Substitutional Alloys Research Articles

Machine learning-driven insights into phase prediction for high entropy alloys

The unique properties of high-entropy alloys (HEAs) have attracted considerable attention, largely due to their dependence on the choice among three distinct phases: solid solution (SS), intermetallic compound (IM), or a blend of both (SS + IM). For this reason, precise phase prediction is key to identifying the optimal element combinations needed to develop HEAs with the required characteristics. Due to large compositional domain of HEAs is opportune to design new HEAs with desired output. A machine learning tool is exploited to discover and characterize high entropy alloys with satisfying targets. Herein, a method of designing substitutional high entropy alloys with optimization of input features and predict their phase formation, using different ML algorithms are proposed. The ML models such as multi layer precreptron MLP, Decision Tree (DT), Random Forest (RF), Gradient Boosting (GB), KNN, XGB nad SVM Classifier algorithm were used for the identifying the phase of HEAs. After assessing the accuracy and tuning of each model, an random forest classifier (accuracy = 0.914. precision = 0.916, ROC-AUC score = 0.97) model showed the best predictive capabilities for phase prediction. The new HEA was designed based on prediction and successfully validated with thermodynamic simulation. Data AvailabilityData will be made available on request

A DFT investigation of Ti-substituted [formula omitted] for tailored photovoltaic properties

Transition Metal Chalcogenide Perovskites (TMCP) have been in the spotlight due to their exceptional optoelectronic properties. CaZrS3 is one among them with an experimental bandgap of 1.90 eV. If its bandgap is tuned to lower values, it can be employed in additional photovoltaic applications, such as solar cell absorbers. In this work, the transition metal element Zr in CaZrS3 is substituted with Ti atoms in different proportions and the optoelectronic properties are investigated using Density Functional Theory (DFT). The optoelectronic calculations are all done using the DFT+U method including the spin–orbit coupling. With substitutional alloying, we successfully tuned the energy gap from 1.91 eV to 1.18 eV and the photovoltaic properties were also observed to be modified. For the substituted CaZr1−xTixS3 samples, large birefringence is observed. This indicates enhancement in optical anisotropy via substitutional alloying which is significant in both linear and nonlinear optoelectronic applications like polarizers, wave plates etc.

Mixed-Metal Alloying in Hybrid Bronzes.

We show that substitutional alloying during the aqueous self-assembly of layered organic-templated metal oxides produces single-phase mixed-metal hybrids. Single-crystal X-ray diffraction, bulk elemental analyses, and vibrational and electronic spectroscopies corroborate a solid solution of Mo and W atoms at lattice sites within the two-dimensional metal oxide layers. Mild postsynthetic reduction then introduces relatively delocalized electrons to afford mixed-metal hybrid bronzes. To our knowledge, this represents the first demonstration of mixed-metal alloying in a hybrid metal oxide and a rare example of solid-solution formation at low temperature. We show this approach yields mixed-metal congeners with optical band gaps over 130 meV smaller than those of single-metal analogs, while achieving activation energies (Ea) of conduction as low as 78.4(2) meV. Further, metal substitution appears to tune collective electronic phenomena by suppressing the non-Arrhenius behavior observed for Mo-based hybrids. This work considerably expands the nascent hybrid bronze platform to help address energy-related challenges and fundamental solid-state physical questions.

STUDY ON SELF-DIFFUSION IN ALLOY AuCuSi WITH FCC STRUCTURE: DEPENDENCE ON TEMPERATURE, PRESSURE, CONCENTRATION OF SUBSTITUTIONAL ATOMS, CONCENTRATION OF INTERSTITIAL ATOMS AND STRAIN

This article presents a self-diffusion theory, including analytical expressions for activation energy, pre-exponential factor, and self-diffusion coefficient, dependent on temperature, pressure, the concentration of substitutional atoms, the concentration of interstitial atoms, and strain for substitutional and interstitial ternary alloys with the FCC structure, based on the statistical moment method (SMM). The theoretical results are applied to numerical calculations for the AuCuSi alloy. The SMM numerical results for AuCuSi are compared with those for AuSi, AuCu, and Au. The variation of self-diffusion quantities with temperature and strain in AuCuSi follows the same patterns as in AuCu, AuSi, and Au. The variation with the interstitial atom concentration in AuCuSi follows the same pattern as in AuSi, while the variation with the substitutional atom concentration follows the same pattern as in AuCu. The SMM numerical results for Au's self-diffusion quantities agree well with experimental data and other calculation results. Other SMM numerical results for self-diffusion quantities are novel and predict future experimental outcomes.

Topological Photonic Alloy.

We present the new concept of photonic alloy as a nonperiodic topological material. By mixing nonmagnetized and magnetized rods in a nonperiodic 2D photonic crystal configuration, we realized photonic alloys in the microwave regime. Our experimental findings reveal that the photonic alloy sustains nonreciprocal chiral edge states even at very low concentration of magnetized rods. The nontrivial topology and the associated edge states of these nonperiodic systems can be characterized by the winding of the reflection phase. Our results indicate that the threshold concentrations for the investigated system within the first nontrivial band gap to exhibit topological behavior approach zero in the thermodynamic limit for substitutional alloys, while the threshold remains nonzero for interstitial alloys. At low concentration, the system exhibits an inhomogeneous structure characterized by isolated patches of nonpercolating magnetic domains that are spaced far apart within a topologically trivial photonic crystal. Surprisingly, the system manifests chiral edge states despite a local breakdown of time-reversal symmetry rather than a global one. Photonic alloys represent a new category of disordered topological materials, offering exciting opportunities for exploring topological materials with adjustable gaps.

Modeling solute drag during austenite–ferrite transformation with ab initio binding energies

The solute drag effect can have a considerable impact on the austenite to ferrite transformation. Most models describing the solute drag effect require knowledge of solute binding energies, which due to lack of accurate data typically are treated as fit parameters. In this study we use density functional theory (DFT) calculations to estimate the effective binding energies of the substitutional alloying elements in an ultra low carbon steel. For the first time these energies are used to calculate the solute drag effect considering site competition for the austenite to ferrite phase transformation. The depletion of elements in solid solution as a result from precipitation is computed and the solute drag contribution of each element is calculated. Comparison of the binding energies with the resulting solute drag effect to literature data shows reasonable agreement. The outlined approach points the way to future alloy development based on interface-controlled integrated computational materials engineering. Another field of application is given by the circular economy driven transition in the steel industry from the BF/BOF- to the EAF-based production route with increased utilization of scrap material introducing new tramp elements.

Iron alloys of volatile elements in the deep Earth's interior.

Investigations into the compositional model of the Earth, particularly the atypical concentrations of volatile elements within the silicate portion of the early Earth, have attracted significant interest due to their pivotal role in elucidating the planet's evolution and dynamics. To understand the behavior of such volatile elements, an established 'volatility trend' has been used to explain the observed depletion of certain volatile elements. However, elements such as Se and Br remain notably over-depleted in the silicate Earth. Here we show the results from first-principles simulations that explore the potential for these elements to integrate into hcp-Fe through the formation of substitutional alloys, long presumed to be predominant constituents of the Earth's core. Based on our findings, the thermodynamic stability of these alloys suggests that these volatile elements might indeed be partially sequestered within the Earth's core. We suggest potential reservoirs for volatile elements within the deep Earth, augmenting our understanding of the deep Earth's composition.

In Ukrainian

The review material reveals the question of the influence of impurities on the physical properties of alloys, as well as the influence of gases on processes in alloys. It has been established that the presence of hydrogen affects diffusion in alloys, and gases have a significant effect on the thermoelectromotive force of metals. The paper describes the addition of a third element to binary systems, which can expand or narrow the domain of existence of an ordered phase. The above examples of adding an impurity of vanadium or molybdenum to an alloy of iron and chromium increase the ordering temperature. It has been established that gases can have a significant effect on the thermoelectromotive force of metals and increase the hardness and reduce the ductility of metals, as well as cause their brittleness and delamination. It has been established that hydrogen (H2) in alloys (for example, Fe-Ni and Au-Cu) affects diffusion and atomic ordering processes. Also, a small concentration of hydrogen, as an impurity, can change the electrical resistance of alloys (for example, in a Pd-Au alloy). It has been established that the addition of a third element to binary systems can change the state diagram, which must be taken into account when solving problems for the production of heat treatment modes for alloys, determining the conditions for phase equilibrium, etc. The paper considers the dependence of hydrogen solubility on temperature in metals (Cu, Fe and Al) and establishes the dependence of hydrogen solubility in a metal on the concentration of impurity metals. The temperature dependence of hydrogen solubility in Fe-V alloys is also shown. The shift of the density of state is studied for different distributions of impurity atoms in the alloy in the order-disorder system, and the dependence of the density of states of disordered and ordered solid solutions is studied. The dependence of the relative solubility of atoms introduced into the pores of alloys with an fcc structure (of the Cu3Au type) is indicated, taking into account the solubility, on the parameter χ, which is proportional to the degree of long-range order in the octahedral and tetrahedral pores of the alloys. The dependence of the relative solubility of intercalated atoms in octahedral pores of alloys with a bcc structure (such as CuZn and Fe3Al) is considered. A plot of the dependence of the concentration of dissolved impurity atoms on the degree of order is shown for substitutional alloys with a bcc lattice, where the M2 coefficient is introduced, which does not depend on the degree of order. The effect of pressure as a measure of long-range order is also studied, where its step decreases or increases, or decreases and then increases and vice versa (order-disorder phase transition). An equally important result is the graphic dependence of solubility (c) on pressure (P) in disordered and ordered alloys.

Phase transformations in WC-Cr3C2-Ni thermal spray coatings under low, medium and high thermal input: Part 2 – DSC analysis and heat-treated coating characterisation

Conventionally, bulk WC and Cr3C2-based carbide compositions have been used independently of each other. However, recent investigations have begun to explore combining these carbides together within the same composite/hard metal coating system. This work explored the diversity of phases produced in the coatings from a 42 wt% WC-42 wt% Cr3C2–16 wt% Ni powder sprayed under “low”, “medium” and “high” thermal input conditions using high velocity oxygen-fuel (HVOF), an argon‑helium (ArHe) plasma and an argon‑hydrogen (ArH2) plasma respectively. The thermally driven solid-state transformations in the coatings were investigated as a function of carbide dissolution/decomposition, using differential scanning calorimetry (DSC). All coatings exhibited a well defined exothermic peak attributed to the transformation of the metastable supersaturated Ni-alloy and WC1-X phases in the as-sprayed coatings, into the equilibrium Ni alloy and a mix of the intermediate metastable carbide phases β-W2C and Cr3C2-Y. At higher temperatures the metastable compounds transformed to the equilibrium phases of ε-W2C and Cr3C2. A defining feature of the XRD analysis was the significant shift in lattice positions of all phases, indicating substitutional alloying. The mechanisms of phase formation are discussed.

Laves phase double substitution alloy design and device filling modification for Ti-based metal hydride hydrogen compressors

The ambition of carbon neutrality has become a common pursuit for researchers worldwide, and hydrogen as a green and clean energy carrier is expected to contribute to this achievement. Since hydrogen fuel cell vehicles require a high input hydrogen pressure, compressors in hydrogen refueling stations are thus indispensable for the large-scale application of hydrogen energy. Compared to conventional mechanical hydrogen compressors, metal hydride based hydrogen compressors have the advantages of high security and low cost, providing a viable solution for hydrogen refueling station development. Herein, two series of Ti0.95Zr0.07Cr1.3-xMnxFe0.4 (x = 0.2, 0.3, 0.4, 0.6) and Ti0.95Zr0.07Cr1.0Mn0.6Fe0.4-yCuy (y = 0.1, 0.15, 0.2, 0.3) alloys were designed to achieve 8–22 MPa hydrogen compression in water bath at 283–353 K. All the as-prepared alloys exhibit the single hexagonal C14 Laves phase structure. In particular, the double substitution of Cr and Fe by Cu and Mn elements in Ti–Zr–Cr–Mn–Fe alloy can enlarge its hydrogen storage capacity without varying the equilibrium pressure. For instance, Ti0.95Zr0.07Cr1.0Mn0.6Fe0.25Cu0.15 alloy has a hydrogenation plateau pressure of 6.16 MPa at 283 K and a dehydrogenation plateau pressure of 24.23 MPa at 353 K, with the max hydrogen capacity of 1.72 wt%. Furthermore, the relative high density molding method of metal hydride bed with 20 g of alloy powder was developed. The binder epoxy resin and the thermal conductor flake graphite were selected and added into the alloys to form metal hydride compacts (MHCs). The structural stability of modified MHCs can be enhanced due to the formation of a three-dimensional grid structure, with a higher resistance to pulverization and degradation. The improved thermal conductivity involving a filling compact strategy can significantly shorten hydrogenation reaction time by 50% and lower the abrupt temperature of metal hydride bed by 20 K with excellent stability over 100 cycles, and provide new insights of the development of safe and efficient hydride hydrogen compressor.

Efficient ab initio stacking fault energy mapping for dilute interstitial alloys

Density Functional Theory (DFT) is the prevalent first principles computational method for determining the stacking fault energy (SFE) of face centered cubic (fcc) metals and alloys. Due to several theoretical and computational challenges, SFE determination for interstitial alloys with alloying elements such as carbon, nitrogen, and hydrogen, has so far been limited to few studies at relatively high interstitial content. We propose a new method, rooted in the axial interaction model, that allows rapid and robust mapping of SFE for virtually arbitrary interstitial contents. Instead of computing the total energy of a very large supercell to represent dilute interstitial solutions, representative interstitial-affected and bulk regions are treated separately at the equivalent volume. The SFE is obtained by balancing the SFE values of the regions with a lever rule approach. The method matches SFE values from the axial interaction model within ≤4 mJ.m−2 error, as validated for non-magnetic fcc Fe-N and paramagnetic fcc Fe-N and AISI 304 alloys. The significantly reduced computational workload and equidistant SFE mapping vs. interstitial content down to extremely low values allows accurate fitting of the SFE vs. interstitial content with only few datapoints. This further improves the computational efficiency. So far DFT-based SFE mapping was limited to purely substitutional alloys; we demonstrate the first-time DFT-based SFE mapping in fcc AISI 304 vs. N and Ni, revealing a non-additive contribution of N and Ni to the SFE. Finally, the remaining challenges and future application for high-throughput DFT SFE computation in interstitial alloys is discussed.

Band-engineered LaFeO3–LaNiO3 thin film interfaces for electrocatalysis of water

Iron and nickel-based perovskite oxides have proven promising for the oxygen evolution reaction (OER) in alkaline environments, as their catalytic overpotentials rival precious metal catalysts when the band alignment is tuned through substitutional doping or alloying. Here, we report the engineering of band alignment in LaFeO3/LaNiO3 (LFO/LNO) heterostructures via interfacial doping that yields greatly enhanced catalytic performance. The 0.2 eV offset (VBO) between the Fermi level in metallic LNO and the valence band in semiconducting LFO that we predict using density functional theory makes LFO a p-type semiconductor, resulting in significantly lower barriers for hole transport through LFO compared to the intrinsic material. Experimental band alignment measured with in situ x-ray photoelectron spectroscopy of epitaxial LFO/LNO heterostructures confirms these predictions, producing a measured VBO of 0.3(1) eV. Furthermore, OER catalytic measurements on these samples in the alkaline solution show an increase in catalytic current density by a factor of ∼275 compared to LFO grown on n-type Nb-doped SrTiO3. These results demonstrate the power of tuning band alignments through interfacial band engineering for improved catalytic performance of oxides.

Understanding fusion zone hardness in resistance spot welds for advanced high strength steels: Strengthening mechanisms and data-driven modeling

The fusion zone (FZ) hardness of resistance spot welds is a crucial factor affecting the performance and durability of the welds. Failure mode transition, tendency to fail in interfacial mode, interfacial failure load, and the impact of liquid metal embrittlement cracks on the weld strength are influenced by the FZ hardness. Therefore, accurately predicting the FZ hardness in resistance spot welds made on automotive steels is essential. A simple thermal model is used to calculate the time required for the temperature to drop from 800 °C to 500 °C (Δt85). With the aid of continuous cooling transformation (CCT) diagrams and experimental confirmation, it is shown that in most automotive steels, the FZ exhibits an almost full martensitic microstructure. Generally, the FZ hardness tends to increase in the order of interstitial-free (IF), drawing quality specially killed (DQSK), high strength low alloy (HSLA), ferrite-martensite dual-phase steels, transformation induced plasticity (TRIP), and quench & partitioning (Q&P) steels. To predict the FZ hardness, data-driven regression-based models have been developed based on carbon content, carbon equivalent concept, and the strengthening mechanisms of the martensite. Among these models, the model based on martensite strengthening mechanisms showed the best performance and robustness in estimating the FZ hardness. The assessment of the strengthening mechanism of the FZ showed that most important factors determining the FZ hardness are dislocation hardening due to carbon atoms, interface hardening due to block boundaries, and solid solution hardening due to substitutional alloying elements, respectively. A simple relation is proposed to estimate the load-bearing capacity of automotive steel spot welds during interfacial failure based on the FZ hardness.

Improved hydrogen storage properties of low-cost Ti–Cr–V alloys by minor alloying of Mn

V-based BCC solid-solution alloys have been considered as promising hydrogen storage materials due to their high hydrogen storage capacity; however, the low effective hydrogen desorption capacity and high cost of pure V have limited their practical application in fuel cell vehicles or devices. Herein, a low-V TiCr1.2(V–Fe0.203)0.6 hydrogen storage alloy with a hydrogen storage capacity of 3.35 wt% has been developed by using a low-cost FeV80 as raw material. To further increase the effective hydrogen desorption capacity, cheap Mn is first used to partially substitute for Cr in FeV80-based alloy. The Mn substitution alloy achieves an effective hydrogen desorption capacity of 2.07 wt% cutting-off at 0.1 MPa with good activation performance. It is ascribed that the minor alloying of Mn increases the dehydrogenation plateau pressure and reduces the hysteresis and the enthalpy change value. The dehydrogenation enthalpy decreases from 34.84 kJ/mol to 31.51 kJ/mol. The synergistic effect of Mn and Fe increases the abundance of the C14 Laves phase, which plays a catalytic role in hydrogen absorption and desorption. The cost of TiCr1.1Mn0.1(V–Fe0.203)0.6 alloy is much lower than that of using pure V, and even lower than some AB2 alloys based on cost estimation. The results of this study would provide a reference for the application of low-cost Ti–Cr-(FeV80) alloys for hydrogen storage.

Grain-boundary segregation and grain growth in nanocrystalline substitutional solid solution alloys

A model for describing solute segregation at grain boundaries has been developed for substitutional solid solution alloys, which integrates multiple factors from atomic to microstructural scales. A concept of molar Gibbs free energy of segregation was introduced to evaluate the segregating capability of the solute elements in a closed system, through which the influences of grain boundary structure, grain size, material composition, and external conditions were described. Based on the evaluation of various energy forms related to solute segregation and grain growth processes, the nature of the thermal stabilization of nanograin structures by solute segregation was disclosed. A criterion for the destabilization of nanostructures, which is determined by the competition of the change rates between the molar Gibbs free energy of segregation and the total energy of grain boundaries with grain size, has been proposed. This study provided guideline to achieve high-temperature stability of nanograin structures of solid solution alloys even for the weakly segregating nanocrystalline systems.

DFT study of the formation of the Sb-Ag(1 1 1) (√3×√3) – R30° surface alloy

According to performed DFT calculations, the (√3×√3) – R30° structure, in which Sb atoms occupy hcp sites on Ag(111), is more energetically favorable than the related substitutional SbAg2 surface alloy. The difference in energies is significant, 0.66 eV per unit cell, and can be attributed to a larger size of Sb atoms compared to Ag. The obtained results of modeling the substitutional process explain the formation of adsorbed Sb layer on the Ag(111) surface by means of Sb deposition instead of the formation of the substitutional surface alloy. In turn, Ag deposition onto the alloy surface will cause a floating up (segregation to the surface) of Sb atoms thus restoring the alloy surface, which may serve as a surfactant in the process of the growth of Ag crystal.

Microstructural diversity, nucleation paths, and phase behavior in binary mixtures of charged colloidal spheres.

We study low-salt, binary aqueous suspensions of charged colloidal spheres of size ratio Γ = 0.57, number densities below the eutectic number density nE, and number fractions of p = 1.00-0.40. The typical phase obtained by solidification from a homogeneous shear-melt is a substitutional alloy with a body centered cubic structure. In strictly gas-tight vials, the polycrystalline solid is stable against melting and further phase transformation for extended times. For comparison, we also prepare the same samples by slow, mechanically undisturbed deionization in commercial slit cells. These cells feature a complex but well reproducible sequence of global and local gradients in salt concentration, number density, and composition as induced by successive deionization, phoretic transport, and differential settling of the components, respectively. Moreover, they provide an extended bottom surface suitable for heterogeneous nucleation of the β-phase. We give a detailed qualitative characterization of the crystallization processes using imaging and optical microscopy. By contrast to the bulk samples, the initial alloy formation is not volume-filling, and we now observe also α- and β-phases with low solubility of the odd component. In addition to the initial homogeneous nucleation route, the interplay of gradients opens various further crystallization and transformation pathways leading to a great diversity of microstructures. Upon a subsequent increase in salt concentration, the crystals melt again. Wall-based, pebble-shaped β-phase crystals and facetted α-crystals melt last. Our observations suggest that the substitutional alloys formed in bulk experiments by homogeneous nucleation and subsequent growth are mechanically stable in the absence of solid-fluid interfaces but thermodynamically metastable.

Nanostructured Ni(Zn)–Al2O3 cermet particle synthesis in high-energy mechanical alloying method (CG-3:IL07)

Ceramic-metallics (cermets) have emerged as an important class of composite material combining favorable properties of their constitutive phases. In this work, nanostructured cermet Ni(Zn)–Al2O3 composition was synthesized using the high-energy mechanical alloying (HE-MA) method to be deployed as a cold spray deposition powder feedstock. Pre-milled Ni(Zn) alloy particles were mechanically mixed followed by milling in a SPEX™ 8000 M High Energy Ball Mill at a fixed 1050 rpm. The material design-of-experiment (DoE) involved two design compositions (30 wt% and 50 wt% α-alumina) with two substitutional alloy compositions, Ni(5 wt%Zn) and Ni(10 wt%Zn), along with milling time as a process variable. Microstructural characterization confirmed the embedment of nanoscale Al2O3 grain in micron-scale Ni(Zn) alloy particles. Additionally, EBSD analysis of Ni(Zn) alloy particles revealed that Ni(5 wt%Zn) experienced more uniform plastic deformation, work hardening and subsequent fracturing when compared to Ni(10 wt% Zn) alloy particles. The particle size measurement was carried out by laser diffraction showed that the cermet particles milled for 4 h had the desired size range for the objective of cold spraying. Also, the embedment of alumina into the Ni(Zn) alloy matrix and thereby, homogenization of cermet particles enhanced with the increase in milling time.

Thermophysical Properties of Tantalum–Tungsten Substitution and Interstitial Alloys in the Temperature Range 1000‒2500 K

Thermophysical Properties of Tantalum–Tungsten Substitution and Interstitial Alloys in the Temperature Range 1000‒2500 K

Study on density and lattice compression of Al, Cu and AlCu in high temperature and pressure from statistical moment method

We derive analytic expressions of Helmholtz free energy, crystal parameters, nearest neighbor distance and mean nearest neighbor distance between two atoms, volume and equation of state of metal and substitutional alloy with FCC structure on the basis of the statistical moment method. We perform numerical calculations of density and ratio of volume at pressure [Formula: see text] to volume at zero pressure for metals Al, Cu and substitutional alloy AlCu at temperature up to 1500 K and pressure up to 350 GPa when using the Mie–Lennard–Jones [Formula: see text] potential and the coordination sphere method. Our calculation results are compared with the experimental data, other theoretical calculations and molecular dynamics simulations.