Superlattice Alloys Research Articles

Oxidation behavior of an ultra-high strength and ductile Ni-enriched complex concentrated alloy

Recent research work revealed that Ni43.9Co22.4Fe8.8Al10.7Ti11.7B2.5 HEA is one of the ultra-high strength and ductile superlattice alloys. In this work, high-temperature oxidation behavior of the as-cast Ni43.9Co22.4Fe8.8Al10.7Ti11.7B2.5 alloy was investigated at 1000 ℃ up to 100 h. The oxidized samples were characterized using X-ray Diffractometer, Energy Dispersive Spectroscopy, and X-ray Photoelectron Spectroscopy. The results revealed that the initial microstructure of the alloy consists of face centered cubic (FCC) and L12 structures. High-temperature exposure resulted in the formation of Al2O3 and TiO2 scales during the initial hours of oxidation, which eventually spall-off after 25 h of exposure allowing further oxidation. The results showed that protective oxide layers such as Al2O3 and TiO2 were not present after 100 h of exposure. The external layer of the 100 h oxidized sample was composed of Fe, Co, and Ni-rich oxides which are known to have mere effective resistance against oxygen ingression. The alloy which has superior strength and ductility may be used for high temperature applications after attaining the thermally stable fine-grain microstructure by suitable thermomechanical processing / providing oxidation resistance coating / by doping with elements having superior oxidation resistance.

Effect of ball milling on the structure and electrochemical hydrogen storage properties of a RE-Mg-Ni alloy

RE-Mg-Ni alloy is considered a highly promising anode material for the next generation of MH-Ni batteries. However, the low cycle life, fast performance, and high manufacturing cost of this ideal MH-Ni battery anode material have become obstacles to its optimization. In this work, an as-cast La0.7Sm0.3MgNi3.6Co0.4 superlattice alloy was prepared by vacuum medium frequency induction melting. Then, La0.7Sm0.3MgNi3.6Co0.4 + 10 wt% Ni (0–2 h) composite hydrogen storage alloys with excellent properties were prepared by adding a catalyst (Ni) and conducting ball milling. XRD, SEM, and HRTEM were conducted to analyse the phase compositions, microstructures and macrostructures of the samples. The experimental results showed that with the increase in ball milling time, the microstructural characteristics of the alloy changed, such as the phase abundance, grain refinement, particle size reduction, and amorphous structure increase, directly leading to the change in electrochemical performance. The discharge capacity (Cmax) of the sample decayed from 264.0 mAh/g to 219.1 mAh/g, the cycle stability (Sn) gradually increased with the extension of the grinding time, and the rate discharge performance first increased and then decreased. When the grinding time was 1 h, the best performance was HRD900 = 36.53%. The critical factors affecting the dynamic performance, such as the charge transfer resistance (Rct), limiting current density (IL), and hydrogen diffusion coefficient (D), all exhibited the same trend and reached their respective optimal values at 1 h.

Microstructure and gas-solid hydrogen storage properties of La1-xCexY2Ni10.95Mn0.45 (x = 0 – 0.75) alloys

RE-Y-Ni-based superlattice alloys have been studied for their high hydrogen storage capacity and excellent activation properties. However, their practical applications in gas-solid hydrogen storage devices are limited by their low hydrogen absorption/desorption platform pressure and the occurrence of a double platform phenomenon. In this study, we investigated the phase structure and gas-solid hydrogen storage properties of La1−xCexY2Ni10.95Mn0.45 (x = 0, 0.15, 0.30, 0.45, 0.60, 0.75) alloys. As the Ce content increases, the content of Ce2Ni7 phase and the platform pressure also increase, while the hydrogen storage capacity and hydrogen desorption enthalpy decrease. When the Ce content increases to 0.45, the Gd2Co7 phase disappears and the LaNi5 phase appears, the hydrogen absorption/desorption platform of the alloy changes from a double platform to a single platform. However, the increase in Ce content leads to a higher amount of residual hydrogen during the dehydrogenation process, attributed to the increased content of the Ce2Ni7 phase. The La0.55Ce0.45Y2Ni10.95Mn0.45 alloy exhibits a hydrogen absorption capacity of 1.61 wt%, with a capacity retention of 97.89% after 100 cycles, and achieves a hydrogen desorption platform pressure of 0.23 MPa. These results indicate that this alloy demonstrates the best overall hydrogen storage performance, characterized by high platform pressure and hydrogen storage capacity.

Prolonging cycling life of AB3-type superlattice alloys by adjusting hydrogen absorption/desorption behaviors of [A2B4] and [AB5] subunits

AB3-type La-Mg-Ni-based hydrogen storage alloys are known for their high electrochemical hydrogen storage capacity. However, their practical application has been hindered by fast capacity degradation. Herein, we managed to improve their cycle life by synchronizing the hydrogen absorption/desorption behaviors of [A2B4] and [AB5] subunits based on a series of AB3-type single-phase La0.60R0.12Mg0.28Ni3 (R = La, Pr, Nd, Gd) superlattice alloys. This is achieved by the preferable occupation of Pr, Nd and Gd in [A2B4] subunits rather than [AB5] subunits compared to La, which decreases the volume difference between [A2B4] and [AB5], significantly reducing the mismatch between [A2B4] and [AB5] during hydrogen absorption/desorption. Resultantly, the microstrain of the R = Gd alloy is only half that of the R = La alloy, leading to enhanced anti-pulverization and anti-oxidation/corrosion resistance, as well as an improved cycle stability. The S100 of the R = Gd alloy is more than 15 % higher than the R = La alloy. Moreover, the plateau pressure is adjusted to a proper level, enabling an increase in the high-rate dischargeability (HRD1500) from 53.6 % (R = La) to 66.4 % (R = Gd). This work provides impressive insights into developing high-performance superlattice hydrogen storage alloys.

Compositional tuning of A2B7-type La0.7–xYxMg0.3Ni3.5 alloys for gaseous hydrogen storage

Rare earth-based superlattice alloys have great potential for gaseous hydrogen storage, as well as successful application as nickel-metal hydride batteries anodes. In this work, Y substitution was carried out to adjust the gaseous hydrogen storage properties of A2B7-type La0.7Mg0.3Ni3.5 alloys. The results indicate a multiphase structure in the alloys comprised of the main rhombohedral Gd2Co7 and PuNi3 phases, with a small amount of CaCu5 phase. Moreover, the Y substitution results in higher abundance of the Gd2Co7 phase. The alloy La0.42Y0.28Mg0.3Ni3.5 exhibits a hydrogen storage capacity of 1.55 wt% at 298 K and a desorption plateau pressure of 0.244 MPa. In addition, this alloy demonstrates a stable cycle life by a capacity retention of 94.2% after 50 cycles, with the main capacity degradation occurring during the initial 20 cycles. This work accentuates the potential of the La–Y–Mg–Ni-based superlattice alloys for applications in solid-state hydrogen storage.

Tuning phase structure and electrochemical hydrogen storage properties of A5B19-type La–Y–Ni–Mn-based superlattice alloys by partial Al substitution

Rare earth-based superlattice alloys are essential anodes for high-performance nickel-metal hydride (Ni-MH) batteries. This study aimed to improve the overall electrochemical hydrogen storage properties of (La0.33Y0.67)5Ni18.1-xMn0.9Alx alloys by adjusting the Al content. The alloys predominantly consist of the 2H–Pr5Co19 phase, with Al atoms mainly occupying the 12k2 site at the [AB5]-1/[AB5]-2 boundary and subsequently the 12k1 site at the [A2B4]/[AB5]-1 boundary. As a result, a minimum subunit volume difference of 2.08 Å3 is observed at x = 0.6. Therefore, the Al substitution is responsible for the decrease in the dehydriding plateau pressure and the increase in discharge capacity. Among them, the alloy (La0.33Y0.67)5Ni17.5Mn0.9Al0.6 presents a discharge capacity of 361.2 mAh/g, a higher capacity retention S200 of 69.80 %, and better rate performance owning to the enhanced hydrogen diffusion rate through the multi-phase interface. This work demonstrates the potential of the (La0.33Y0.67)5Ni17.5Mn0.9Al0.6 alloy as high-performance Ni-MH battery anode.

Designing composition ratio of magnetic alloy multilayer for transverse thermoelectric conversion by Bayesian optimization

We demonstrated the effectiveness of the machine learning method combined with first-principles calculations for the enhancement of the anomalous Nernst effect (ANE) of multilayers. The composition ratio of CoNi homogeneous alloy superlattices was optimized by Bayesian optimization so as to maximize the transverse thermoelectric conductivity (αxy). The nonintuitive optimal composition with a large αxy of ∼10 A K−1 m−1 was identified through the two-step Bayesian optimization using rough and fine candidate pools. The Berry curvature and band dispersion analyses revealed that αxy is enhanced by the appearance of the flat band near the Fermi level due to the multilayer formation. The magnitude of the energy derivative of the anomalous Hall conductivity increases owing to the large Berry curvature near the flat band along the R-M high symmetry line, which emerges only in the optimized superlattice, leading to the αxy enhancement. The effective method verified here will broaden the choices of ANE materials to more complex systems and, therefore, lead to the development of transverse thermoelectric conversion technologies.

Optical properties of (GaAs/InAs)–GaAsySb1−y digital alloy superlattices in the short-wavelength infrared region calculated by an sp3d5s* tight-binding method

Optical properties of (GaAs/InAs)–GaAsySb1−y digital alloy superlattices in the short-wavelength infrared region calculated by an sp3d5s* tight-binding method

Grain-boundary segregation and superior mechanical properties in a multicomponent L12 Ni46.5Co24Fe8Al12.5Ti9 superlattice alloy

Ni3Al superlattice alloys with the L12 structure have garnered much attention due to their attractive high-temperature mechanical properties; however, their grain-boundary brittleness and low ductility in the ambient temperature range have greatly restricted their widespread application. In this study, we developed an L12 structure multicomponent Ni46.5Co24Fe8Al12.5Ti9 (at. %) superlattice alloy that notably suppressed the room-temperature intergranular brittleness and exhibited a large tensile elongation of 17.1% ± 5.2% together with a high ultimate tensile strength of 1,080.2 ± 57.4 MPa. Multiple microstructural examinations reveal an L12 equiaxed-grain microstructure, with the presence of a minor B2 phase. Moreover, the co-segregation of Fe and Co atoms, and the associated reduction or elimination of the L12 chemical order at the grain-boundary regions were characterized, which were proved to be the root cause of the suppression of intergranular brittleness and the high tensile ductility. Further theoretical calculations show that alloying of Fe and Co to binary Ni3Al reduced the ordering energy, which promoted intergranular segregation and associated disordering. This observation demonstrated that the elimination or reduction of interfacial chemical order is an effective ductilizing method for superlattice alloys.

Improving structure design of active region of InAs quantum dots by using InAs/GaAs digital alloy superlattice

A 1.3-μm InAs quantum dot laser has been successfully fabricated on a GaAs(100) substrate by molecular beam epitaxy (MBE) technique through using InAs/GaAs digital alloy superlattices instead of the conventional InGaAs layer. The samples grown by conventional growth method and the digital alloy superlattice growth method are characterized by atomic force microscope (AFM) and photoluminescence (PL) spectroscopy. It is found that 8-period sample possesses a low quantum dot density and poor luminescence performance. With the increase of the number of growth periods, the quantum dot density of the sample increases and the luminous performance improves. This indicates that the quality of the grown sample improves with the increase of InAs/GaAs period of the InGaAs layer. When the total InAs/GaAs period is 32, the quantum dot density of the sample is high and the luminescence performance is good. After the experimental measurement, the sample DAL-0 fabricated by conventional growth method and the sample DAL-32 (32-periods InAs/GaAs digital alloy superlattices) are utilized to fabricate quantum dot laser by standard process. The performances of two types of quantum dot lasers obtained with different growth methods are characterized. It is found that the InAs quantum dot lasers fabricated by the sample grown by digital alloy superlattice method have good performances. Under continuous wave operation mode, the threshold current is 24 mA corresponding to a threshold current density of 75 A/cm<sup>2</sup>. The highest operation-temperature reaches 120 ℃. In addition, InAs quantum dot laser using digital alloy superlattice has good temperature stability. Its characteristic temperature is 55.4 K. Compared with the traditional laser, the InAs quantum dot laser grown by InAs/GaAs digital alloy superlattice has good performance in terms of threshold current density, output power and temperature stability, which indicates that high-quality laser can be obtained by this growth method. Using the InAs/GaAs digital alloy superlattice growth method, the InGaAs composition can be changed without changing the temperature of the source oven. Thus InAs quantum dot lasers with different luminescence wavelengths can be obtained through this growth method. The InAs/GaAs digital alloy superlattice structure can be used to realize different averaging of In content in the growth structure. The method provides a new idea for designing and growing the active region of quantum dot laser.

Three-dimensional strain imaging of irradiated chromium using multi-reflection Bragg coherent diffraction

Radiation-induced materials degradation is a key concern in limiting the performance of nuclear materials. The formation of nanoscale void and gas bubble superlattices in metals and alloys under radiation environments can effectively mitigate radiation-induced damage, such as swelling and aid the development of next generation radiation tolerant materials. To effectively manage radiation-induced damage via superlattice formation, it is critical to understand the microstructural changes and strain induced by such superlattices. We utilize multi-reflection Bragg coherent diffraction imaging to quantify the full strain tensor induced by void superlattices in iron irradiated chromium substrate. Our approach provides a quantitative estimation of radiation-induced three-dimensional (3D) strain generated at the microscopic level and predicts the number density of defects with a high degree of sensitivity. Such quantitative evaluation of 3D strain in nuclear materials can have a major impact on predicting materials behavior in radiation environments and can revolutionize design of radiation tolerant materials.

Incoherent-to-coherent crossover in thermal transport through III–V alloy superlattices

We report on time-domain thermoreflectance measurements of cross-plane thermal conductivity of In0.63Ga0.37As/In0.37Al0.63As superlattices with interface densities ranging from 0.0374 to 2.19 nm−1 in the temperature range 80–295 K. The measurements are complemented by a three-dimensional finite-difference time-domain solution to the elastic wave equation, in which the rms roughness and correlation length at heterointerfaces are varied, and the parameters yielding best agreement with experiment are determined using machine learning. Both experimental measurements and simulations demonstrate the existence of a minimum in the cross-plane thermal conductivity as a function of interface density, which is evidence of a crossover from incoherent to coherent phonon transport as the interface density increases. This minimum persists with increasing temperature, indicating the continued dominance of the temperature-independent interface and alloy-disorder scattering over the temperature-dependent three-phonon scattering in thermal transport through III–V alloy superlattices.

Structural evolution and electrochemical hydrogen storage properties of single-phase A5B19-type (La0.33Y0.67)5Ni17.6Mn0.9Al0.5 alloy

La–Y–Ni-based superlattice alloys demonstrate the potential as high-capacity anodes for Nickel-metal hydride batteries. However, the usual multi-phase structure formed in A5B19-type La–Y–Ni-based alloys makes precise investigations of composition-structure-performance relationships difficult. This work optimizes the quenching temperature to obtain the single-phase (La0.33Y0.67)5Ni17.6Mn0.9Al0.5 alloy with a hexagonal 2H–Pr5Co19 structure. The single-phase alloy exhibits better overall electrochemical properties than multi-phase alloys, such as the maximum discharge capacity of 368.8 mAh g−1, the capacity retention at the 200th cycle of 73.4%, and the rate performance at a high discharge current of 1500 mA g−1 of 58.6%. The ex-situ X-ray diffraction research reveals that during charging, the single-phase alloy sequentially forms β and γ hydrides, with the β hydride exhibiting the highest lattice strain, mainly due to the asynchronous hydrogenation and expansion between [A2B4] and [AB5]-2 subunits. This work offers meaningful guidance for developing high-performance La–Y–Ni-based electrode alloys.

Simultaneous enhancement of strength and ductility via microband formation and nanotwinning in an L12-strengthened alloy

Metallic alloys with high strength and large ductility are required for extreme structural applications. However, the achievement of ultrahigh strength often results in a substantially decreased ductility. Here, we report a strategy to achieve the strength-ductility synergy by tailoring the alloy composition to control the local stacking fault energy (SFE) of the face-centered-cubic (fcc) matrix in an L12-strengthened superlattice alloy. As a proof of concept, based on the thermodynamic calculations, we developed a non-equiatomic CoCrNi2(Al0.2Nb0.2) alloy using phase separation to create a near-equiatomic low SFE disordered CoCrNi medium-entropy alloy matrix with in situ formed high-content coherent Ni3(Al, Nb)-type ordered nanoprecipitates (∼ 12 nm). The alloy achieves a high tensile strength up to 1.6 GPa and a uniform ductility of 33%. The low SFE of the fcc matrix promotes the formation of nanotwins and parallel microbands during plastic deformation which could remarkably enhance the strain hardening capacity. This work provides a strategy for developing ultrahigh-strength alloys with large uniform ductility.

Stable Multi‐Wavelength Lasing in Single Perovskite Quantum Dot Superlattice

AbstractSingle perovskite alloy micro‐nano structure capable of emitting multi‐wavelength tunable lasing is desirable for its potential application in highly integrated photonic devices. However, owing to the soft and dynamic crystal lattice of perovskite and the low mobility energy of halide anion, it is still a challenge to obtain alloy micro‐nano structures with a stable bandgap gradient. Herein, based on the highly ordered superlattice consisting of well‐separated individual quantum dots and precise anion exchange strategy, a single perovskite alloy superlattice is obtained with a widely tunable bandgap. The photoluminescence (PL) spectra results indicate that the stability of the bandgap difference of the alloy superlattice sample is approximately 10 times higher than that of the previously reported perovskite single‐crystal alloy nanowires. By combination of quantitative study based on confocal PL measurements and theoretical calculations, the kinetics and atomic‐scale anion exchange mechanism are analyzed, the latter being responsible for the formation of stable bandgap gradient in the alloy superlattice. Multi‐wavelength lasing is further realized in single alloy superlattice. In addition, carrier transportation dynamics reveals the energy‐transfer process in the alloy superlattice, explaining the difficulties of realizing multicolor lasing. The work provides a new approach to solving problems in the field of stable multicolor microlasers.

Cover Picture: Soft Alloys Constructed with Distinct Mesoatoms via Self‐Sorting Assembly of Giant Shape Amphiphiles (Angew. Chem. Int. Ed. 19/2022)

Metal alloy superlattices with large volume asymmetry were constructed based on self-sorting assembly of binary giant shape amphiphiles, as reported by Zebin Su, Stephen Z. D. Cheng et al. in their Research Article (DOI: 10.1002/anie.202200637). MgZn2, NaZn13 and CaCu5 superlattices were observed in soft matters, and the thermodynamic origin of their formation was quantitatively analyzed.

Modulating superlattice structure and cyclic stability of Ce2Ni7-type LaY2Ni10.5-based alloys by Mn, Al, and Zr substitutions

It is a great challenge to achieve high capacity and long cyclic life for hydrogen storage electrode alloys in Ni-MH batteries, especially for superlattice alloys. This work investigates three series of single-phase LaY2Ni10.5-based alloys to tune the Ce2Ni7-type superlattice structure and electrochemical hydrogen storage properties by Mn, Al, and Zr substitutions. The Mn, Al substitutions for Ni could reduce the volume difference between [A2B4] and [AB5] subunits, thus greatly improving the cyclic performance. The maximum discharge capacity for the LaY2Ni9.7Mn0.5Al0.3 alloy is 384.1 mAh g−1, and the capacity retention S200 is as high as 76.1%. However, the partial replacement of Y with Zr dramatically reduces the discharge capacity and cyclic stability, although the subunit volume difference is even zero in the LaY1.75Zr0.25Ni9.7Mn0.5Al0.3 alloy. Detailed structural analysis shows that LaY1.75Zr0.25Ni9.7Mn0.5Al0.3 alloy exhibits greater a-axis expansion (Δa/a = 4.52%) and cell volume change (Δc/c = 15.46%) during charging than the Mn, Al-substituted alloys. The unusual expansion at the basal plane of [A2B4] and [AB5] subunits should be responsible for a considerable lattice strain (1.42%) and poor cyclic performance in the Zr-substituted alloys. This work provides new insights into the design of high capacity and long-life superlattice electrode alloys.

A short review of defect superlattice formation in metals and alloys under irradiation

Irradiation damage drives complex and coupled phenomena in materials at far-from-equilibrium conditions. The self-organization of nanoscale defects in materials under irradiation shows great potential to tailor the physical properties of materials by controlling nanopatterned microstructures. Irradiation-induced gas bubble and void superlattices are two important ordered nanostructures of great scientific interest. Although both types of superlattices have been investigated extensively, a consensus has yet to be reached on their formation mechanisms. In this review article, the current research status of gas bubble and void superlattices in metals and alloys and their characterization, structural stability, and mechanistic modeling are summarized. The fundamental research goals to advance the mechanistic understanding of gas bubble and void superlattices are outlined.

A new strategy for enhancing the cycling stability of superlattice hydrogen storage alloys

For the first time, a strategy to equalize the subunit volumes of [A 2 B 4 ] and [AB 5 ], and meanwhile increase the oxidation/corrosion resistance for superlattice hydrogen storage alloys is proposed. • A novel sublattice modification strategy for supperlattice HSAs is proposed. • V [A2B4] and V [AB5] are equalized in a superlattice alloy for the first time. • Oxidation/corrosion resistance is enhanced by adding Gd with high electronegativity. • Cycling stability is significantly improved without losing capacity or HRD. Superlattice hydrogen storage alloys are one of the most potential candidates for the negative electrode materials of nickel-metal hydride batteries. However, their crystal structure damage and oxidation/corrosion during battery cycling severely deteriorate their cycle life. In this work, we propose a new strategy based on the alloys’ structure characteristics analysis and First-principles calculation to enhance their cycling stability. The strategy equalizes the sublattice volumes of [A 2 B 4 ] and [AB 5 ] by atomic selective occupation and meanwhile increases the alloys’ electronegativity. The feasibility of the strategy is validated experimentally by designing a series of single-phase superlattice alloys with simple elemental compositions of La 0.75- x Gd x Mg 0.25 Ni 3.5 ( x = 0, 0.05, 0.1, 0.15). It is found that when appropriate amount of Gd is added, the sublattice volumes of [LaMgNi 4 ] and [LaNi 5 ] become equal, so that the alloy maintains a stable crystal structure in the long-term battery cycling. Moreover, the alloys’ oxidation/corrosion resistance is enhanced due to the high electronegativity of Gd. The alloys’ capacity retention after 100 cycles is improved from 82.1% (La 0.75 Mg 0.25 Ni 3.5 ) to 88.2% (La 0.60 Gd 0.15 Mg 0.25 Ni 3.5 ) without sacrificing the maximum discharge capacity, and meanwhile the high rate dischargeability is also increased. The design strategy provides new insights for overcoming the weakness in the cycling stability of superlattice hydrogen storage alloys, thus promoting their practical applications.

Phase transformation and hydrogen storage properties of LaY2Ni10.5 superlattice alloy with single Gd2Co7-type or Ce2Ni7-type structure

Abstract The structural evolution of as-cast LaY2Ni10.5 multi-phase alloy with the annealing temperature has been investigated, and the single-phase alloy with rhombohedral Gd2Co7-type or hexagonal Ce2Ni7-type structure was firstly prepared. It is found that the rhombohedral structure is more stable than the hexagonal polymorph at the low temperature, which is reversed at the high annealing temperature. Structural refinement indicates that Y atoms preferably substitute La in the [A2B4] subunit of both hexagonal and rhombohedral superlattice structures. Compared with the rhombohedral polymorph, the hexagonal phase possesses a relatively smaller volume of [A2B4] subunit and thus demonstrates higher structural stability during the hydrogen charging-discharging cycling. This work offers the guideline for the preparation process and composition optimization of single-phase rare earth-based A2B7-type alloys for nickel-metal hydride battery applications.