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High‐throughput combinatorial approach expedites the synthesis of a lead‐free relaxor ferroelectric system
Abstract Developing novel lead‐free ferroelectric materials is crucial for next‐generation microelectronic technologies that are energy efficient and environment friendly. However, materials discovery and property optimization are typically time‐consuming due to the limited throughput of traditional synthesis methods. In this work, we use a high‐throughput combinatorial synthesis approach to fabricate lead‐free ferroelectric superlattices and solid solutions of (Ba 0.7 Ca 0.3 )TiO 3 (BCT) and Ba(Zr 0.2 Ti 0.8 )O 3 (BZT) phases with continuous variation of composition and layer thickness. High‐resolution x‐ray diffraction (XRD) and analytical scanning transmission electron microscopy (STEM) demonstrate high film quality and well‐controlled compositional gradients. Ferroelectric and dielectric property measurements identify the “optimal property point” achieved at the composition of 48BZT–52BCT. Displacement vector maps reveal that ferroelectric domain sizes are tunable by varying {BCT–BZT} N superlattice geometry. This high‐throughput synthesis approach can be applied to many other material systems to expedite new materials discovery and properties optimization, allowing for the exploration of a large area of phase space within a single growth. image
Simulation of the Impact of Point Defects and Edge Dislocations on X-Ray Diffraction in Hexagonal (Ni,Co) 1+2 x Ti 1– x O 3 Thin Films
In this work, a computer code for simulating high-resolution X-ray diffraction (XRD) data from disordered crystals with arbitrary spatial composition and local lattice parameters is developed. Simulated patterns are compared with the experimental data collected on a single phase, highly crystalline (Ni 0.42 Co 0.58 ) 2.22 Ti 0.39 O 3 solid-solution thin film exhibiting a large number of subsidiary minima. As a case study, the edge dislocations in hexagonal (Ni,Co) 3 O 3 thin films with Burgers vector parallel and perpendicular to the a-axis and c-axis, respectively, are modeled. No peak profiles are assumed and thus issues related to profile fitting are avoided. Both macroscopic features, such as film thickness, and atomic-scale structure, such as dislocations, are simultaneously modeled. Simulations are run on a desktop machine. Commonly applied database-based phase identification routines can result in wrong phase identification, unless intensities are properly modeled. Modeling tools of diffractometer manufacturers are compared with the present approach. An example of how simulated reciprocal lattice patterns can be used to choose relevant measurement geometries in defected thin films is given.
Creation of Multi-Principal Element Alloy NiCoCr Nanostructures via Nanosecond Laser-Induced Dewetting
The multi-principal element alloy nanoparticles (MPEA NPs), a new class of nanomaterials, present a highly rewarding opportunity to explore new or vastly different functional properties than the traditional mono/bi/multimetallic nanostructures due to their unique characteristics of atomic-level homogeneous mixing of constituent elements in the nanoconfinements. Here, in this paper, the successful creation of NiCoCr nanoparticles, a well-known MPEA system is reported, using ultrafast nanosecond laser-induced dewetting of alloy thin films. Nanoparticle formation occurs by spontaneously breaking the energetically unstable thin films in a melt state under laser-induced hydrodynamic instability and subsequently accumulating in a droplet shape via surface energy minimization. While NiCoCr alloy shows a stark contrast in physical properties compared to individual metallic constituents, i.e., Ni, Co, and Cr, yet the transient nature of the laser-driven process facilitates a homogeneous distribution of the constituents (Ni, Co, and Cr) in the nanoparticles. Using high-resolution chemical analysis and scanning nanodiffraction, the environmental stability and grain arrangement in the nanoparticles are further investigated. Thermal transport simulations reveal that the ultrashort (≈100 ns) melt-state lifetime of NiCoCr during the dewetting event helps retain the constituent elements in a single-phase solid solution with homogenous distribution and opens the pathway to create the unique MPEA nanoparticles with laser-induced dewetting process.
N‐Doping Effects On Electrocatalytic Water Splitting of Non‐Noble High‐Entropy Alloy Nanoparticles Prepared by Inert Gas Condensation
Abstract The unique catalytic activities of high‐entropy alloys (HEAs) emerge from the complex interaction among different elements in a single‐phase solid solution. As a “green” nanofabrication technique, inert gas condensation (IGC) combined with laser source opens up a highly efficient avenue to develop HEA nanoparticles (NPs) for catalysis and energy storage. In this work, the novel N‐doped non‐noble HEA NPs are designed and successfully prepared by IGC. The N‐doping effects of HEA NPs on oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) are systematically investigated. The results show that N‐doping is conducive to improving the OER, but unfavorable for HER activity. The FeCoNiCrN NPs achieve an overpotential of 269.7 mV for OER at a current density of 10 mA cm −2 in 1.0 M KOH solution, which is among the best reported values for non‐noble HEA catalysts. The effects of the differences in electronegativity, ionization energy and electron affinity energy among mixed elements in N‐doped HEAs are discussed as inducing electron transfer efficiency. Combined with X‐ray photoelectron spectroscopy and the extended X‐ray absorption fine structure analysis, an element‐design strategy in N‐doped HEAs electrocatalysts is proposed to improve the intrinsic activity and ameliorate water splitting performance.
How Atomic Bonding Plays the Hardness Behavior in the Al–Co–Cr–Cu–Fe–Ni High Entropy Family
A systematic study on a face-centered cubic-based compositionally complex alloy system Al–Co–Cr–Cu–Fe–Ni in its single-phase state is carried out, where a mother senary compound Al 8 Co 17 Cr 17 Cu 8 Fe 17 Ni 33 and five of its suballoys, obtained by removing one element at a time, are investigated and exhaustively analyzed determining the contribution of each alloying element in the solid solution. The senary and the quinaries are compared using experimental techniques including X-ray absorption spectroscopy, X-ray diffraction, transmission electron microscopy, and first principles hybrid Monte Carlo/molecular dynamics simulations. Chemical short-range order and bond length distances have been determined both at the experimental and computational level. Electronic structure and local atomic distortions up to 5.2 Å have been correlated to the microhardness values. A linear regression model connecting hardness with local lattice distortions is presented.
Non‐Linear Optical Properties of the ( RE ) 3 CuGeS 7 Family of Compounds
Abstract Non‐linear optical materials must possess a balanced combination of laser‐induced damage threshold (LDT) and second‐harmonic generation (SHG) and be phase matchable. In our previous work, chiral and polar La 3 CuGeS 7 was identified as a promising non‐linear optical material. Herein, we report the optimization of non‐linear optical properties through replacement of La with smaller lanthanides. It is determined that Gd 3 CuGeS 7 exhibits the best combination of SHG (1.6× AgGaS 2 at 88–105 μm particle size) and LDT (3× AgGaS 2 , 89 MW/cm 2 ) and is phase matchable. Based on changes in metal‐sulfur bond lengths and angles, we further propose structural optimization through solid‐solution formation and doping.
Mechanical Behavior of High-Entropy Alloys: A Review
High-entropy alloys (HEAs) are materials that consist of equimolar or near-equimolar multiple principal components but tend to form single phases, which is a new research topic in the field of metallurgy, and have attracted extensive attention in the past decade. The HEAs families contain the face-centered-cubic (fcc), body-centered-cubic (bcc), and hexagonal-close-packed (hcp)-structured HEAs. On the one hand, mechanical properties, e.g., hardness, strength, ductility, fatigue, and elastic moduli, are essential for practical applications of HEAs. Scientists have explored in this direction since the advent of HEAs. On the other hand, the pursuit of high strength and good plasticity is the critical research issue of materials. Hence, strengthening of HEAs is a crucial issue.In this chapter, we reviewed the recent work on the room-temperature elastic properties and mechanical behavior of HEAs, including the mechanisms behind the plastic deformation of HEAs at both low and high temperatures. Furthermore, the present work examined the strengthening strategies of HEAs, e.g., strain hardening, grain-boundary strengthening, solid-solution strengthening, and particle strengthening. The fatigue, creep, and fracture properties were briefly introduced. Lastly, the future scientific issues and challenges of HEAs were discussed.
Performance of Wrought Superalloys in Extreme Environments
As power generation systems move towards higher efficiency operation above 700 °C, wrought superalloys are the leading structural alloy candidates, including precipitation strengthened (PS) alloys 740 and 282 for the highest temperatures. To evaluate the performance of these alloys for these applications, a range of 500–5000 h evaluations have been conducted in environments including steam, supercritical CO2 (sCO2) and simulated combustion exhaust with H2O and/or SO2 at 700–800 °C and compared to baseline exposures in laboratory air and 1 bar CO2. These alloys primarily rely on the formation of an external Cr-rich oxide layer or scale for environmental protection and the reaction rates in all of these conditions are similar and relatively low. However, compared to a conventional solid solution strengthened alloy, like 625, the mass gains are higher for PS alloys due to the internal oxidation of the γ' forming additions, Al and Ti. Post-exposure characterization has quantified the reaction products and the depth of internal oxidation is not a concern and does not appear to increase above the baseline behavior in laboratory air. Likewise, there is no indication of internal carburization in the sCO2 environment at 750 °C/300 bar. The addition of 0.1% SO2 in CO2-10% H2O at 800 °C actually suppressed the internal oxidation at 1 bar but SO2 may be a concern when the total pressure is higher.
Competing Grain Boundary and Interior Deformation Mechanisms with Varying Sizes
In typical coarse-grained alloys, the dominant plastic deformations are dislocation gliding or climbing, and material strengths can be tuned by dislocation interactions with grain boundaries, precipitates, solid solutions, and other defects. With the reduction of grain size, the increase of material strengths follows the classic Hall-Petch relationship up to nano-grained materials. Even at room temperatures, nano-grained materials exhibit strength softening, or called the inverse Hall-Petch effect, as grain boundary processes take over as the dominant deformation mechanisms. On the other hand, at elevated temperatures, grain boundary processes compete with grain interior deformation mechanisms over a wide range of the applied stress and grain sizes. This book chapter reviews and compares the rate equation model and the microstructure-based finite element simulations. The latter explicitly accounts for the grain boundary sliding, grain boundary diffusion and migration, as well as the grain interior dislocation creep. Therefore the explicit finite element method has clear advantages in problems where microstructural heterogeneities play a critical role, such as in the gradient microstructure in shot peening or weldment. Furthermore, combined with the Hall-Petch effect and its breakdown, the above competing processes help construct deformation mechanism maps by extending from the classic Frost-Ashby type to the ones with the dependence of grain size.
Thermal behavior of polyhalite: a high-temperature synchrotron XRD study
As an accessory mineral in marine evaporites, polyhalite, K 2 MgCa 2 (SO 4 ) 4 ·2H 2 O, coexists with halite (NaCl) in salt formations, which have been considered as potential repositories for permanent storage of high-level nuclear wastes. However, because of the heat generated by radioactive decays in the wastes, polyhalite may dehydrate, and the released water will dissolve its neighboring salt, potentially affecting the repository integrity. Thus, studying the thermal behavior of polyhalite is important. In this paper, a polyhalite sample containing a small amount of halite was collected from the Salado formation at the WIPP site in Carlsbad, New Mexico. To determine its thermal behavior, in situ high-temperature synchrotron X-ray diffraction was conducted from room temperature to 1066 K with the sample powders sealed in a silica-glass capillary. At about 506 K, polyhalite started to decompose into water vapor, anhydrite (CaSO 4 ) and two langbeinite-type phases, K 2 Ca x Mg 2-x (SO 4 ) 3 , with different Ca/Mg ratios. XRD peaks of the minor halite disappeared, presumably due to its dissolution by water vapor. With further increasing temperature, the two langbeinite solid solution phases displayed complex variations in crystallinity, composition and their molar ratio and then were combined into the single-phase triple salt, K 2 CaMg(SO 4 ) 3 , at ~919 K. Rietveld analyses of the XRD data allowed determination of structural parameters of polyhalite and its decomposed anhydrite and langbeinite phases as a function of temperature. Finally, from the results, the thermal expansion coefficients of these phases have been derived, and the structural mechanisms of their thermal behavior been discussed.
High-pressure crystal structure and equation of state of ferromagnesian jeffbenite: implications for stability in the transition zone and uppermost lower mantle
Jeffbenite, ideally Mg 3 Al 2 Si 3 O 12 , has been identified as inclusions in super-deep diamonds originating from depths that exceed 300 km. Although Mg-end member jeffbenite has limited stability at upper-mantle conditions, iron-bearing jeffbenite may have broader P–T stability that extends to the transition zone or uppermost lower mantle, incorporating significant amounts of ferric iron. Using synchrotron-based, single-crystal X-ray diffraction (XRD) and synchrotron Mössbauer spectroscopy (SMS) at pressures up to 29 GPa, we report the crystal structure, compressibility, and likely spin transition of iron in ferromagnesian jeffbenite (Mg 2.32 Al 0.03 Fe 2+ 1.28 Fe 3+ 1.77 Si 2.85 O 12 ). High-pressure structure refinements reveal that Fe 3+ substitution for Si in the T2 site, which shares edges with the M2 octahedron, likely stabilizes jeffbenite at high pressure, because it increases the cation-to-cation distance between these sites. Although ferromagnesian jeffbenite does not undergo a structural phase transition below 30 GPa, SMS hyperfine parameters suggest the onset of an electronic spin transition of iron from high-spin (HS) to low-spin (LS) at around 22 GPa, which may increase its stability at high pressures. Pressure–volume data were fit to a third order Birch–Murnaghan equation of state, resulting in V o = 816.54(9), K To = 181.54(1.39), and K' T0 = 2.76(14). These equation of state parameters are applicable to evaluating the encapsulation pressures of super-deep diamonds. The density and bulk modulus of ferromagnesian jeffbenite are similar to or higher than pyrope–almandine, pyrope–majorite, and skiagite–majorite solid solution garnets, further suggesting that jeffbenite may be an important ferric–iron silicate in the deeper parts of the mantle transition zone and uppermost lower mantle. Furthermore, future studies on the influence of temperature and oxidation state on the stability and equations of state of iron-bearing jeffbennite are still needed to determine what role, if any, jeffbenite plays in transition-zone mineralogy.
Synthesis and Characterization of MoNb Films Superconducting at 100-200 mK
We have developed a new transition-edge sensor material with critical temperature (T-c) in the range 100-200 mK. The new material is a solid solution of two superconducting components, MoxNb1-x, co-sputtered from two high-purity single-component targets (Mo and Nb). The T-c has a minimum (dT(c)/dx = 0) at an intermediate concentration of the components. We have optimized the deposition parameters and composition to provide films with a sharp superconducting transition at ~ 150 mK. We investigated structural features of the films and surface morphology using X-ray diffraction (XRD) and scanning electron microscopy. The XRD measurements indicate that the grown films are polycrystalline, with a preferred orientation along the (110) crystal direction and a clear correlation between superconducting properties and film microstructure.
Threshold Stress Creep Behavior of Alloy 617 at Intermediate Temperatures
Creep of Alloy 617, a solid solution Ni-Cr-Mo alloy, was studied in the temperature range of 1023 K to 1273 K (750 °C to 1000 °C). Typical power-law creep behavior with a stress exponent of approximately 5 is observed at temperatures from 1073 K to 1273 K (800 °C to 1000 °C). Creep at 1023 K (750 °C), however, exhibits threshold stress behavior coinciding with the temperature at which a low volume fraction of ordered coherent y' precipitates forms. The threshold stress is determined experimentally to be around 70 MPa at 1023 K (750 °C) and is verified to be near zero at 1173 K (900 °C)—temperatures directly correlating to the formation and dissolution of y' precipitates, respectively. The y' precipitates provide an obstacle to continued dislocation motion and result in the presence of a threshold stress. TEM analysis of specimens crept at 1023 K (750 °C) to various strains, and modeling of stresses necessary for y' precipitate dislocation bypass, suggests that the climb of dislocations around the y' precipitates is the controlling factor for continued deformation at the end of primary creep and into the tertiary creep regime. As creep deformation proceeds at an applied stress of 121 MPa and the precipitates coarsen, the stress required for Orowan bowing is reached and this mechanism becomes active. At the minimum creep rate at an applied stress of 145 MPa, the finer precipitate size results in higher Orowan bowing stresses and the creep deformation is dominated by the climb of dislocations around the y' precipitates.
Creep Deformation of Allvac 718Plus
The creep deformation behavior of Allvac 718Plus was studied over the temperature range 650° to 732°C at initial applied stress levels ranging from 517 to 655 MPa. Over the entire experimental temperature stress regime this alloy exhibits Class M type creep behavior with all creep curves exhibiting a decelerating strain rate with strain or time throughout primary creep. However, unlike pure metals or simple solid solution alloys this gamma prime strengthened superalloy does not exhibit steady state creep. Rather, primary creep is instantly followed by a long duration of accelerating strain rate with strain or time. These creep characteristics are common amongst the gamma prime strengthened superalloys. Allvac 718Plus also exhibits a very high temperature dependence of creep rate. Detailed TEM examination of the deformation structures of selected creep samples reveals dislocation mechanisms similar to those found in high volume fraction gamma prime strengthened superalloys. Strong evidence of microtwinning is found in several of the deformation structures. The presence of microtwinning may account for the strong temperature dependence of creep rate observed in this alloy. In addition, due to the presence of Nb and thus, grain boundary delta phase, matrix dislocation activity which is not present in non Nb bearing superalloys occurs in this alloy. The creep characteristics and dislocation mechanisms are presented and discussed in detail.
Transient-Liquid-Phase Bonding of H230 Ni-Based Alloy Using Ni-P Interlayer: Microstructure and Mechanical Properties
Transient-liquid-phase bonding using Ni-P as an interlayer has been developed for H230 Ni-Cr-W solid-solution-strengthened Ni-based alloy. Two process parameters—composition of the interlayer and bonding time—have been varied to optimize the mechanical properties. H230 has been bonded into two sets of stacks (set I and II) for 8 and 4 hours using Ni-12P and Ni-6P interlayer, respectively, (wt pct) at 1423 K (1150 °C) and 12.7 MPa. The microstructure of both the stacks has three distinct regions—the joint centerline which showed the presence of pores, an isothermally solidified zone (ISZ) which did not have any carbide precipitates and base H230. Transmission electron microscopy and atom probe tomography showed a uniform microstructure, and an absence of any deleterious phases at the joint and in ISZ. Set I and set II had a yield strength of 76 and 86 pct of that of the H230 sheet, tested at 1023 K (750 °C). Here, the measured elongation at fracture was negligible, but the fracture surfaces revealed a ductile cup-and-cone-type fracture occurring through the ISZ/joint region. Examination of broken tensile samples revealed that the plastic strain was constrained to within one joint region through which fracture occurred.
Microstructural, Oxidation, and Mechanical Behavior of NbTi-Based Refractory Alloys with 5 to 10 Pct Co, Cr, and Ni Additions
NbTi-based refractory alloys with additions of Co, Cr, and Ni represent an interesting medium-entropy alloy system with potential for protective oxide film formation, high strength, and ductility. This study investigates the microstructural evolution, oxidation behavior, and mechanical properties of NbTi-based alloys containing 5 to 10 at. pct Co, Cr, and Ni. CALPHAD predictions suggest that this composition range can be heat treated to obtain a predominantly body-centered cubic matrix phase. Mechanical properties, including microhardness, yield strength, maximum strength, and specific strength are evaluated through isothermal compression tests conducted between room temperature and 800 °C. The oxidation kinetics of these alloys are assessed through discontinuous oxidation tests. Parabolic oxidation kinetics were observed for NbTi–10Ni and NbTi–5Co, while linear oxidation kinetics were found for NbTi–10Cr and NbTi–10(CoCrNi). Microstructures and oxide layers are characterized using X-ray diffraction, electron backscatter diffraction, energy-dispersive X-ray spectroscopy, and scanning electron microscopy. All alloys exhibit significant mechanical softening between room temperature and 800 °C, with elastic-perfectly plastic flow observed at 800 °C. The addition of 10 pct Cr to NbTi resulted in two BCC phases up to 1050 °C, conflicting with CALPHAD predictions of a single-phase solid solution at this temperature, and resulting in higher flow stress at 800 °C. NbTi–10(CoCrNi) exhibited the lowest flow stress at 800 °C despite having more ‘cocktail effect’ potential and insufficient molar fractions of Co, Cr, or Ni to form a desirable protective oxide film.
Creep-Resistant Ferritic-Martensitic Steels for Power Plant Applications
Creep-resistant ferritic-martensitic steels are essential materials used in power plants for electricity generation. The efficiency and emissions of the plant are closely related to the operating temperature. Materials used in the hottest sections are one of the main limiting factor for plant operation at 650 °C. This review describes the recent development of creep-resistant ferritic-martensitic steels from a large dataset of in-house research. The findings are tied to available literature for discussion. A first steel, CPJ-7, was developed with an operating temperature approaching 650 °C, and a refined design, JMP, showed the potential to operate at 650 °C. Approximately fifty different ingots, each weighing ~ 7 kg, were vacuum induction melted and processed. Overall, wrought and cast versions of CPJ-7 present superior creep properties when compared to wrought and cast versions of COST alloys for steam turbine and P91/92 for boiler applications. The prolonged creep life was attributed to slowing down the process of the destabilization of the MX and M 23 C 6 precipitates at 650 °C. The cast version of CPJ-7 also revealed superior mechanical performance, above commercially available cast 9% Cr martensitic steel or derivatives. Following the work on CPJ-7, the JMP steels were designed with higher Co for increased solid solution strengthening, Si for oxidation resistance and increased W for matrix strength and stability. The JMP steels showed increases in creep life compared to CPJ-7 between 118 and 150% at 650 °C for testing at various stresses between 138 and 207 MPa. On a Larson–Miller plot, the performance of the JMP steels surpasses that of state-of-the-art MARBN steel. The influence of various elements within the composition of the alloys on the microstructure and mechanical properties is discussed. This review presents approximately 420,000 h of in-house creep testing, the equivalent of almost 50 years of cumulative creep tests.