Chemical Order Research Articles

Microstructure and texture control of Ni-Mn-Ga magnetic shape memory alloys manufactured by laser powder bed fusion

The Laser Powder Bed Fusion (LPBF) process was used to manufacture Ni-Mn-Ga polycrystalline samples. The initial powders with a fine particle size of about 20 µm were firstly prepared by a ball milling from melt-spun ribbons. The microstructure and texture evolution of Ni-Mn-Ga alloy manufactured by LPBF were investigated employing SEM, TEM, and synchrotron radiation diffraction. Using two different scanning strategies, optimization of laser parameters and post-processing heat treatment, a homogeneous microstructure with a strong crystallographic texture was obtained. The localized heating/cooling conditions allow obtaining a layered structure with preferred <100> fiber orientations along the growth direction. The crystal structure and crystallographic texture drastically change when the laser oscillation mode is chosen. The strong crystalline anisotropy and layered-type microstructure have a significant impact on twining flow behavior giving rise to an anisotropic mechanical response. The variant reorientation during mechanical training is realized by the so-called orthogonal shearing. It is also shown that the resulting crystal structure and characteristic transformation temperatures are strongly dependent on chemical composition related to Mn losses and internal stresses along with chemical order creating metastable phases due to the extremely high cooling rate upon the 3D printing. Therefore, all the above parameters are thoroughly monitored during the three-stage fabrication process.

Dichroism of plasmonic chiral nanoalloys by rational design.

Time-dependent density functional theory (TDDFT) simulations are conducted on a series of chiral gold/silver alloy nanowires to explore whether silver doping can produce an enhancement of circular dichroism at the plasmon resonance in these systems, and to identify the quantum-mechanical origin of the observed effects. We find a strong plasmonic dichroism when one or two helixes of gold atoms are substituted by silver in a linear chiral nanotube, whose pure gold counterpart does not display any plasmonic dichroism, and we rationalize this finding in terms of "decoupling" the destructive interference of excitations in the pure gold nanotube via alloying. However, further attempts to increase the plasmonic dichroism by considering multi-shell gold nanowires in which one entire shell is doped with silver did not produce the desired effect, but rather a decrease in circular dichroism. We show that this latter result is due to a more severe destructive interference in the dipole excitation contributions, and suggest that further amplification should be possible in principle by properly tuning simultaneously the nanowire structure and chemical ordering.

Insights into the origin of the first sharp diffraction peak in amorphous silica from an analysis of chemical and radial ordering

Insights into the origin of the first sharp diffraction peak in amorphous silica from an analysis of chemical and radial ordering

Effects of Zr dopants on properties of PtNi nanoparticles for ORR catalysis: A DFT modeling.

Pt-based alloys, such as Pt3Ni, are among the best electrocatalysts for oxygen reduction reaction (ORR) in polymer electrolyte membrane fuel cells. Doping of PtNi alloys with Zr was shown to enhance the durability of the operating ORR catalysts. Rationalizing these observations is hindered by the absence of atomic-level data for these tri-metallic materials, even when not exposed to the fuel cell operation conditions. This study aims at understanding structure-property relations in Zr-doped PtNi nanoparticles as a key to their ORR function. In particular, we calculated, using a method based on density functional theory, the most stable chemical orderings of pristine and Zr-doped Pt3Ni particles containing over 400 atoms. We thus clarify (i) preferential location and charge states of Zr atoms in the Pt3Ni NPs; (ii) effect of doping Zr atoms on the stability of the Pt skin of the Pt3Ni NPs; (iii) charge redistribution induced by Zr dopants; (iv) layer-by-layer atomic ordering in the Pt3Ni/Zr NPs with the increasing Zr content; and (v) effect of Zr atoms on the adsorption energies of O and OH species as indicators of the ORR activity.

High-pressure study of synthesized α-Bi2O3/NiBi3 nanostructured composite: In-situ synchrotron XRD measurements, DFT calculations and PDF approach

We investigated a nanostructured composite of α-Bi2O3/NiBi3, which was synthesized through high-energy milling, under high-pressure conditions of up to 30 GPa. To track its structural changes, we employed in-situ synchrotron angle-dispersive X-ray diffraction measurements in conjunction with density functional theory calculations. Crystallographic information was derived using the Rietveld method and DFT computations. Additionally, we examined the chemical short-range order using pair distribution functions and determined the compressibility parameters through the Birch-Murnaghan equation of state.We explored the high-pressure behavior of a nanostructured composite of α-Bi2O3/NiBi3, applying pressures up to 30 GPa. The composite sample, synthesized by mechanical alloying, underwent thorough structural characterization by X-ray diffraction, high-resolution transmission electron microscopy and Raman spectroscopy. The sample's response to applied pressure was studied by analyzing crystallographic data obtained from in-situ synchrotron angle-dispersive X-ray diffraction measurements and density functional theory calculations. Crystallographic information from experiments was refined using the Rietveld method. Additionally, chemical short-range order was examined using pair distribution functions, and compressibility parameters were determined via the Birch-Murnaghan equation of state. This investigation into the high-pressure behavior of NiBi3 provides valuable insights for future studies and potential applications of similar materials in extreme pressure environments.

Graph Neural Network-Accelerated Multitasking Genetic Algorithm for Optimizing PdxTi1-xHy Surfaces under Various CO2 Reduction Reaction Conditions.

Palladium (Pd) hydride-based catalysts have been reported to have excellent performance in the CO2 reduction reaction (CO2RR) and hydrogen evolution reaction (HER). Our previous work on doped PdH and Pd alloy hydrides showed that Ti-doped and Ti-alloyed Pd hydrides could improve the performance of the CO2 reduction reaction compared with pure Pd hydride. Compositions and chemical orderings of the surfaces with only one adsorbate under certain reaction conditions are linked to their stability, activity, and selectivity toward the CO2RR and HER, as shown in our previous work. In fact, various coverages, types, and mixtures of the adsorbates, as well as state variables such as temperature, pressure, applied potential, and chemical potential, could impact their stability, activity, and selectivity. However, these factors are usually fixed at common values to reduce the complexity of the structures and the complexity of the reaction conditions in most theoretical work. To address the complexities above and the huge search space, we apply a deep learning-assisted multitasking genetic algorithm to screen for PdxTi1-xHy surfaces containing multiple adsorbates for CO2RR under different reaction conditions. The ensemble deep learning model can greatly speed up the structure relaxations and retain a high accuracy and low uncertainty of the energy and forces. The multitasking genetic algorithm simultaneously finds globally stable surface structures under each reaction condition. Finally, 23 stable structures are screened out under different reaction conditions. Among these, Pd0.56Ti0.44H1.06 + 25%CO, Pd0.31Ti0.69H1.25 + 50%CO, Pd0.31Ti0.69H1.25 + 25%CO, and Pd0.88Ti0.12H1.06 + 25%CO are found to be very active for CO2RR and suitable to generate syngas consisting of CO and H2.

Dynamic Nanoindentation and Short-Range Order in Equiatomic NiCoCr Medium-Entropy Alloy Lead to Novel Density Wave Ordering.

Chemical short-range order is believed to be a key contributor to the exceptional properties of multicomponent alloys. However, its direct validation and confirmation has been highly elusive in most compounds. Recent studies for equiatomic NiCoCr alloys have shown that thermal treatments (i.e., annealing and aging) may facilitate and manipulate such ordering. In this work, by using molecular simulations, we show that nanomechanical probes, such as nanoindentation, may be utilized toward further manipulation of chemical short-range order, providing explicit validation pathways. By using well established interatomic potentials, we perform hybrid molecular-dynamics-MonteCarlo at room temperature to demonstrate that particular dwell nanoindentation protocols can lead, through thermal MonteCarlo equilibration, to local reorganization under the indenter tip, toward a density-wave stripe pattern. We characterize the novel density-wave structures, which are highly anisotropic and dependent on local, nanoindentation-induced stress concentrations, and we show how they deeply originate from intrinsic features of interelemental interactions. Furthermore, we show that these novel patterns consistently scale with the incipient plastic zone, under the indenter tip, justifying their observation at experimentally feasible nanoindentation depths.

Ab-initio calculations, Debye–Wang model and thermodynamic modeling for the short-range order and crystal chemistry characterization of the [formula omitted]8-Al[formula omitted]Fe[formula omitted]Si[formula omitted] solid solution

This paper presents a comprehensive examination of the τ8-Al2Fe3Si4 solid solution, investigating its crystal chemistry and short-range order. To do so, Density functional theory (DFT) calculations were performed, simulating partial substitutions of Al and Si on the four mixed sites of the structure. Our results indicates a negligible contribution from the SRO, allowing the Bragg–Williams approximation to be used to describe its crystal chemistry. Subsequently, systematic substitutions of Al and Si on all mixed sites generated a complete set of ordered structures, called end-members. The formation enthalpies, equation of state and elastic constants for each end-member were calculated, allowing the Gibbs free energy of the solid solution to be derived using the Debye–Wang model. On the basis of this new theoretical insights, a thermodynamic model was developed, predicting the Si occupancy factors in all the mixed sites of the structure at different temperatures, allowing the quantification of its chemical ordering.

A Coherent Pd–Pd16B3 Core–Shell Electrocatalyst for Controlled Hydrogenation in Nitrogen Reduction Reaction

AbstractThe manipulation of surface catalytic sites has rarely been explored for metal borides, and the subsurface effects on the electrocatalytic activity of the nitrogen reduction reaction (NRR) remain unknown. Herein, this work develops a core–shell nanoparticle catalyst with a Pd core that ensures high electron transfer rates and an Pd16B3 atomical shell that possess tunable active sites for regulating the NRR. The atomic structural evolution from Pd to Pd16B3 is investigated by precisely controlling the B atom diffusion, molecular rearrangement, and d–sp orbital hybridization. Pd/Pd16B3 core–shell nanocrystals exhibit an exceptional NRR performance with a high NH3 Faradaic efficiency of 30.8%, which is superior to those of pristine Pd (1.2%) and B‐doped Pd (4.8%) under identical conditions, and a yield rate of 0.81 µmol h−1 cm−2. This work discovers that the Pd16B3 shell could promote the NRR selectivity by separating the separating the hydrogen evolution reaction proceeded on hole sites and NRR proceeded on bridge sites, and the Pd core could provide the excellent conductivity to Pd16B3 shell through regulated electron interactions. Consequently, the controlled chemical ordering of palladium boride on palladium surfaces provides insight into the synthesis of advanced NRR electrocatalysts.

Effects of Chemical Short-Range Order and Temperature on Basic Structure Parameters and Stacking Fault Energies in Multi-Principal Element Alloys

In the realm of advanced material science, multi-principal element alloys (MPEAs) have emerged as a focal point due to their exceptional mechanical properties and adaptability for high-performance applications. This study embarks on an extensive investigation of four MPEAs—CoCrNi, MoNbTa, HfNbTaTiZr, and HfMoNbTaTi—alongside key pure metals (Mo, Nb, Ta, Ni) to unveil their structural and mechanical characteristics. Utilizing a blend of molecular statics and hybrid molecular dynamics/Monte Carlo simulations, the research delves into the impact of chemical short-range order (CSRO) and thermal effects on the fundamental structural parameters and stacking fault energies in these alloys. The study systematically analyzes quantities such as lattice parameters, elastic constants (C11, C12, and C44), and generalized stacking fault energies (GSFEs) across two distinct structures: random and CSRO. These properties are then evaluated at diverse temperatures (0, 300, 600, 900, 1200 K), offering a comprehensive understanding of temperature’s influence on material behavior. For CSRO, CoCrNi was annealed at 350 K and MoNbTa at 300 K, while both HfMoNbTaTi and HfNbTaTiZr were annealed at 300 K, 600 K, and 900 K, respectively. The results indicate that the lattice parameter increases with temperature, reflecting typical thermal expansion behavior. In contrast, both elastic constants and GSFE decrease with rising temperature, suggesting a reduction in resistance to stability and dislocation motion as thermal agitation intensifies. Notably, MPEAs with CSRO structures exhibit higher stiffness and GSFEs compared to their randomly structured counterparts, demonstrating the significant role of atomic ordering in enhancing material strength.

Variation of first pop-in loads in nanoindentation to detect chemical short-range ordering in the equiatomic Cr-Co-Ni medium-entropy alloy

The variations of the first pop-in load in nanoindentation with annealing temperature and time have been investigated both by experiment and theoretical molecular dynamics (MD) simulations for the equiatomic Cr-Co-Ni medium-entropy alloy (MEA). Regardless of the loading rates, the first pop-in load tends to increase with the increase in electrical resistivity consistently by experiment and MD simulations. The trend in the variation of pop-in load with annealing temperature/time coincides with what is observed in the variation of electrical resistivity in our previous study, indicating that the first pop-in load in nanoindentation can thus be used to qualitatively detect the evolution of short-range ordering (SRO) in the Cr-Co-Ni MEA, and that the peak temperature for SRO for the equiatomic Cr-Co-Ni MEA is 673 K. MD simulations indicate that the first pop-in corresponds to the nucleation of partial dislocation loops beneath the indenter for both annealing conditions and that the increased first pop-in load in nanoindentation is attributed to the higher energy barrier for the nucleation of dislocation loops due to the higher degree of SRO.

Low-temperature diffusion in thin-film Pt-(Au-)-Co heterostructures: a structural and magnetic characterization

Formation of functional thin films for nanoelectronics and magnetic data storage via thermally induced diffusion-driven structural phase transformations in multilayer stacks is a promising technology-relevant approach. Ferromagnetic thin films based on Co Pt alloys are considered as a material science platform for the development of various applications such as spin valves, spin orbit torque devices, and high-density data storage media. Here, we study diffusion processes in Pt–Co-based stacks with the focus on the effect of layers inversion (Pt/Co/substrate versus Co/Pt/substrate) and insertion of an intermediate Au layer on the structural transitions and magnetic properties. We demonstrate that the layer stacking has a pronounced effect on the diffusion rate at temperatures, where the diffusion is dominated by grain boundaries. We quantify effective diffusion coefficients, which characterize the diffusion rate of Co and Pt through the interface and grain boundaries, providing the possibility to control the homogenization rate of the Pt–Co-based heterostructures. The obtained values are in the range of 10−16–10−13 cm2 s−1 for temperatures of 150 °C–350 °C. Heat treatment of the thin-film samples results in the coercivity enhancement, which is attributed to short-range chemical ordering effects. We show that introducing an additional Au intermediate layer leads to an increase of the coercive field of the annealed samples due to a modification of exchange coupling between the magnetic grains at the grain boundaries.

Controlling the Optical Properties of Transparent Auxetic Liquid Crystal Elastomers.

Determining the tunability of the optical coefficients, order parameter, and transition temperatures in optically transparent auxetic liquid crystal elastomers (LCEs) is vital for applications, including impact-resistant glass laminates. Here, we report measurements of the refractive indices, order parameters, and transition temperatures in a family of acrylate-based LCEs in which the mesogenic content varies from ∼50 to ∼85%. Modifications in the precursor mixture allow the order parameter, ⟨P2⟩, of the LCE to be adjusted from 0.46 to 0.73. The extraordinary refractive index changes most significantly with composition, from ∼1.66 to ∼1.69, in moving from a low to high mesogenic content. We demonstrate that all LCE refractive indices decrease with increasing temperature, with temperature coefficients of ∼10-4 K-1, comparable to optical plastics. In these LCEs, the average refractive index and the refractive index anisotropy are tunable via both chemical composition and order parameter control; we report design rules for both.

Effects of Chemical Short-Range Order and Lattice Distortion on Crack-Tip Behavior of Medium-Entropy Alloy by Atomistic Simulations

It is well demonstrated that the complex chemical fluctuations on high/medium-entropy alloys (H/MEAs) play critical roles in their deformation process, but there are few reports related to the effect of such complex chemical fluctuations on the crack behavior. In this paper, the effects of chemical short-range order (CSRO) and lattice distortion (LD) on the crack-tip behavior of CrCoNi MEAs under mode I loading at room temperature are investigated by carrying out molecular dynamics (MD) simulation, hybrid MD/Monte-Carlo (MC) simulation and the J-integral method. The results reveal that CSRO can improve the J-integral value without significant changes in the localized deformation zone size. On the contrary, LD can lower the J-integral value with an increase in the localized deformation zone size. The energetic analysis shows that CSRO improves the activation energy barrier of Shockley partial dislocation from the crack-tip while LD reduces the activation energy barrier. Our work is a step forward in understanding the effects of CSRO and LD on the crack-tip behavior and deformation mechanisms of CrCoNi MEAs.

Break the strength and ductility trade-off in novel NbC reinforced Fe40.5(CoCr)25Mn17.5Ni10Si5 high entropy alloy

In this study, a novel NbC reinforced Fe40.5(CoCr)25Mn17.5Ni10Si5 high entropy alloy (HEA) was designed, which exhibits high strength and ductility at room temperature. This alloy was prepared through hot forging and annealing treatments, which exhibited the FCC microstructure with NbC and discrete nano-sized M23C6 carbides precipitation. The studied HEA retained a tensile strength of 863 MPa, and an outstanding ductility of 58 %. The microstructure evolution and deformation mechanisms were further investigated. The deformation mechanism was mainly governed by nano-scale defect heterogeneity of stacking faults (SFs), deformation twins and dislocations. The improved strength-ductility synergy is attributed to the formation of multiple SFs, deformation twining, dislocation cells, and nano-sized NbC precipitation. Moreover, the TEM observation elucidates the presence of chemical short-range order (SRO) domain in the studied HEA, which also has a major contribution in strength and ductility increment. The present findings shed light on developing the HEAs through proper compositional design and heat treatments, resulting in the combine effect of precipitation strengthening, SFs mediated plasticity, twinning induced plasticity (TWIP), and SRO effect.

Effect of Al related chemical short-range order on the irradiation resistance of Al0.5CoCrFeNi multi-principal element alloy: A molecular dynamics simulation

Multi-principal element alloys (MPEAs) have received much attention for future nuclear materials due to their exceptional radiation resistance. This work investigated the effect of Al related chemical short-range ordered (CSRO) which caused by its large atomic size in face-centered cubic (FCC) Al0.5CoCrFeNi alloy on its irradiation damage behavior through mixed molecular dynamics and Monte Carlo simulations. The calculation results indicate that the CSRO structure effectively suppresses the generation of point defects during irradiation in Al0.5CoCrFeNi alloy, and this inhibitory effect increases with increasing irradiation energy. The enhanced radiation resistance of CSRO alloy is attributed to the high vacancy formation energy, interstitial atom formation energy, and vacancy migration energy barrier of Al. Moreover, the Al atoms in CSRO alloy can also act as defect barriers, preventing the migration of vacancy defects; The formation energy of interstitial atoms in the CSRO alloy exhibits a strong element dependence, and there is a significant difference in the formation energy of interstitial atoms between different points, which strengthens the hysteresis diffusion effect of MPEAs and further enhances the radiation resistance of MPEAs.

Role of chemical short-range order in friction and wear behaviors in medium-entropy alloy CoCrNi

Chemical short-range order (SRO) can effectively retard the dislocation motion, allowing for a substantive hardening effect. Notwithstanding, fundamental mechanisms behind subsequent friction and wear processes at the atom level are insufficiently understood. Meanwhile, it is still a tall order to experimentally characterize this nanoscale chemical inhomogeneity. Here, through conducting hybrid Monte-Carlo (MC) and molecular dynamic (MD) simulations, the impact of chemical SRO on the friction and wear behaviors in a CoCrNi medium-entropy alloy (MEA) was investigated. The friction weakening after introducing SRO was demonstrated to be the consequence of a combination of several factors apart from a reduction in penetration depth, which encompassed a suppression of the pile-up effect and lowered densities in total and immobile dislocation ahead of tip. Meanwhile, the SRO-dependent repression of sub-surface deformation behaviors was verified to effectively improve wear resistance. Particularly, the presence of Ni-rich domains was validated to impose additional resistance to dislocations slipping toward the interior and increase the interlocking likelihood between dislocations parallel to the surface and those slipping deeper into the sub-surface, effectively weakening the sub-surface damage. In addition, a strong SRO was corroborated to further decrease friction and wear damage. These findings are expected to provide important insights into understanding chemical SRO-related anti-friction and wear-resisting behaviors in a CoCrNi MEA.

The impact of chemical short-range order on the thermophysical properties of medium- and high-entropy alloys

The unusual behavior observed in the coefficient of thermal expansion and specific heat capacity of CrFeNi, CoCrNi, and CoCrFeNi medium/high-entropy alloys is commonly referred to as the K-state effect. It is shown to be independent of the Curie temperature, as demonstrated by temperature-dependent magnetic moment measurements. CoCrFeNi alloy is chosen for detailed characterization; potential reasons for the K-state effect such as texture, recrystallization, and second-phase precipitation are ruled out. An examination of the electronic structure indicates the formation of a pseudo-gap in the Density of States, which suggests a specific chemical interaction between Ni and Cr atoms upon alloying. Hybrid Monte Carlo/Molecular Dynamic (MC/MD) simulations indicate the presence of non-negligible chemical short-range order (CSRO). Local lattice distortions are shown to be negligible, although deviations around Cr and Ni elements from those expected in a fully disordered structure are experimentally observed by X-ray absorption spectroscopy. The determined bonding distances are in good agreement with MC/MD calculations. A mechanism is proposed to explain the anomalies and calorimetric experiments and their results are used to validate the mechanism.

The Philippines’ EcoWaste Coalition cites 10th year of Chemical Control Order banning lead in paints

The Philippines’ EcoWaste Coalition cites 10th year of Chemical Control Order banning lead in paints

Impacts of local chemical ordering on the primary radiation damage in Cr–Fe–Ni multi-principal element alloys

Face-centered cubic (FCC) single-phase multi-principal element alloys (MPEAs) have demonstrated promising irradiation resistance. The generation and evolution of irradiation defects are integral aspects to comprehend, as they offer valuable insights into the radiation resistance of materials. In this work, we use molecular dynamics simulations to investigate the primary radiation damage of CrFeNi FCC MPEAs, while focusing on the impact of local chemical ordering (LCO) on the generation and evolution of defects. Our simulation results reveal that the LCO reduces the probability of defect production, consequently mitigating the maximum number of defects. Furthermore, the LCO increases defect migration barriers, thereby impeding the migration of interstitial atoms and leading to an increased number of residual defects subsequent to primary damage. However, the dispersed distribution of defects throughout space facilitates the uniform nucleation of dislocations in subsequent irradiation cascades. Additionally, the increased migration energy acts as a barrier to the movement of defect clusters, impeding their growth and thereby enhancing the radiation resistance of MPEAs. Our findings underscore the potential for enhancing the radiation resistance of MPEAs through the deliberate tailoring of LCOs.