCP, with important implications for mechanical properties. Very low migration barriers for Li diffusion that rival those of superion conductors are predicted, both in pure Li and in Li-M intermetallics. However, vacancy concentration, which is crucial for substitutional diffusion, is predicted to be low in metallic Li and most Li-M intermetallics. Compounds such as B32 LiAl and LiGa as well as D03 Li3Sb and Li3Bi exhibit structural vacancies at higher ends of their voltage windows, which together with low migration barriers leads to exceptionally high Li mobilities. In the Li-Mg solid solution, the addition of Mg is found to decrease the vacancy tracer diffusion coefficient by an order of magnitude.

Grain refinement in solidification of undercooled Fe2B intermetallic compound alloy

Spontaneous grain refinement widely occurs in solidification of highly undercooled solid solution alloys, but the case in stoichiometric intermetallic compound alloys is seldom concerned. In this paper, Fe2B alloy was undercooled up to 336 K to investigate the solidification structure’s evolution with undercooling. It is shown that the equilibrium peritectic reaction L + FeB → Fe2B is completely suppressed and there is only an Fe2B phase to solidify at various undercoolings. As the undercooling increases, Fe2B crystals change from a faceted into non-faceted morphology, and grain refinement takes place from 48 K undercooling until a fully refined solidification structure is obtained above 92 K undercooling. When the sample solidified at a large undercooling is immediately annealed at high temperature, the grains are coarsened quickly. In combination with the electron backscattered diffraction and transmission electron microscopy analysis results, it is suggested that the solidification stress-induced recrystallization triggers the grain refinement in Fe2B alloy.

Anomalous eutectic formation in undercooled Ni−P eutectic alloy

To investigate the relationship between anomalous eutectic and the remelting of primary phase, thin samples of Ni−19.6at.%P eutectic alloy were solidified over a range of undercoolings with temperature recalescence depressed. Fully regular eutectic structures were obtained. The thin samples were then rapidly heated to certain temperatures between the nucleation and eutectic temperatures, resulting in anomalous eutectic structure. It was found that the volume fraction of anomalous eutectic increased with increasing target temperature. α-Ni particulates were in principle randomly oriented while β-Ni3P phase was well oriented in the anomalous eutectic formed upon heating. Considering that α-Ni is a solid solution while β-Ni3P is a stoichiometric intermetallic compound, it is unambiguous that anomalous eutectic forms due to remelting of the primary regular eutectic.

The effect of CO treatment on the surface structure of bimetallic Pd-Au/HOPG and Pd-In/HOPG nanoparticles: A comparative study

Bimetallic nanoparticles have been attracting more and more attention as catalysts in recent years since they often provide improved catalytic performance when compared to their monometallic analogues. Trying to design the optimum configuration of active sites in such catalytic systems, researchers resort to different strategies, one of which consists of a specific pretreatment of pre-synthesized nanoparticles in a particular gaseous environment prior to the catalytic reaction, resulting in adsorption-induced segregation. However, such a strategy does not always lead to any significant changes in the surface structure, so the identification of all factors responsible for the segregation process is of high fundamental and applied interest for the targeted design of bimetallic catalytically active nanoparticles. Here we show that for two types of bimetallic nanosystems, a stoichiometric intermetallic compound of Pd and In, and a substitutional solid solution of Pd and Au, the exposure to a CO atmosphere leads to completely different results, i.e., a noticeable surface segregation of Pd atoms being observed exclusively for the latter system. Driven by adsorption of CO molecules, the process of segregation is enhanced in the studied Pd-Au/HOPG catalyst (initial Pd/Au ratio of 1.48) when the temperature of the CO treatment is increased to 150 °C. In contrast, the Pd-In/HOPG nanoparticles (initial Pd/In ratio of 1.35) show no considerable change in the apparent surface atomic composition under a CO atmosphere, irrespective of temperature. Our results, alongside previous findings, demonstrate that CO adsorption-induced segregation as a tool for target-oriented modification of active sites in bimetallic nanoparticles for catalytic applications is appropriate only for substitutional solid solutions having a reasonable number of contacts between atoms of one type prone to segregation.

Rapid solidification of a FeSi intermetallic compound in undercooled melts: dendrite growth and microstructure transitions

Rapid solidification of a FeSi stoichiometric intermetallic compound was studied using the glass fluxing method. The recalescence process was in situ observed for the first time by the infrared high-speed high-resolution cameras. The dendrite envelope was found to be non-isothermal during the recalescence process. The growth velocity increased first, then decreased and finally held nearly constant. The average dendrite growth velocity for the recalescence process increased monotonically with undercooling and was described well by the dendrite growth model for a stoichiometric intermetallic compound. At low undercooling, the microstructure transition from coarse dendrites to refined grains was consistent with the dendrite fragmentation model and the chemical superheating model. At high undercooling, dendrite deformation triggers stress accumulation upon rapid solidification, thus providing the driving force for recrystallization. However, there were no evidences for annealing twins accompanied by recrystallization as well as random textures due to recrystallization nucleation. From the local misorientation map, the grain refinement mechanism was suggested to be stress-induced dendrite fragmentation. This study is helpful for not only understanding the intrinsic mechanisms of microstructure transitions in theory but also controlling microstructures and performance of intermetallic compounds in practical applications.

Thermodynamic description of the Ce–Ga and Ga–Gd systems

The Ce–Ga and Ga–Gd systems are critically assessed by means of the calculation of phase diagram (CALPHAD) technique. The excess Gibbs energy of the solution phases [liquid, orthorhombic (Ga), fcc (Ce), bcc (Ce or Gd), and hcp (Gd)] is modeled with the Redlich–Kister equation. In the Ce–Ga and Ga–Gd systems, the intermetallic compounds, CeGa, Ce3Ga2, Ce3Ga, α-CeGa6, β-CeGa6, GaGd, Ga6Gd, Ga2Gd3, and Ga3Gd5 are treated as stoichiometric compounds. The intermetallic compounds CeGa2 and Ga2Gd with a homogeneity range, are treated using a two-sublattice model as the formula (Ce,Ga)0.333333(Ce,Ga)0.666667, and (Ga,Gd)0.666667(Ga,Gd)0.333333, respectively. In the Ce–Ga system, there are nine invariant reactions, and five stoichiometric intermetallic compounds and the compound CeGa2 with a homogeneity domain ranging from about 66.67 to 77.94 at.% Ga. Some improvements are obtained based on the experimental data in the present work, the liquidus line is more close to the most experimental points, and the homogeneity range of the compound CeGa2 seems more reasonable at full temperature range. In the Ga–Gd system, there are eight invariant reactions, and four stoichiometric intermetallic compounds and the compound Ga2Gd with a homogeneity domain ranging from about 19.64 to 33.33 at.% Gd. The present phase diagram and thermodynamic properties of the system are well reproduced with the experimental data.

Anomalous eutectics in intermediately and highly undercooled Ni-29.8at.%Si eutectic alloy

The transition from regular eutectics to anomalous eutectics is a well-known non-equilibrium phenomenon for undercooled eutectic alloys, the mechanism of which has been studied extensively but is still controversial. In this work, a Ni-29.8at.%Si eutectic alloy, as an ideal modeling system whose eutectic products are two stoichiometric intermetallic compounds, whose rapid solidification does not suffer any phase-selection and whose transformation path does not include any solid-state phase transformation, was undercooled by a melt fluxing technique and observed in-situ by a high-speed camera. A common transition from uncoupled eutectics to anomalous eutectics and a unique transition from uncoupled coarse lamellar-eutectics to anomalous eutectics were found at intermediate and high undercooling, respectively. The formation of coarse lamellar-eutectics is highly related to the abrupt increase of growth velocity at ΔT ≈ 149 K, which is accompanied by a transition from dual-recalescence to single-recalescence. An electron back-scattering diffraction analysis shows that epitaxial growth of the second δ-Ni2Si phase on the primary γ-Ni31Si12 phase follows a particular eutectic orientation relationship. Anomalous eutectics were concluded to be formed by uncoupled eutectic growth with the reduction of interfacial energy but not chemical superheating as the driving force.

Phase selection and re-melting-induced anomalous eutectics in undercooled Ni–38 wt% Si alloys

The Ni–38 wt% Si alloy whose eutectic products are two stoichiometric intermetallic compounds (i.e., NiSi and NiSi2) was undercooled by the melt fluxing technique. After in situ observations of the recalescence processes using a high-speed camera and by electron back-scattering diffraction analysis of the solidification microstructures, the crystal growth velocities, phase selection, and microstructure evolutions were studied. Due to a growth-controlled mechanism, the primary phase changes from the NiSi to the NiSi2 phase at a critical undercooling ΔT ≈ 48 K. Even in the absence of the driving force of chemical superheating, the transition from regular eutectics to anomalous eutectics happens. The reason is that the single-phase dendrite of NiSi2 phase solidifies firstly and then the NiSi phase grows epitaxially to form an uncoupled eutectic-dendrite at high undercooling. The present work provides further experimental evidences for the dual origins of anomalous eutectics (e.g., uncoupled eutectic-dendrite growth during the recalescence stage and coupled lamellar eutectic growth at low undercooling during the post-recalescence stage) and is helpful for understanding of non-equilibrium phenomena in undercooled melts.

Precipitation of Sn phase from stoichiometric intermetallic compound in Sn-40 at%Mn peritectic alloy

The precipitation of pure Sn particles from the peritectic MnSn2 phase which is stoichiometric intermetallic compound in equilibrium condition has been observed for the first time in directionally solidified Sn-40at%Mn peritectic alloy. The Sn particles are inclined to nucleate at the primary Mn3Sn2/peritectic MnSn2 interface at higher growth velocity. Besides, analysis shows that the solubility range is not constant even for the intermetallic compound which is stoichiometric in equilibrium condition. The actual solubility range of the peritectic MnSn2 phase which is dependent on temperature has been determined through measuring the solute concentrations of the transverse sections with different temperatures. The solidification characteristic of this peritectic alloy also contributes to the precipitation of Sn particles.

Screened moments and absence of ferromagnetism in FeAl

While the stoichiometric intermetallic compound FeAl is found to be paramagnetic in experiment, standard band-theory approaches predict the material to be ferromagnetic. We show that this discrepancy can be overcome by a better treatment of electronic correlations with density functional plus dynamical mean field theory. Our results show no ferromagnetism down to 100 K and since the susceptibility is decreasing at the lowest temperatures studied we also do not expect ferromagnetism at even lower temperatures. This behavior is found to originate from temporal quantum fluctuations that screen short-lived local magnetic moments of 1.6 $\mu_B$ on Fe.

Femtosecond laser ablation and nanoparticle formation in intermetallic NiAl

The ablation behavior of a stoichiometric intermetallic compound β-NiAl subjected to femtosecond laser pulsing in air has been investigated. The single-pulse ablation threshold for NiAl was determined to be 83±4mJ/cm2 and the transition to the high-fluence ablation regime occurred at 2.8±0.3J/cm2. Two sizes of nanoparticles consisting of Al, NiAl, Ni3Al and NiO were formed and ejected from the target during high-fluence ablation. Chemical analysis revealed that smaller nanoparticles (1–30nm) tended to be rich in Al while larger nanoparticles (>100nm) were lean in Al. Ablation in the low-fluence regime maintained this trend. Redeposited material and nanoparticles remaining on the surface after a single 3.7J/cm2 pulse, one hundred 1.7J/cm2 pulses, or one thousand 250mJ/cm2 pulses were enriched in Al relative to the bulk target composition. Further, the surface of the irradiated high-fluence region was depleted in Al indicating that the fs laser ablation removal rate of the intermetallic constituents in this regime does not scale with the individual pure element ablation thresholds.

Remelting-induced anomalous eutectic formation during solidification of deeply undercooled eutectic alloy melts

Anomalous eutectics are argued to form due to the remelting of the primary solid during solidification of deeply undercooled eutectic alloy melts. As an indicator of the tendency for anomalous eutectic formation, the remelted fraction of the primary eutectic was analyzed systematically based on the eutectic dendrite growth theory. For eutectic alloys with either larger equilibrium solute distribution coefficients or gentle liquidus slopes, the primary eutectic is highly supersaturated with solute and more prone to remelting. When the eutectic composition is set to different values, (e.g. the eutectic point is closer to one phase of the eutectic), the two eutectic phases under rapid growth change their compositions simultaneously, but their remelted fractions during temperature recalescence do not vary significantly. Three representative binary eutectic alloys Ag–39.9at.%Cu, Ni–19.6at.%P and Pd–16.0at.%P – their eutectic products are solid solution–solid solution, solid solution–stoichiometric intermetallic compound and stoichiometric intermetallic compound–stoichiometric intermetallic compound, respectively, were solidified at large undercooling. Anomalous eutectics were observed in the first two eutectic alloys whose eutectic structure consists of at least one solid solution phase, whereby the stoichiometric intermetallic compound phase remained highly oriented whereas the solid solution phase had a near random distribution of orientations. For the Pd–16.0at.%P alloy, however, the primary eutectic retained its original morphology as the final solidification structure regardless of the degree of undercooling. All these experimental results validate the argument that anomalous eutectics result from the remelting of the primary solid.

Solute redistribution during planar growth of intermetallic compound with nil solubility

Planar growth of a stoichiometric intermetallic compound, Al9Co2, is first experimentally confirmed in directionally solidified Al-11at%Co alloy. Based on the principle of solute conservation, the solute redistribution equations during planar growth of a stoichiometric intermetallic compound are obtained. When diffusion is the solute transport mode in the melt, a linear equation is obtained to evaluate the variation of the solute concentration in the liquid at the solid/liquid interface front with the freezing distance. For planar growth of intermetallic compound with nil solubility, there is no steady state boundary layer at the solid/liquid interface front, unless the initial composition of the melt is equal to the composition of the intermetallic compound. Experimental and theoretical work demonstrates that planar growth of the two constituent intermetallic compound phases can lead to a banded structure in a peritectic system.

Computation of concentrations of characteristic atoms of alloys in BCC structure

In this paper, taking Nb-Mo alloy system as an example, the equations of concentration of characteristic atoms of alloys in BCC structure were obtained on the basis of the idea of systematic science of alloys and the number of coordination atoms. The concentrations of characteristic atoms in B2-NbMo type ordered alloys were calculated as functions of ordering degree(s) and composition xMo. When s=smax, the concentrations of characteristic atoms of stoichiometric B2-NbMo intermetallic compound are equal to that of alloys, that is, Nb8Nb = 0.5 at., x0Mo = 0.5 at. As ordering degree decreases, characteristic atoms A8Nb and A0Mo of B2-NbMo type ordered alloy split. And the degree of splitting of characteristic atoms increases with the ordering degree decreasing. Therefore, disordered alloys and various types of ordered alloys can be designed.

Golden Mean analysis of bulk metallic glasses with critical diameter over half-inch for their mole fractions of compositions

Eighteen bulk metallic glasses (BMGs) with critical diameters over ∼15 mm in Pd-, Zr-, Mg-, Y-, La-, Pt- and Ca-based multicomponent alloys were analyzed for their mole fractions of compositions with Golden Mean ( ϕ ∼ 1.618). The results proved that the 18 BMG compositions are describable with mϕ − n or mΦ − n where Φ − n is ϕ −( n+1) plus ϕ −( n−1) and m and n are positive integers. The maximum difference in fractions between the analyzed and experimental compositions was 0.038. The irrational nature of ϕ makes the compositions of the 18 BMGs different from those of the stoichiometric intermetallic compounds and near eutectic compositions, leading to the high glass-forming ability. The possible prototypes of the 18 BMGs have the ability to form critically-percolated local atomic arrangements for both Metal–Metal and Metal–Metalloid types of BMGs. Golden Mean analysis provides a new alloy design for fractions of alloying elements owing to its irrationality.

Détermination sur lame du statut d’HER2 : bilan en France

Based on the random solution model and stoichiometric intermetallic compound Gibbs free energy model, the driving forces for the crystallization from the undercooled liquid of Cu-Zr binary alloys were evaluated by using Turnbull and Thompson-Spaepen (TS) approximate equations, respectively. Using Davies-Uhlmann kinetic formulations within the frame of continuous nucleation theory (CNT), two groups of time-temperature transformation (TTT) curves of eight compositions of the binary system were calculated, and thereby the corresponding critical cooling rates were obtained. It is shown that both groups of the calculated values for the glass-forming ability (GFA) are in good agreement with the experimental data. The critical cooling rates evaluated by using TS equation are in good agreement with the experimental data compared with that evaluated by using Turnbull equation.

Prediction of the Glass-Forming Ability of Cu-Zr Binary Alloys

Based on the random solution model and stoichiometric intermetallic compound Gibbs free energy model, the driving forces for the crystallization from the undercooled liquid of Cu-Zr binary alloys were evaluated by using Turnbull and Thompson-Spaepen (TS) approximate equations, respectively. Using Davies-Uhlmann kinetic formulations within the frame of continuous nucleation theory (CNT), two groups of time-temperature transformation (TTT) curves of eight compositions of the binary system were calculated, and thereby the corresponding critical cooling rates were obtained. It is shown that both groups of the calculated values for the glass-forming ability (GFA) are in good agreement with the experimental data. The critical cooling rates evaluated by using TS equation are in good agreement with the experimental data compared with that evaluated by using Turnbull equation.

Field-induced first-order magnetic phase transition in an intermetallic compoundNd7Rh3: Evidence for kinetic hindrance, phase coexistence, and percolative electrical conduction

The compound ${\mathrm{Nd}}_{7}{\mathrm{Rh}}_{3}$, crystallizing in the ${\mathrm{Th}}_{7}{\mathrm{Fe}}_{3}$-type hexagonal structure, was previously known to exhibit two magnetic transitions, one at $32\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and the other at $10\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ (in a zero magnetic field). Here we report the existence of a field-induced first-order antiferromagnetic-to-ferromagnetic transition at $1.8\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ in this compound. On the basis of the measurements of isothermal magnetization and magnetoresistance, we provide evidence for the occurrence of kinetic hindrance, proposed in the recent literature, resulting in phase coexistence (supercooled $\text{ferromagnetic}+\text{antiferromagnetic}$) and percolative electrical conduction in this stoichiometric intermetallic compound. A point of emphasis, as inferred from ac susceptibility data, is that such a coexisting phase is different from spin glasses thereby clarifying a question raised in the field of phase separation.

Fracture toughness of polycrystalline YCu, DyCu, and YAg

Room temperature fracture toughness was measured for YCu, DyCu, and YAg. Standard tensile tests indicated that YCu was the least ductile of these three intermetallics (11% elongation at failure); DyCu had an intermediate ductility (16% elongation at failure); while YAg was the most ductile (20% elongation at failure). The K IC value was determined directly for YCu using ASTM test method E 399-90; J IC values were measured using ASTM test method E 813-89 for DyCu and YAg, and these J IC values were converted to K IC values. The K IC values were found to be 12.0 MPa m for YCu, 25.5 MPa m for DyCu, and 19.1 MPa m for YAg. These values are relatively high for polycrystalline, fully ordered, stoichiometric intermetallic compounds tested in room air of normal humidity. Factors that may contribute to the high fracture toughness of these materials are discussed.

Anisotropic spin-glass-like and quasi-one-dimensional magnetic behavior in the intermetallic compoundTb2PdSi3

We report temperature-dependent ac susceptibility $(\ensuremath{\chi})$ measurements on a high-quality single crystal of ${\mathrm{Tb}}_{2}{\mathrm{PdSi}}_{3},$ crystallizing in a ${\mathrm{AlB}}_{2}$-derived hexagonal structure. This compound is found to exhibit features attributable to quasi-low-dimensional magnetism at high temperatures and anisotropic spin-glass-like behavior at low temperatures $(<10\mathrm{K})$ with an unusually large frequency dependence of peak temperature in ac $\ensuremath{\chi}(T).$ This compound thus presents a novel situation in metallic magnetism, considering that the former phenomenon is normally encountered only among insulators, whereas the anisotropic spin-glass-like behavior, to our knowledge, has not been known in stoichiometric intermetallic compounds.