Random Solid Solution Research Articles

Enantiospecific equilibrium adsorption and chemistry of d-/l-proline mixtures on chiral and achiral Cu surfaces.

A fundamental understanding of the enantiospecific interactions between chiral adsorbates and understanding of their interactions with chiral surfaces is key to unlocking the origins of enantiospecific surface chemistry. Herein, the adsorption and decomposition of the amino acid proline (Pro) have been studied on the achiral Cu(110) and Cu(111) surfaces and on the chiral Cu(643)R&S surfaces. Isotopically labelled 1-13 C-l-Pro has been used to probe the Pro decomposition mechanism and to allow mass spectrometric discrimination of d-Pro and 1-13 C-l-Pro when adsorbed as mixtures. On the Cu(111) surface, X-ray photoelectron spectroscopy reveals that Pro adsorbs as an anionic species in the monolayer. On the chiral Cu(643)R&S surface, adsorbed Pro enantiomers decompose with non-enantiospecific kinetics. However, the decomposition kinetics were found to be different on the terraces versus the kinked steps. Exposure of the chiral Cu(643)R&S surfaces to a racemic gas phase mixture of d-Pro and 1-13 C-l-Pro resulted in the adsorption of a racemic mixture; i.e., adsorption is not enantiospecific. However, exposure to non-racemic mixtures of d-Pro and 1-13 C-l-Pro resulted in amplification of enantiomeric excess on the surface, indicative of homochiral aggregation of adsorbed Pro. During co-adsorption, this amplification is observed even at very low coverages, quite distinct from the behavior of other amino acids, which begin to exhibit homochiral aggregation only after reaching monolayer coverages. The equilibrium adsorption of d-Pro and 1-13 C-l-Pro mixtures on achiral Cu(110) did not display any aggregation, consistent with prior scanning tunneling microscopy (STM) observations of dl-Pro/Cu(110). This demonstrates convergence between findings from equilibrium adsorption methods and STM experiments and corroborates formation of a 2D random solid solution.

Improved optoelectronic properties in CdSexTe1−x through controlled composition and short-range order

We employ first principles methods based on density functional theory and beyond to study CdSexTe1−x alloys in the zincblende and wurtzite structures. From the cluster expansion formalism, we provide a detailed phase diagram showing a consolute temperature of 325 K, above which miscibility may be achieved. In the random solid solution, a zincblende-to-wurtzite phase boundary is found to range from Se concentrations of x = 0.5–0.6, in agreement with experiment, owing to increasing ionic character of the Cd-anion bonds. Disordered CdSexTe1−x configurations are modeled using special quasirandom structures, for which optoelectronic properties are computed with the hybrid HSE06 functional. Alloying is shown to cause strong bowing effects in the band gap and effective electron/hole masses, which we attribute to local structural distortions as illustrated by analysis of bond length distributions. Downward bowing in the band gap and effective hole mass of the zincblende structure is highlighted for its potential benefits in photovoltaics through increased net photocurrent. Absorption coefficients and reflectivity are also reported, showing promising results in zincblende CdSexTe1−x as indicated by substantial optical absorption throughout all Se concentrations. Lastly, we identify the presence of short-range order in CdSexTe1−x characterized by clustering among like atoms in order to minimize strain. The degree of clustering, which may be tuned by temperature, also controls the magnitude of the band gap. Therefore, we propose both composition and short-range order as effective tools to be utilized in the design and synthesis of improved solar cell absorber layer materials.

Predicting magnetization of ferromagnetic binary Fe alloys from chemical short range order

Among the ferromagnetic binary alloys, body centered cubic (bcc) Fe-Co is the one showing the highest magnetization. It is known experimentally that ordered Fe-Co structures show a larger magnetization than the random solid solutions with the same Co content. In this work, based on density functional theory (DFT) studies, we aim at a quantitative prediction of this feature, and point out the role of the orbital magnetic moments. Then, we introduce a DFT-based analytical model correlating local magnetic moments and chemical compositions for Co concentrations ranging from 0 to 70 at.%. It is also extended to predict the global magnetization of both ordered and disordered structures at given concentration and chemical short range orders. The latter model is particularly useful for interpreting experimental data. Based on these models, we note that the local magnetic moment of a Fe atom is mainly dictated by the Co concentration in its first two neighbor shells. The detailed local arrangement of the Co atoms has a minor effect. These simple models can fully reproduce the difference in magnetization between the ordered and disordered Fe-Co alloys between 30% and 70% Co, in good agreement with experimental data. Finally, we show that a similar model can be established for another bcc binary Fe alloy, the Fe-Ni, also presenting ferromagnetic interactions between atoms.

An optimized random structures generator governed by chemical short-range order for multi-component solid solutions

Multi-component random solid solutions have caused much interest for research because they often exhibit excellent mechanical properties. Many computational efforts have been made through various length scales, by first principles, molecular dynamics or finite elements. Here, a quasi-simulated-annealing (Markov Chain Monte Carlo algorithm) with prior burn-in procedure using chemical short-range order parameters as input values is developed for searching the global optimal distribution of species with regard to the phase stability of face-centered cubic CoCrNi alloys. The results show that this new approach could effectively generate the stable structures with a desired concentration and moreover, the stacking fault energies of these configurations have less uncertainties statistically.

Kinetically driven ordering in phase separating alloys

It is shown that in substitutional alloys, peculiar ordered patterns can result from neighborhood-dependent diffusion activation barriers even when there are no metastable ordered phases. Lattice gases with pure phase separation character are shown to exhibit transient ordered structures that can be retained almost indefinitely, although these structures are not at thermodynamic equilibrium. It is shown that such structures can come about relatively easily by quenching from the high-temperature configurationally random solid solution.

Strengthening in multi-principal element alloys with local-chemical-order roughened dislocation pathways

High-entropy and medium-entropy alloys are presumed to have a configurational entropy as high as that of an ideally mixed solid solution (SS) of multiple elements in near-equal proportions. However, enthalpic interactions inevitably render such chemically disordered SSs rare and metastable, except at very high temperatures. Here we highlight the wide variety of local chemical ordering (LCO) that sets these concentrated SSs apart from traditional solvent-solute ones. Using atomistic simulations, we reveal that the LCO of the multi-principal-element NiCoCr SS changes with alloy processing conditions, producing a wide range of generalized planar fault energies. We show that the LCO heightens the ruggedness of the energy landscape and raises activation barriers governing dislocation activities. This influences the selection of dislocation pathways in slip, faulting, and twinning, and increases the lattice friction to dislocation motion via a nanoscale segment detrapping mechanism. In contrast, severe plastic deformation reduces the LCO towards random SS.

Phase diagrams and mechanical properties of TiC-SiC solid solutions from first-principles

First-principles calculations were carried out in order to investigate the stability and properties of the random Ti1-xSixC solid solutions (alloys) with both the B1 and B3 structures. Lattice parameter, total energy, formation energy, phonon spectra, and elastic properties were studied as functions of composition. The phase diagram, in particular, binodal and spinodal curves were calculated. It was established that at 0 K the B1 alloys are energetically favorable at 0 ≤ x < 0.5, while the B3 alloys are favorable at 0.5 ≤ x ≤ 1.0. It was found that the contribution to the Gibbs free energy coming from the lattice vibrations strongly reduces the critical temperature of stabilization of the solid solutions. Calculated elastic moduli, Debye temperature, and Vickers hardness do not point to any strength enhancement of the alloys compared to TiC and SiC. Analysis of the spatial distribution of the Young and shear moduli shows that the B3 alloys exhibit much more spatial anisotropy of the elastic moduli than the B1 alloys.

Short-range ordering in a commercial Ni-Cr-Al-Fe precision resistance alloy

Short-range order has been thought to be the primary reason of the abnormal increase of electrical resistivity after aging treatment in the Ni-20Cr based precision resistance alloys. However, there is no report on the microstructures of this alloy, to say nothing of the lattice type of the short-range order. In this work, the microstructure of a commercial Ni-20Cr-3.2Al-2.5Fe precision resistance alloy was investigated using X-ray diffraction, transmission electron microscopy and atom probe tomography techniques. A random solid solution was initially formed after quenching treatment. Short-range order with the L12-type structure, which was formed due to aging treatment, resulted in a decrease in lattice parameter and an increase in electrical resistivity. The short-range order microdomains distributed in a random matrix coherently with the same composition.

A first-principles study of the stability and mechanical properties of ternary transition metal carbide alloys

First-principles calculations were carried out to investigate electronic structure, phase stability, elastic properties, Debye temperature, and hardness of the TiC–HfC and TiC–TaC random solid solutions as functions of composition. For TiC–HfC, significant miscibility gaps with consolute temperatures about 1975 K are revealed in the binodal and spinodal curves. The negative deviation of the elastic moduli and hardness from linearity are obtained for TiC–HfC, whereas, for TiC–TaC, these characteristics are above their linear interpolation between the end members. Concentration dependences of the Debye temperature for both systems have a negative curvature. To clarify a possible mechanism of stabilization or destabilization of these solid solutions and other similar carbide systems, mixing energies of the M11−xM2xC alloys, where M1 and M2 are the transition metals of the IV, V, and partially VI groups, were calculated. It is found that the behavior of mixing energies for the M1C–M2C alloys with M1 and M2 of different groups depending on composition is determined by the difference between cell volumes of the end members, ΔVC, the degree of occupancy of the metal band, and the shape of the density of states in the metal band region. Values of ΔVC mainly are responsible for positive mixing energies of the alloys with valence electron concentrations (VECs) equal to 8 and 9 for which occupancy of the metal band weakly changes with composition. The maximum hardness of the solid solutions for which VECs of the end members are different is predicted to be reached for the compositions with VECs = 8.5–8.75.

Energetic Stability of Vacancy‐Carbon Clusters in Solid Solution Alloys: The Fe‐Cr‐C Case

In carbon‐containing alloys, the energetic stability of vacancy‐carbon (v‐C) clusters plays a key role in the post‐irradiation damage recovery. So far, all interpretations of post‐irradiation annealing experiments are based on the theoretical knowledge established for the iron‐carbon alloy. For the interpretation of high‐Cr steels, the stability of v‐C clusters in the presence of a solid solution background is essential. Therefore, a simple and efficient model to estimate the stability of v‐C clusters in random solid solution alloys is proposed. The model is parameterized for the Fe‐Cr‐C alloy using density functional theory. The results are discussed accounting for available experimental evidences.

High-cycle fatigue and tensile deformation behaviors of coarse-grained equiatomic CoCrFeMnNi high entropy alloy and unexpected hardening behavior during cyclic loading

High entropy alloy (HEA), a new class of materials, has received attention as a substance that can potentially replace conventional alloys. Equiatomic CoCrFeMnNi HEA is an attractive material with excellent strength-ductility combination and corrosion resistance, and it achieves greater performance in low temperatures. This study investigated the HCF and tensile deformation behavior of equiatomic CoCrFeMnNi HEA. In order to suggest the possibility of the material's application in an as-homogenized state, coarse-grained (CG) equiatomic CoCrFeMnNi HEA was prepared. Microstructural observation measured an average grain size of 245.5 μm, and it was confirmed to have a face-centered cubic (FCC) random solid solution. A tensile test confirmed that the yield strength and tensile strength are 293.1 MPa and 625.6 MPa, respectively, and change in the work hardening rate according to deformation twin (DT) evolution during tensile deformation was observed. A high-cycle fatigue results shows fatigue strength of 280 MPa, which is close to its yield strength, and this confirmed that the material has outstanding high-cycle fatigue properties considering its yield strength. DT is uniquely formed at cycle loading with a stress level lower than the critical twinning stress (σT), and this can improve yield strength by approximately 95% and tensile strength by approximately 17%. Based on the above findings, this study discussed the role of DTs which affect the high-cycle fatigue and deformation behavior of coarse-grained HEA.

Direct observation of dendrite fragmentation in the solidification of undercooled melts

The fragmentation of dendrites immediately following the recalescence phase of growth during the solidification of undercooled melts has been invoked to explain various rapid solidification microstructures. Despite this, little direct evidence of such a fragmentation process usually survives in the as-solidified material. We report on the rapid solidification of the single phase, congruently melting intermetallic β-Ni3Ge. During equilibrium solidification this material solidifies to the chemically ordered L12 crystal structure. Conversely, during rapid solidification, disorder trapping results in solidification to a random fcc solid solution, thereby providing a means to distinguish the rapidly solidified structures. We present results which show a range of microstructures in which the dendrite fragmentation process has been captured in progress. Results from EBSD Euler mapping reveal that dendrite fragmentation is a potential, but not particularly efficient, route to grain refinement.

Discovery of a FeCoNiPdCu High‐Entropy Alloy with Excellent Magnetic Softness

The authors report on the discovery of a magnetically soft high‐entropy alloy of composition FeCoNiPdCu, which performs comparably to the best commercial soft magnets for static and low‐frequency applications. Properly heat‐treated FeCoNiPdCu develops nanostructure that can be viewed as a two‐phase bulk nanocomposite of randomly intermixed FeCoNi magnetic domains and PdCu nonmagnetic “spacers”, both of 2–5 nm cross dimensions. Due to the nanometric size, the FeCoNi domains are magnetically single‐domain particles, and since the particles are exchange‐coupled across the boundaries, exchange averaging of magnetic anisotropy takes place, resulting in an almost vanishing coercive field and excellent magnetic softness. The formation of a two‐phase nanostructure favorable for the exchange averaging of magnetic anisotropy is a consequence of specific values of the binary mixing enthalpies for the chosen elements. Though high‐entropy alloys are generally considered to be random solid solutions of multiple elements on a topologically ordered crystal lattice, clustering of the atoms into preferential chemical environments on a nanoscale essentially determines their magnetic properties. Experimentally, the magnetic properties of the FeCoNiPdCu high‐entropy alloy are compared to the commercial, magnetically soft non‐oriented silicon electrical steel.

Chemical Short Range Order Strengthening in a Model FCC High Entropy Alloy

In order to understand the role of chemical short-range order on deformation mechanism in FCC compositionally complex alloys, a random model alloy (Co30-Fe16.67-Ni36.67-Ti16.67) is annealed at various temperatures using Hybrid Molecular-dynamics/Monte-Carlo simulations. The simulations produce significant chemical short-range order (CSRO) that increases with decreasing annealing temperature. Annealing tends to homogenize regions of high enthalpy due to: (1) chemical species redistributing into more compact configurations, and (2) pairs of atoms forming chemical bonds that lower the overall energy of the system; the composition explored here shows significant amount of ordering in Ti-Fe pairs with respect to random distributions as described by pairwise (EAM) potentials due to Johnson and Zhou. An energy topology approach is used to assess the local strengthening behavior in random solid solutions and annealed systems, where an interesting interplay is observed between misfit components and chemical short-range order affecting the overall critical resolved shear stress. The role of short-range order on the critical yield stress is quantified and compared with current solid solution models. Finally, we propose and validate an extension to the Labusch-Varvenne class of high-concentration solid-solution analytic models that incorporates the effects of chemical short range order.

Effect of aluminum addition on solid solution strengthening in CoCrNi medium-entropy alloy

Solid solution strengthening effect was investigated in AlxCoCrNi medium-entropy alloy (MEA) with different Al content (x = 0–7 at.%). The result of X-ray diffraction indicates that the fully recrystallized phase was a single face-centered cubic (FCC) structure for all compositions. The average grain size regarding a twin boundary as a grain boundary was approximately 16 μm. Tensile testing shows that the yield strength increases with the addition of Al. Here, Labusch's approach to random solid solution alloys is applied to confirm the strengthening mechanism in AlxCoCrNi MEA by using lattice constants and elastic moduli. The experimental results were found to be in good agreement with quantitative predictions of the solid solution contribution in AlxCoCrNi MEA.

Synergistic effects on thermoelectric properties of Sn0.5Ge0.4875Te with Pb alloying

In this contribution, we report a significant enhancement of thermoelectric properties of Sn0.5Ge0.4875Te alloy through three synergistic effects with Pb alloying: the significant reduction of the lattice thermal conductivity of the high entropy alloy (HEA) main phase (Sn, Ge, Pb)Te and lamellae microstructures produced from the spinodal decomposition in the alloy; the significant reduction of the carrier thermal conductivity and the significant increases of the Seebeck coefficient by decreasing the carrier concentration with Pb alloying. The high entropy random solid solution (Sn, Ge, Pb)Te single phase with NaCl-type structure forms in the alloys (Sn0.5Ge0.4875)1-xPbxTe with x = 0.2 and 0.8, while the minor cubic GeTe-based phase arises in the (Sn, Ge, Pb)Te matrix in the alloys with x = 0.33, 0.5 and 0.6, forming the lamellae microstructures. All alloys keep p-type conduction. The hole concentration decreases, but carrier mobility increases and carrier thermal conductivity decreases with Pb content x increasing. The thermoelectric properties of the alloys can be enhanced significantly with synergistic effects with proper Pb alloying. We eventually obtained the maximum figure of merit ZT of 1.61 at 773 K in the alloy (Sn0.5Ge0.4875)1-xPbx with x = 0.5.

Effect of atomic order/disorder on Cr segregation in Ni-Fe alloys

Recent irradiation experiments on concentrated random solid solution alloys (CSAs) show that some CSAs can undergo disorder-to-order transition, i.e., the atoms that are initially randomly distributed on a face centered cubic crystal lattice undergo ordering (e.g., L10 or L12) due to irradiation. In this work, we elucidate that the atomic structure could affect the segregation properties of grain boundaries. While working on Ni and Ni-Fe alloys, from static atomistic simulations on 138 grain boundaries, we show that despite identical alloy composition, Cr segregation is higher in the disordered structures compared to ordered structures in both Ni0.50Fe0.50 and Ni0.75Fe0.25 systems. We also show that grain boundary (GB) energy could act as a descriptor for impurity segregation. We illustrate that there is a direct correlation between Cr segregation and grain boundary energy, i.e., segregation increases with the increase in the GB energy. Such correlation is observed in pure Ni and in the Ni-Fe alloys studied in this work.

Entropy contributions to phase stability in binary random solid solutions

High entropy alloys contain multiple elements in large proportions that make them prone to phase separation. These alloys generally have shallow enthalpy of mixing which makes the entropy contributions of similar magnitude. As a result, the phase stability of these alloys is equally dependent on enthalpy and entropy of mixing and understanding the individual contribution of thermodynamic properties is critical. In the overall vision of designing high entropy alloys, in this work, using density functional theory calculations, we elucidate the contributions of various entropies, i.e., vibrational, electronic and configurational towards the phase stability of binary alloys. We show that the contribution of electronic entropy is very small compared to the vibrational and configurational entropies, and does not play a significant role in the phase stability of alloys. The configurational and vibrational entropies can either destabilize or can collectively contribute to stabilize the solid solutions. As a result, even those systems that have negative mixing enthalpy can show phase instability, revealed as a miscibility gap; conversely, systems with positive mixing enthalpy can be phase stable due to entropic contributions. We suggest that including entropic contributions are critical in the development of theoretical framework for the computational prediction of stable, single-phase high entropy alloys that have shallow mixing enthalpies, unlike ordered intermetallics.

Stability of SnSe1−xSx solid solutions revealed by first-principles cluster expansion

The configurational thermodynamics of a pseudo-binary alloy SnSe1−xSx in the Pnma phase is studied using first-principles cluster-expansion method in combination with canonical Monte Carlo simulations. We find that, despite the alloy having a tendency toward a phase decomposition into SnSe and SnS at 0 K, the two constituent binaries readily mix with each other to form random SnSe1−xSx solid solutions even at a temperature below room temperature. The obtained isostructural phase diagram of SnSe1−xSx reveals that the alloy is thermodynamically stable as a single-phase random solid solution over a whole composition range above 200 K. These findings provide a fundamental understanding on the alloying behavior of SnSe1−xSx and bring clarity to the debated clustering tendency in this alloy system.

Pyramidal II to basal transformation of ⟨c + a⟩ edge dislocations in Mg-Y alloys

Transitions of pyramidal ⟨c + a⟩ dislocations to sessile structures contribute to poor ductility in pure Mg. Mg-3 wt% Rare Earth (RE) alloys have good ductility, possibly due to ⟨c + a⟩ dislocation stabilization upon addition of RE solutes. Here, ⟨c + a⟩ stability is investigated in a model Mg-3 at.%Y random solid solution alloy using molecular dynamics simulations. Favorable fluctuations of Y solutes lower all dislocation energies and have no appreciable effects on the transition mechanism, energy barrier, or time. Enhanced ⟨c + a⟩ activity and improved ductility in Mg-3 wt%RE alloys are thus not likely associated with solute-stabilization of pyramidal ⟨c + a⟩ dislocations.