Single-phase Solid-solution Alloys Research Articles

Chelating Co-reduction Strategy for the Synthesis of High-Entropy Alloy Aerogels.

Aerogels, as three-dimensional porous materials, have attracted much attention in almost every field owing to their unique structural properties. Designing high-entropy alloy aerogels (HEAAs) to quinary and above remains an enormous challenge due to the different reduction potentials and nucleation/growth kinetics of different constituent metals. Herein, a novel and universal chelating co-reduction strategy to prepare HEAAs at room temperature in the water phase is proposed. The addition of chelators (ethylenediaminetetraacetic acid tetrasodium salt, sodium citrate, salicylic acid, and 4,4'-bipyridine) with a certain strong coordination capacity can adjust the reduction potential of different metal components, which is the key to synthesize single-phase solid solution alloys successfully. The optimized AgRuPdAuPt HEAA can be an excellent electrocatalyst for hydrogen evolution reaction (HER) with an ultrasmall overpotential of 22 mV at 10 mA cm-2 and excellent stability for 24 h in an alkaline solution. In situ Raman spectroscopy unveils the enhanced hydrogen evolution reaction mechanism of HEAAs. Overall, this work provides a novel chelating co-reduction strategy for the facile and versatile synthesis and design of advanced HEAAs and broadens the development and utilization of multi-elemental alloy electrocatalysts.

Interdiffusion and atomic mobility of the Mg–Ag–Zn system

Diffusion couples for the Mg–Ag–Zn system were prepared using hcp-Mg single-phase solid solution alloys and pure magnesium (Mg). Their diffusion behaviour at 675–753K was analysed and the related diffusion coefficients were obtained. Combining these experimental results and the thermodynamic description, the atomic mobility parameters were optimised, and the kinetic database of the Mg–Ag–Zn ternary system was constructed for the first time. According to the thermodynamic and kinetic descriptions, the diffusion coefficients for concentration range of hcp-Mg (silver (Ag): 0–3 at% and zinc (Zn): 0–3 at%) were calculated. Finally, the composition and temperature dependence of the interdiffusion coefficients was further discussed using first-principles calculations and thermodynamic factors.

Exploring the limits of mechanical properties of Ti-Zr binary alloys

In this paper, we prepared Ti50Zr50 single-phase solid solution alloys using copper mold spray casting process. The single-phase structure changed from the steady-state hexagonal close-packed (HCP) phase (α phase) to body-centered cubic (BCC) phase (β phase) as the cooling rate increased. The β single-phase alloy exhibited the highest yield strength (1437 MPa) and hardness (631.9 Hv) while maintained an elongation of 20.8%, which is due to the strengthening effect of the fine grains. Moreover, the Young’s modulus of the alloys decreased with the phase transition. The β single-phase alloy shows a modulus of 71 GPa, which is nearly 30% lower compared with conventional α-type Ti-Zr binary alloys.

Recent development of coloring alloys

In this study, we systematically reviewed the strategy of the tailoring intrinsic color characteristics of chromatic and achromatic metallic materials. Its scientific contribution involves defining and expanding the range of coloration of metallic materials, including single-phase solid-solution alloys and multiphase alloys that includes specific phases with chromatic colors. In chromatic single-phase alloys, the modulation of optical properties depends on the transition energy of the solid solution phase. The additional alloying elements significantly influence the optical properties, i.e., the reflectivity in the visible spectrum, by varying the transition energy of the solid solution phase. Another coloring alloy design strategy varies and realizes the intrinsic color based on the multiphase system. In particular, the microstructural design based on the multiphase forming alloy system, including the chromatic coloring phase, effectively improves the chromaticity of the alloys by blending effect from the constitutive phases. A methodological framework is subsequently suggested to realize chromatic characteristics based on the juxtaposition method. The novel alloy design method using the juxtaposition method can be considered an important strategy for developing novel coloring alloys with high chromatic color durability from achromatic metallic materials.

Dislocation loop evolution and radiation hardening in nickel-based concentrated solid solution alloys

Effects of chemical composition, ion irradiation dose and temperature on unfaulting of irradiation induced Frank dislocation loops to perfect loops in two nickel based single-phase solid solution alloys, Ni–20Fe and NiFe–20Cr, have been studied. The fraction of Frank loops decreases with irradiation dose from 7.2 to 38.4 dpa at 500°C, but with more Frank loops remaining in the ternary alloy. However, perfect loops and dislocation networks become the dominant features of defects at 580°C in both alloys. The results indicate a thermally assisted loop unfaulting process that may be hindered by more sluggish defect motion in the alloy with more chemical components. Nano-indentation with both continuous stiffness method and single indentation method are used to measure radiation hardening. Loop unfaulting in both alloys irradiated at 580°C reduced radiation hardening while significant hardening is observed after irradiation at 500°C. The quasi-static single indentation method exhibits lower hardness results compared to continuous stiffness method, because dislocations induced from the cyclic loading in the latter method get relaxed and stabilized, resulting in higher resistance to the indenter.

Using machine learning and feature engineering to characterize limited material datasets of high-entropy alloys

The prediction of the phase formation of high entropy alloys (HEAs) has attracted great research interest recent years due to their superior structure and mechanical properties of single phase. However, the identification of these single phase solid solution alloys is still a challenge. Previous studies mainly focus on trial-and-error experiments or thermodynamic criteria, the previous is time consuming while the latter depends on the descriptors quality, both provide unreliable prediction. In this study, we attempted to predict the phase formation based on feature engineering and machine learning (ML) with a small dataset. The descriptor dimensionality is augmented from original small dimension to high dimension by non-linear combinations to characterize HEAs. The results showed that this method could achieve higher accuracy in predicting the phase formation of HEAs than traditional methods. Except the prediction of HEAs, this method also can be applied to other materials with limited dataset.

First Principle and Experimental Study for Site Preferences of Formability Improved Alloying Elements in Mg Crystal

Some alloying elements (Al, Er, Gd, Li, Mn, Sn, Y, Zn) were proved recently by calculations or experiments to improve the formability of Mg alloys, but ignoring their site preference in Mg crystals during the calculated process. A crystallographic model was built via first principle calculations to predict the site preferences of these elements. Regularities between doping elements and site preferences were summarized. Meanwhile, in the basis of the crystallographic model, a series of formulas were deduced combining the diffraction law. It predicted that a crystal plane with abnormal XRD peak intensity of the Mg-based solid solutions, compared to that of the pure Mg, prefers to possess solute atoms. Thus, three single-phase solid solution alloys were then prepared through an original In-situ Solution Treatment, and their XRD patterns were compared. Finally, the experiment further described the site preferences of these solute atoms in Mg crystal, verifying the calculation results.

Thermodynamic mixing energy and heterogeneous diffusion uncover the mechanisms of radiation damage reduction in single-phase Ni-Fe alloys

Understanding and predicting radiation damage is of central importance to develop radiation-tolerant structural materials for current and next-generation nuclear systems. Single-phase solid solution alloys constitute attractive choices due to their promising mechanical properties and radiation tolerance. Here, by examining radiation-induced defect production and evolution in single-phase Ni-Fe alloys, we show that radiation damage resistance directly correlates with thermodynamic mixing energy and heterogeneity of defect diffusion. We found that radiation damage in materials decreases linearly with lowering mixing energy, and the relationship holds true for all studied Ni-Fe compositions. The damage reduction with varying composition is further ascribed to the increasing heterogeneity of point defect migration across a complex potential energy landscape that enhances defect recombination. This new insight into the dynamical evolution of radiation defects points to a thermodynamic criterion for designing radiation-tolerant materials.

Designing high entropy alloys employing thermodynamics and Gaussian process statistical analysis

High entropy alloys (HEAs), a category of highly concentrated multicomponent alloys, have become a subject of interest in the past years due to their combination of properties. The development of these single phase solid solution alloys, containing between 5% and 35% of at least five different elements, has mainly relied on trial-and-error experiments, and more recently on modelling. The latter has notably focused on criteria to guide the formation of a single solid solution: (1) Hume-Rothery rules or their modification based on elemental variations in atomic radius, electronegativity, valence or number of itinerant electrons; (2) the use of thermodynamic concepts relying on estimates of enthalpy or entropy of mixing, and/or on melting or spinodal decomposition temperatures; (3) criteria based on lattice distortion; and (4) computational thermodynamics using the CALculation of PHAse Diagrams (CALPHAD) method. However, none of these criteria or methods, taken alone, can reliably predict the formation of a single solid solution. Instead, based on a critical assessment and a Gaussian process statistical analysis, a robust strategy to predict the formation of a single solid solution is proposed, taking into account most of the previously proposed criteria simultaneously. The method can be used as a guide to design new HEAs.

Thermal activation mechanisms and Labusch-type strengthening analysis for a family of high-entropy and equiatomic solid-solution alloys

To understand the underlying strengthening mechanisms, thermal activation processes are investigated from stress-strain measurements with varying temperatures and strain rates for a family of equiatomic quinary, quaternary, ternary, and binary, face-center-cubic-structured, single phase solid-solution alloys, which are all subsystems of the FeNiCoCrMn high-entropy alloy. Our analysis suggests that the Labusch-type solution strengthening mechanism, rather than the lattice friction (or lattice resistance), governs the deformation behavior in equiatomic alloys. First, upon excluding the Hall-Petch effects, the activation volumes for these alloys are found to range from 10 to 1000 times the cubic power of Burgers vector, which are much larger than that required for kink pairs (i.e., the thermal activation process for the lattice resistance mechanism in body-center-cubic-structured metals). Second, the Labusch-type analysis for an N-element alloy is conducted by treating M-elements (M < N) as an effective medium and summing the strengthening contributions from the rest of N-M elements as individual solute species. For all equiatomic alloys investigated, a qualitative agreement exists between the measured strengthening effect and the Labusch strengthening factor from arbitrary M to N elements based on the lattice and modulus mismatches. Consequently, the Labusch strengthening factor provides a practical critique to understand and design such compositionally complex but structurally simple alloys.

Critical Assessment 14: High entropy alloys and their development as structural materials

The field of high entropy alloys has exploded in its first 10 years. Vast opportunities for new compositions and microstructures are offered by this idea, but current efforts have become surprisingly focused on a narrow set of systems and on the search for single phase solid solution alloys. This perspective outlines the challenge to re-engage the full range of compositional and microstructural complexity in the search for new structural metals. We draw on extensive experience in the development of exquisite multiphase microstructures and define new high throughput experiments and computations that must be integrated to respond to this challenge. Broadening the scope is expected to provide dramatic new opportunities in the field of complex concentrated alloys.

Accelerated exploration of multi-principal element alloys for structural applications

A strategy for accelerated discovery and exploration of multi-principal element alloys was developed and used to identify new alloys within a design window of desired microstructures and properties. As an example, the strategy was applied to analyze thousands of 3- 4-, 5- and 6-component alloys at equiatomic compositions of the alloying elements. Currently available thermodynamic databases were used to assess equilibrium phase diagrams for these alloys. The validity and reliability of the calculated phase diagrams were estimated based on the extent of experimental binary and ternary data used to build the respective thermodynamic databases. Alloys with specific characteristics, such as single-phase solid solution alloys with the use temperature above 1000°C, were identified using an automated analysis of the calculated phase diagrams. The density, elastic moduli and costs of these alloys were estimated using the rule of mixtures of pure elements and were used as additional criteria for alloy selection. This approach allowed rapid, albeit preliminary, screening of many thousands of alloys and identification of promising candidate compositions, some of which are reported in this paper, for more time intensive experimental validations and assessments.

The development of non-equilibrium solidification theories

In recent years, the non-equilibrium solidification technology is developing very fast while the designing of solidification processes still relies on classical theories and empirical relations, which can no longer satisfy the increasing need for controlling of solidification conditions. The classical solidification theories are based on several major assumptions, such as linear liquidus/solidus, ideal solution, local equilibrium and they usually focus on model alloys instead of real industrial concentrated multi-component alloys. The discrepancies between classical theories and practical conditions demand us to abandon these classical assumptions so that the solidification theories can be applied to industrial processes. The current paper reviewed the recent development of non-equilibrium solidification theories for single-phase solid solution alloys. For dilute binary alloys, the microscopic interface kinetics, interface stability, dendrite growth and the micro-macro overall transformation kinetics were reviewed and the significances of nonlinear liquidus/solidus were illustrated. For concentrated binary and multi-component alloys, the interface kinetics, interface stability and dendrite growth were reviewed with an emphasis on the importance of interaction between the components. The newly-developed non-equilibrium theories can give more accurate description of complicated solidification processes, thus promote the application of the solidification theories to industrial processes.

Effect of back diffusion on overall solidification kinetics of undercooled single-phase solid-solution alloys

Departing from the volume-averaging method, an overall solidification kinetic model for undercooled single-phase solid-solution alloys was developed to study the effect of back diffusion on the solidification kinetics. Application to rapid solidification of undercooled Ni-15%Cu (mole fraction) alloy shows that back diffusion effect has significant influence on the solidification ending temperature but possesses almost no effect on the volume fraction solidified during recalescence. Inconsistent with the widely accepted viewpoint of Herlach, solidification ends at a temperature between the predictions of Lever rule and Scheil's equation, and the exact value is determined by the effect of back diffusion, the initial undercooling and the cooling rate.

Role of yttrium in the microstructure and texture evolution of Mg

Abstract Single-phase solid solution alloys, Mg–2.9Y and Mg–2.9Zn (in wt%), and two-phase (α-Mg solid solution + intermetallic) Mg–Zn–Y alloys were deformed in uniaxial hot compression (350 °C) to investigate the origin of texture weakening upon the addition of Y rare earth element into Mg. The separate effects of the type of solutes and intermetallics during deformation and subsequent isothermal annealing were studied via X-ray texture measurement and EBSD techniques. It was found that the ternary Mg–5Zn–2Y and Mg–6Zn–1.2Y alloys exhibit recrystallization of new grains at intermetallics. Furthermore, the progress of dynamic recrystallization with straining is severely retarded in the Mg–2.9Y alloy, with respect to all the other alloys. A nearly similar basal texture develops in all the alloys at the strain of 1 (i.e., steady state region of flow curves) despite the difference observed in the volume fraction of dynamically recrystallized grains. On subsequent annealing of the as-deformed alloys, a significant increase in the average size of the recrystallized grains is observed, but the texture is weakened only in the Mg–2.9Y alloy.

Modeling grain refinement for undercooled single-phase solid-solution alloys

A grain-refinement model for rapid solidification of undercooled single-phase solid-solution alloys was developed in which a combination of the model of dendrite fragmentation with the model of overall solidification kinetics allows dendrite fragmentation to occur along with the solidification process (e.g. during mushy zone solidification). Compared with Karma’s model, the dendrite break-up time as well as the time for the interdendritic liquid to be solidified completely can be predicted. Thus, the model can predict, in the absence of experimental data (e.g. the plateau duration), the microstructure transition. The model was applied to describe the rapid solidification of undercooled Ni–15 at.% Cu alloy, and a good agreement between the model predictions and the experimental results was obtained. On this basis, the effects of capillarity and back-diffusion on dendrite fragmentation during mushy zone solidification, as well as the effects of back-diffusion and cooling rate on the prediction of the microstructure transition, were found.

Modeling the overall solidification kinetics for undercooled single-phase solid-solution alloys. II. Model application

An overall solidification kinetic model was applied to undercooled Ni–15 at.% Cu alloy. A good agreement between the model predictions and the measured cooling curves was obtained by adopting a “phenomenological” heat boundary condition. Applying numerical calculations, it was demonstrated that solute is uniformly distributed at the purely thermal-controlled growth stage; a transition from non-equilibrium to near-equilibrium solidification occurs at the mainly thermal-controlled growth stage only if sufficiently high initial undercooling is available; and a transition from recalescence to post-recalescence occurs at the solutal-controlled growth stage wherein the current model reduces to Scheil’s equation. The volume fraction solidified during recalescence is the same as the ratio of the initial undercooling to the hypercooling limit, and solidification should end generally at a temperature between the prediction of Scheil’s equation and that of the Lever rule if back diffusion is considered.

Modeling the overall solidification kinetics for undercooled single-phase solid-solution alloys. I. Model derivation

Departing from the volume-averaging method, the equiaxed solidification model was extended to describe the overall solidification kinetics of undercooled single-phase solid-solution alloys. In this model, a single grain, whose size is given assuming site saturation, is divided into three phases, i.e. the solid dendrite, the inter-dendritic liquid and the extra-dendritic liquid. The non-equilibrium solute diffusion in the inter-dendritic liquid and the extra-dendritic liquid, as well as the heat diffusion in the extra-dendritic liquid, is considered. The growth kinetics of the solid/liquid interface is given by the solute or heat balance, where a maximal growth velocity criterion is applied to determine the transition from thermal-controlled growth to solutal-controlled growth. A dendrite growth model, in which the nonlinear liquidus and solidus, the non-equilibrium interface kinetics, and the non-equilibrium solute diffusion in liquid are considered, is applied to describe the growth kinetics of the grain envelope. On this basis, the solidification path is described.

Metastable phase formation in rapidly solidified Al–Mo alloys

In Al-Mo alloys, rapidly quenched from the melt at rates of approximately 106 KS−1, it has been demonstrated that single phase solid solution alloys can be obtained in materials with up to 1·3 at.-...

A first report on fracture toughness ofbcc iron alloys as influenced by solutes: opposite effects of silicon and cobalt

The effect of solutes on resistance to fracture of body centred cubic iron single-phase solid-solution alloys has been investigated. TheJ-integral method has been used for the measurement of ductile fracture toughness. TheJ IC values so determined quantitatively indicate the extent of degradation in fracture toughness due to the addition of hardening solute silicon. Cobalt addition results in alloy softening. The measuredJ IC values clearly demonstrate the toughening effect of cobalt addition as a solute, which result renders the case of Fe-Co solid-solution alloys interesting.