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Homogeneous crystal nucleation is prone to formation of defects and often experiences heterogeneities, the inferences of which are crucial in processing crystalline materials and controlling their physical properties. It has been debated in literature whether the associated heterogeneities are an integral part of the homogenous nucleation. In this study by integrating a probabilistic approach with large-scale molecular dynamics simulations based on the most advanced high-temperature interatomic potentials, we attempt to address the ambiguity over the sources and mechanisms of heterogeneities in homogenous nucleation during solidification of pure melts. Different classes of structured metals are investigated for this purpose, including face-centered cubic aluminum, body-centered cubic iron, and hexagonal close-packed magnesium. The results reveal, regardless of the element type or the solidified crystal structure, that the densification process of liquid metals is accompanied by short-range orderings of atoms prior to the formation of crystals, controlling the heterogeneities during homogenous nucleation.

In this work, the capability of Si2BN monolayers (Si2BN-MLs) to sense CO2 and COCl2 molecules was investigated by analyzing the structural, electronic, mechanical and gas sensing properties of defect-free and defective Si2BN-ML structures.

Elastocaloric cooling, which exploits superelastic transitions of shape memory alloys to pump heat, has recently emerged as a frontrunner in alternative cooling technologies. Despite its intrinsic high efficiency, elastocaloric materials exhibit hysteresis associated with input work, a common attribute of caloric cooling materials. In this study, the authors created a Ni-Ti-based elastocaloric material by additive manufacturing nanocomposite materials using a laser directed-energy- deposition system. The material exhibited exceptional stability and unusual operational efficiency derived from the unique and intricate nanocomposite structures made by additive manufacturing. This demonstration shows the potential for using additive manufacturing to optimize caloric cooling by providing a highly desirable topology flexibility into materials components that serve as both refrigerants and heat exchangers.

Effects of atomic defects on electronic properties, mechanical stability, and elemental diffusivity in CuS (or covellite) were investigated by first-principles calculations. The metallic CuS shows higher structural and mechanical stabilities compared to CuxS phases. Strong covalent bonds in pristine CuS restrict the self-diffusion of Cu and S atoms; however, different band orientations in presence of atomic vacancies result in a high degree of self-diffusion. Specially, sulfur vacancies play a prominent role in increasing the diffusion of both Cu and S. Calculated self-diffusion coefficients between 100 °C and 150 °C are of the order of 10−6 cm2/s, comparable to experiments.

Most known two-dimensional materials lack a suitable wide-bandgap, and hydrogenation can be effectively utilized to tune the bandgap of some 2D materials. By employing density functional theory calculations, we investigate the effect of hydrogenation on the electronic and optical properties of a recently reported anisotropic penta-Pt2N4 monolayer. The results show that penta-Pt2N4 is thermally and mechanically stable after hydrogenation and also possesses anisotropic Young's modulus and Poisson's ratio. The electronic property analysis using the hybrid functional reveals that penta-Pt2N4 exhibits a bandgap of 1.10 eV, and the hydrogenation significantly enhances the bandgap to 2.70 eV. Furthermore, the hydrogenated Pt2N4 displays a strong optical absorption of up to 6.45 × 105 cm-1 in the ultraviolet region, and low absorption and low reflectivity in the visible region. Our results strongly suggest that the hydrogenated Pt2N4 has tunable electronic and optical properties for applications as a hole-transport material layer in solar cells in the visible region, and as an ultraviolet detector in the ultraviolet region of the electromagnetic spectrum.

We propose an elasto-plastic phase-field model (PFM) to conduct the first microscopic computational study of shape memory effect (SME), pseudoelasticity, stress assisted two-way memory effect (SATWME), and thermomechanical training of CuAlBe shape memory alloy (SMA). This non-isothermal PFM model considers the effects of temperature dependent properties, latent heat, grain boundaries, and asymmetric transformation and plasticity. PFM simulations demonstrate the capacity of our model to capture the thermomechanical and purely mechanical shape recovery in the SMA. When considering transforming grain boundaries, grain refinement generates a higher transformation stress, a steeper transformation hardening, and smaller hysteresis loops for both SME and pseudoelasticity. The results also point out slightly higher transformation stress when geometrical grain boundaries are used. The simulations of SATWME highlight an augmentation of the residual martensite as the hold stress increases, which is consistent with experimental observations. This is the first PFM that can mimic the thermal training within several SATWME cycles, showing an asymptotic increase of plastic strain and the related retained transformation strain until the fourth cycle, and their stabilization thereafter. The activation of plasticity occurs always after initiation of phase transformation. Although plasticity results in more stress relaxation, it deteriorates the shape recovery for both SME and pseudoelasticity by hindering the reverse transformation. Comparison between tension and compression demonstrates the capacity of this PFM to account for, for the first time, the nonsymmetrical transformation and plastic responses of SMAs.

In this work, the capability of Si

We proposed a boron–phosphorus monolayer (BP-ML) and investigated its gas sensing properties by density functional theory including the van der Waals dispersion correction term. Electronic property...

We propose an elastoplastic phase-field (PF) model to investigate the mechanics of tetragonal-to-monoclinic phase transformation (TMPT) and elastoplastic deformation of polycrystalline yttria-stabilized tetragonal zirconia (YSTZ). A Landau polynomial with non-vanishing chemical energy at the equilibrium temperature is introduced to account for the actual formation energies of the phases. The effects of different grain orientations, latent heat, and temperature on TMPT and deformation mechanisms are considered. The suppressive transformation effects of the grain boundaries (GBs) is modeled using an inhomogeneous kinetic coefficient in the bulk and GBs. The simulation results for single crystals demonstrate the capability of the model to reproduce the orientation-dependent compressive deformation of YSTZ similar to atomistic simulations and micropillar experiments. The single crystal with [100] crystallographic orientation along the loading direction (SC[100]) displays both TMPT and plasticity, SC[101] experiences only phase transformation, while SC[001] undergoes only plastic yielding. The TMPT induced by compressive loading exhibits shape memory effect (SME) below the equilibrium transformation temperature and pseudoelasticity above it, while the critical transformation stress increases with increasing loading temperature. The irrecoverable plastic strain is found to trap a part of the monoclinic phase, which prevents a complete reverse transformation. The polycrystalline cases also display SME and PE at low and high temperatures, respectively. Due to the orientation differences between grains and the stress concentrations at geometric nonlinearities, plastic deformation occurs in polycrystalline YSTZ for an applied load less than the yield stress. The results suggest a possible limitation of plasticity and an improvement of the shape recovery of YSTZ if one can control the orientation of the grains and/or increase the density of stacking faults at the GBs during material processing.

First-principles density functional theory (DFT) computations were adopted to assess the potential application of a boron carbide (BC3) monolayer with point and topological defects as an anode mate...

In annealing of yttria-stabilized tetragonal zirconia (YSTZ), {011}-specific twins and sub-surface defects are often observed, however their effects on the martensitic phase transformation and deformation behavior of YSTZ have never been investigated. In this work, the roles of twin boundaries (TBs) and pre-existing defects in determining the mechanical properties and subsequent deformation mechanisms of YSTZ nanopillars are studied. Using large-scale molecular dynamics simulations, we show that Young’s modulus and strength of YSTZ decrease with the increase of TB density, but the ductility of YSTZ pillars increases. Phase transformation behavior is found to be correlated to TB density. The sensitivity of mechanical responses of twinned structures to pre-existing defects is also studied. A competitive mechanism between TB-induced phase transformation and void-induced phase transformation is observed. When the diameter of a pre-existing void is smaller than a critical value, only TB-induced phase transformation occurs, which leads to void-insensitive mechanical properties.

A phase-field approach is used to study the evolution of edge dislocations in single crystals at the nanoscale. The characteristics of an advanced phase-field approach for dislocation evolution are investigated, and some advancements are made to make it more accurate in predicting the dislocation evolution. To verify the model and numerical procedure, the height of a slip system, the Burgers vector, and the distance between the cores of dislocations are calculated, which show a very good agreement with those of the existing theoretical solutions. The analytical and numerical solutions for the equilibrium shear stress versus order parameter are obtained, and in contrast to the previous models, the current model represents a physical model for the dislocation growth. Different methods are investigated to prevent dislocation widening, revealing that the periodic step-wise function of crystalline energy coefficient performs better than the periodic step-wise function of the Burgers vector and can keep dislocations inside their physical height. Several functions for the coefficient of normal gradient energy are investigated to prevent dislocation localization inside the dislocation band. The sample size effect on the dislocation evolution is also studied which reveals a non-linear variation of the number of dislocations inside the slip system versus the sample size. The presented model and results are useful for understanding and predicting the dislocation evolution and its interaction with other phenomena such as phase transformation.

Effects of crystal orientation and pre-existing defects on tensile properties of yttria-stabilized tetragonal zirconia (YSTZ) thin films are investigated by large-scale molecular dynamics simulations. The tensile strength and strain show clear orientation dependence. Under uniaxial tensile loading, the YSTZ thin films are found to fail through fracture along {110} cleavage planes. dislocations are observed to form in the [100]-, [010]- and [001]-oriented models. Besides, the {110} cleavage planes are noticed to be rough, twisted and tangled around the center of the [100]- and [001]-oriented films, which is responsible for large strains at tensile strength. The simulated Young’s modulus and tensile strength are comparable to the experimental and first principle values. Overall, pre-existing defects could change the fracture pathway and negatively affect the tensile strength and strain in most of the studied cases.

We studied the oxidation behavior of face-centered cubic Al0.3CoCrCuFeNi high entropy alloy through first-principles calculations. Three surface orientations were chosen for oxidation, and all the possible combinations of atomic positions at these surfaces were considered. The adsorption energy of oxygen adhesion to the studied surfaces was the lowest for the sites with more neighboring Cr atoms, and the second most favorite site for oxygen adsorption had more neighboring Al atoms. On the other hand, the calculated cohesive energies of oxides indicated that Al formed the most stable oxide among other alloying elements, and Cr formed the second most stable oxide.

Specimen- and grain-size effects on nanoscale plastic deformation mechanisms and mechanical properties of polycrystalline yttria-stabilized tetragonal zirconia (YSTZ) nanopillars are studied by molecular dynamics simulations. Through uniaxial compression of YSTZ columnar nanopillars, intergranular and transgranular deformation mechanisms are investigated. Cooperative intergranular deformations including grain boundary sliding and migration, grain rotation, and amorphous phase formation at grain boundaries are revealed. Results also reveal formation of partial dislocations, which act as splitters of large grains and play a significant role in facilitating the rotation of grains, and consequently promote amorphous-to-crystalline phase transition in-between neighboring grains. An increase in free surface-to-volume ratio is found to be responsible for specimen size-induced softening phenomenon, where a decrease in Young's modulus and strength is observed when the specimen width decreases from 30 nm to 10 nm. Also, a decrease in Young's modulus and strength is revealed with the decrease of average grain size from 15 nm to 5 nm. Grain boundary density is identified to be responsible for the observed grain size-induced softening behavior in polycrystalline YSTZ nanopillars. A transition in dominant deformation mechanism is observed from amorphous phase formation at grain boundaries to a competition between intergranular grain boundary sliding and transgranular phase transformation. Furthermore, an inverse Hall-Petch relationship is revealed describing the correlation between grain size and strength for polycrystalline YSTZ nanopillars with grain sizes below 15 nm.

The effects of kinetic and capillary anisotropies on crystal morphology and growth rate during solidification of titanium are studied using atomistically-informed phase field simulations. Molecular dynamics (MD) is employed to calculate the anisotropic kinetic coefficient and crystal-melt interface free energy using the free solidification and capillary methods. The phase field simulation results for solidification velocity and interface temperature are in quantitative good agreement with experimental and analytical data for undercoolings below 150 K. As the role of interface kinetic effects increases with undercooling the use of a modified phase field model allowed the extension of its quantitative prediction capability to higher undercoolings. In addition, the effect of MD calculated kinetic and capillary anisotropy parameters on dendrite shape and tip and solidification velocity was investigated.

Formation of solidification defects and their evolution in uniaxial tensile deformation of solidified polycrystalline aluminum (Al) were investigated by molecular dynamics (MD) simulations. First, solidification process was simulated both isothermally and with different quench rates. At the initial stages of nucleation, coherent twin boundaries and/or fivefold twins formed depending on the quench rate or the undercooling temperature. The solidified polycrystalline Al consisted of randomly distributed grains, twin boundaries, and vacancies. Evolution of nanostructures and defects in uniaxial tensile deformation of solidified Al under different temperatures and strain rates were studied. Void formation at grain boundaries and detwinning of preexisting solidification twins and deformation twins were observed during the uniaxial deformation. It was also found that the temperature of deformation has a stronger effect than the applied strain rate on the strength of solidified samples. For solidified cases with grain sizes lower than 10 nm, the yield strength and Young’s modulus increased with increasing grain size, indicating an inverse Hall-Petch relationship. Similar to experimental data, MD simulations showed a higher yield strength for single crystal Al and a large plastic deformation for polycrystalline Al.

Shape memory ceramics are a unique family of shape memory materials with a wide variety of applications, such as ultra-high energy dissipation and high-temperature actuation. Along with significant progress in the experimental study of zirconia-based shape memory ceramics in recent years, computational simulations have exhibited powerful capabilities in revealing nano/microstructure-dependent deformation and failure mechanisms in these materials. In this work, we review the recent progress in understanding the phase transformation behavior in shape memory ceramics, focusing on computational modeling of zirconia-based ceramics. Electronic structure calculations have provided new data on the phase stability and surface properties of shape memory ceramics. At the nanometer length scale, molecular dynamics simulations have captured fundamental information about martensitic phase transformation and the effects of grain boundaries and defects on mechanical response of bi- and polycrystalline zirconia-based ceramics. At the micrometer length scale, advanced phase-field models have shown the ability to predict morphological evolution of microstructures and the corresponding mechanical responses in good agreement with experimental observations. Despite the recent multiscale computational advancements, further developments are required to establish processing-structure-property-performance relations that will lead to reliable and practical strategies for designing zirconia-based (or other) ceramics with improved and sustained shape memory responses. This article critically reviews current computational modeling techniques and provides an outlook for the study and design of the next generation of shape memory ceramics.

Most known two-dimensional materials lack a suitable wide-bandgap, and hydrogenation can be effectively utilized to tune the bandgap of some 2D materials. By employing density functional theory calculations, we investigate the effect of hydrogenation on the electronic and optical properties of a recently reported anisotropic penta-Pt2N4 monolayer. The results show that penta-Pt2N4 is thermally and mechanically stable after hydrogenation and also possesses anisotropic Young's modulus and Poisson's ratio. The electronic property analysis using the hybrid functional reveals that penta-Pt2N4 exhibits a bandgap of 1.10 eV, and the hydrogenation significantly enhances the bandgap to 2.70 eV. Furthermore, the hydrogenated Pt2N4 displays a strong optical absorption of up to 6.45 × 105 cm-1 in the ultraviolet region, and low absorption and low reflectivity in the visible region. Our results strongly suggest that the hydrogenated Pt2N4 has tunable electronic and optical properties for applications as a hole-transport material layer in solar cells in the visible region, and as an ultraviolet detector in the ultraviolet region of the electromagnetic spectrum.

The effective fracture toughness (EFT) of ZrB2-C ceramics with different engineered microarchitectures was numerically evaluated by phase-field modeling. To verify the model, fibrous monoliths (elongated hexagonal ZrB2-rich cells in a continuous C-rich matrix) with different volume fractions of a C-rich phase were considered. Architectures containing 10 and 30 vol% of C-rich phase showed EFT values about 42% more than that of pure ZrB2. Increasing the C-rich phase to 50 vol%, dropped toughness significantly, which is in agreement with the experimental results. Replacing hexagonal cells with cylindrical, triangular, or square cells of the same cross-sectional area changed the toughening mechanism and EFT. The orientation of the interface between the soft and hard phases with respect to the crack orientation also affected the energy required for crack propagation, and in some cases resulted in a higher EFT (even up to 70% of pure ZrB2 fracture toughness) either by suppressing uniform crack propagation or making crack cranking. Results not only show that the model can predict fracture toughness but also provide insight to improve toughness by engineering different microarchitectures.

Oxide growth and the induced stresses in the high-temperature oxidation of steel were studied by a multiphase field model. The model incorporates both chemical and elastic energy to capture the coupled oxide kinetics and generated stresses. Oxidation of a flat surface and a sharp corner are considered at two high temperatures of 850 &deg, C and 1180 &deg, C to investigate the effects of geometry and temperature elevation on the shape evolution of oxides and the induced stresses. Results show that the model is capable of capturing the oxide thickness and its outward growth, comparable to the experiments. In addition, it was shown that there is an interaction between the evolution of oxide and the generated stresses, and the oxide layer evolves to reduce stress concentrations by rounding the sharp corners in the geometry. Increasing the temperature may increase or decrease the stress levels depending on the contribution of eigen strain in the generated elastic strain energy during oxidation.

We investigate the evolution of solid-liquid interfaces in Mg and Mg–9 at % Al during directional solidification by molecular dynamics (MD) simulations. At the initial stages of solidification, several solidification defects such as twins, stacking faults, and grain boundaries form, and at the final stages of solidification no new defects or grain boundaries form. The directional solidification in Mg–Al contains a considerable amount of heterogeneity due to formation of several Mg17Al12 precipitates.

Second nearest neighbor modified embedded-atom method (2NN-MEAM) interatomic potentials are developed for binary aluminum (Al) alloys applicable from room temperature to the melting point. The binary alloys studied in this work are Al-Cu, Al-Fe and Al-Ni. Sensitivity and uncertainty analyses are performed on potential parameters based on the perturbation approach. The outcome of the sensitivity analysis shows that some of the MEAM parameters interdependently influence all MEAM model outputs, allowing for the definition of an ordered calibration procedure to target specific MEAM outputs. Using these 2NN-MEAM interatomic potentials, molecular dynamics (MD) simulations are performed to calculate low and high-temperature properties, such as the formation energies of stable phases and unstable intermetallics, lattice parameters, elastic constants, thermal expansion coefficients, enthalpy of formation of solids, liquid mixing enthalpy, and liquidus temperatures at a wide range of compositions. The computed data are compared with the available first principle calculations and experimental data, showing high accuracy of the 2NN-MEAM interatomic potentials. In addition, the liquidus temperature of the Al binary alloys is compared to the phase diagrams determined by the CALPHAD method.

A quantitative phase-field model based on the regularized formulation of Griffith’s theory is presented for crack propagation in homogenous and heterogeneous brittle materials. This model utilizes correction parameters in the total free energy functional and mechanical equilibrium equation in the diffusive crack area to ensure that the maximum stress in front of the crack tip is equal to the stress predicted by classical fracture mechanics. Also, unlike other phase-field models, the effect of material strength on crack nucleation and propagation was considered independent of the regularization parameter. The accuracy of the model was benchmarked in two ways. First, the stress and strain fields around the crack tip in single-phase ZrB2 were compared with the analytical solutions in classical linear elastic fracture mechanics. Second, the crack path and force–displacement responses were examined against experimental results for concrete in the form of fracture of L-shaped plate and wedge splitting tests. To demonstrate the capability of the model in multi-phase materials, crack propagation was simulated for laminates composed of alternating layers of ZrB2 and carbon. The results showed that the proposed modifications in the phase-field model were necessary to predict crack deflection along carbon layers similar to the experimental observations.

Synthesis and processing of two- or three-dimensional covalent organic frameworks (COFs) have been limited by solvent intractability and sluggish condensation kinetics. Here, we report on the electrochemical deposition of poly(5,10,15,20-tetrakis(4-aminophenyl)porphyrin)-covalent organic frameworks (POR-COFs) via formation of phenazine linkages. By adjusting the synthetic parameters, we demonstrate the rapid and bottom-up synthesis of COF dendrites. Both experiment and density functional theory underline the prominent role of pyridine, not only as a polymerization promoter but as a stabilizing sublattice, cocrystallizing with the framework. The crucial role of pyridine in dictating the structural properties of such a cocrystal (Py-POR-COF) is discussed. Also, a structure-to-function relationship for this class of materials, governing their electrocatalytic activity for the oxygen reduction reaction in alkaline media, is reported.

Designing high-efficiency electrocatalysts for water oxidation has become an increasingly important concept in the catalysis community due to its implications in clean energy generation and storage. In this respect transition-metal-doped mixed-metal selenides incorporating earth-abundant elements such as Ni and Fe have attracted attention due to their unexpectedly high electrocatalytic activity toward the oxygen evolution reaction (OER) with low overpotential in alkaline medium. In this article, quaternary mixed-metal selenide compositions incorporating Ni-Fe-Co were investigated through combinatorial electrodeposition by exploring the ternary phase diagram of Ni-Fe-Co systems. The OER electrocatalytic activity of the resultant quaternary and ternary mixed-metal selenide compositions was measured in order to systematically investigate the trend of catalytic activity as a function of catalyst composition. Accordingly, the composition(s) exhibiting the best catalytic efficiency for the quaternary Fe-Co-Ni m...

Effect of the addition of carbon on phase formations in AlxCoCrFeNiCu (x = 0.3, 1.5, 2.8) high-entropy alloys (HEAs) was studied. Free diffusion of carbon from graphite crucible resulted in the partitioning of the entire Cr from the matrix and the formation of the (Cr,Fe)23C6 phase in all HEAs. No other metal-carbide phase was detected. The formation of (Cr,Fe)23C6 enhanced the overall hardness of the HEAs. By increasing the amount of Al, the Cr amount decreased resulting in the reduction of carbon diffusion and volume fraction of the (Cr,Fe)23C6 phase in HEAs. The hardness of matrix phases and the overall hardness of HEAs increased with an increase in the amount of Al. Impact statement The detailed phase analysis reveals that C addition to AlxCoCrFeNiCu HEAs leads to the formation of the (Cr,Fe)23C6 phase. The overall hardness can be controlled by the amount of C and/or Al.

The compressive mechanical responses of silicon nanoparticles with respect to crystallographic orientations are investigated by atomistic simulations. Superelastic and abrupt hardening-stiffening behaviors are revealed in [110]-, [111]- and [112]-oriented nanoparticles. The obtained hardness values of these particles are in good agreement with the experimental results. In particular, [111]-oriented particle is extremely hard since its hardness (∼33.7 GPa) is almost three times greater than that of the bulk silicon (∼12 GPa). To understand the underlying deformation mechanisms, metastable phase transformation is detected in these particles. Deformation twinning of the metastable phase accounts for the early hardening-stiffening behavior observed in [110]-oriented particle. The twin phase then coalescences and undergoes compression to resist further deformation, and leads to the subsequent re-hardening and re-stiffening events. The same metastable phase is also detected to form in [111]- and [112]-oriented particles. The compression of such metastable phase is responsible for their hardening-stiffening behavior. In contrast, the crystal lattice of diamond cubic silicon is merely elastically deformed when compressing along [100] direction. Throughout the simulations, no perfect tetragonal β-tin silicon phase formed due to the deconfinement status of nanoparticle comparing to the bulk silicon. A size effect on hardness of silicon nanoparticles, i.e., “smaller is harder”, is also revealed.

Designing efficient electrocatalysts has been one of the primary goals for water electrolysis, which is one of the most promising routes towards sustainable energy generation from renewable sources. In this article, we have tried to expand the family of transition metal chalcogenide based highly efficient OER electrocatalysts by investigating nickel telluride, Ni3Te2 as a catalyst for the first time. Interestingly Ni3Te2 electrodeposited on a GC electrode showed very low onset potential and overpotential at 10 mA cm−2 (180 mV), which is the lowest in the series of chalcogenides with similar stoichiometry, Ni3E2 (E = S, Se, Te) as well as Ni-oxides. This observation falls in line with the hypothesis that increasing the covalency around the transition metal center enhances catalytic activity. Such a hypothesis has been previously validated in oxide-based electrocatalysts by creating anion vacancies. However, this is the first instance where this hypothesis has been convincingly validated in the chalcogenide series. The operational stability of the Ni3Te2 electrocatalyst surface during the OER for an extended period of time in alkaline medium was confirmed through surface-sensitive analytical techniques such as XPS, as well as electrochemical methods which showed that the telluride surface did not undergo any corrosion, degradation, or compositional change. More importantly we have compared the catalyst activation step (Ni2+ → Ni3+ oxidation) in the chalcogenide series, through electrochemical cyclic voltammetry studies, and have shown that catalyst activation occurs at lower applied potential as the electronegativity of the anion decreases. From DFT calculations we have also shown that the hydroxyl attachment energy is more favorable on the Ni3Te2 surface compared to the Ni-oxide, confirming the enhanced catalytic activity of the telluride. Ni3Te2 also exhibited efficient HER catalytic activity in alkaline medium making it a very effective bifunctional catalyst for full water splitting with a cell voltage of 1.66 V at 10 mA cm−2. It should be noted here that this is the first report of OER and HER activity in the family of Ni-tellurides.