Shock induced stepwise phase transitions in reactive high entropy alloy

In recent studies, it has been established that higher-order interactions in coupled oscillators can induce a process from continuous to explosive phase transition. In this study, we identify a phase transition, termed the stepwise explosive phase transition, characterized by the emergence of multiple critical phase plateaus in a globally frequency-weighted coupled pendulum model. This transition bridges the continuous and explosive phase transitions, arising from a delicate balance between attractive higher-order interactions and repulsive pairwise interactions. Specifically, the stepwise explosive phase transition occurs when the higher-order coupling is moderate, neither large nor small, while the pairwise interactions remain repulsive. Our analysis shows that stronger attractive higher-order interactions necessitate weaker repulsive pairwise interactions, leading to partial frequency locking among oscillators and triggering the stepwise transition. We construct an analytical framework using self-consistent equationsto provide an approximation of the steady-state behavior. This study uncovers an alternative pathway to the desynchronization, and it provides additional insights into the phase transitions in coupled dynamical networks.

The properties of layered intercalation hybrids are closely related to interlayer molecular packing. To develop functional intercalation hybrids, it is essential to gain deep insights into interlayer molecular packing. This work reports a new comprehensive insight into the controllable multiphase interlayer molecular packing in 4-(phenylazo)benzoate anion-intercalated layered zinc hydroxide (LZH-4-PAB intercalation hybrids). The new insight breaks up the general understanding that the interlayer molecular packing of anions is usually single-phase, lacking diversity and controllability. Furthermore, it uncovers an interesting stepwise rather than the generally expected continuous phase transition of the interlayer molecular packing. The intercalated 4-PAB anions initially organize into the horizontal monolayer packing (θ = 0°, Phase I), which stepwise transforms to the tilted interdigitated antiparallel bilayer packing (θ ≈ 50°, Phase II) along with an increased intercalation loading and eventually to the vertical interdigitated antiparallel bilayer packing (θ = 90°, Phase III). The LZH-4-PAB hybrids exhibited a greatly enhanced interlayer molecular packing-dependent UV-vis absorption. This study provides helpful guidance for developing property-tailored intercalation hybrids. It may attract new interest in more layered intercalation hybrids. New and rich intercalation chemistry might be discovered in more functional intercalation hybrids beyond the 4-PAB anion-intercalated layered zinc hydroxide.

As illustrated by recent extensive crystallographic studies, the structural richness of the ternary Li/Si/P system offers a unique variety of physical properties. Taking advantage of 3D networks enabling fast ionic conduction properties, new phosphorus silicides based electrodes have recently emerged, exhibiting attractive performance in Li-ion as well as in Na-ion batteries. To date, SiP2 reaction mechanism vs Li or Na has nevertheless not been elucidated. The combination of operando X-ray diffraction and ex situ 31P, 7Li, 23Na, and 19F MAS NMR spectroscopy allows here probing of both long-range structural information and evolution of local environments. These complementary techniques lead to deeper understanding and depiction of the mechanism behind the lithiation/delithiation and sodiation/desodiation of SiP2. The versatility of connection of tetrahedral SiP4 anions, from three-dimensional network to isolated SiP4 units, stabilized by an increasing number of counterions as a function of the state of charge/discharge is the driving force of the new SiP2/Li and SiP2/Na batteries. This richness of SiP4 anions’ crystallographic network allows stepwise phase transitions during lithiation/delithiation which limit the detrimental effects of volume expansion.

A new class of materials created at the beginning of the 21st century – high–entropy alloys – attracts the attention of researchers in the field of physical materials science. Based on the analysis of recent literature data, the current state of the problem of creating and researching medium- and high-entropy high-speed steels is considered. Due to solid-solution hardening and nano-precipitation hardening based on medium- and high-entropy alloys of complex composition, it is possible to create high-speed steels with high hardness, thermal resistance and impact strength. The presented results of studies of tribological characteristics and microhardness of high-speed steels indicate the dependence of these characteristics on entropy. The lowest values of cutting forces and contact temperatures are typical for cutting tools made of high-speed steel with a high level of entropy. Thus, when developing new high-speed grades, preference should be given to the compositions with a high level of entropy, since they provide better tribological characteristics and higher wear resistance. The structural and phase state of surfacing of high-entropy high-speed molybdenum steel of non-equiatomic composition on medium-carbon steel in a nitrogen medium was studied by the methods of modern physical materials science. X-ray spectral analysis methods determined the elemental composition of surfacing outer layer, and X-ray phase analysis revealed that solid solutions based on α-iron (88 wt. %) and γ-iron (12 wt. %) are the main phases of the deposited layer material. The calculation of the configuration entropy of this high-speed high-entropy steel gives a value of 1.93R (where R is the universal gas constant). The conclusion is made about the relevance and prospects of the development and research of high-energy alloys.

Evaluation of dissimilar metal joining of aluminum alloy to stainless steel using the filler metals with a high-entropy design

28aTK-13 Stepwise neutral-ionic phase transitions in a covalently-bonded donor/acceptor chain compound, [{Ru_2(O_2CPh-F_4)_4}(DMDCNQI)

The past two decades have seen a great enhancement of piezoelectric coefficients (d33) to higher than 570 pC/N in (K, Na)NbO3 (KNN) piezoelectrics, but one notoriously unresolved issue is their severe temperature instability, obstructing them toward practical applications. The present work demonstrates a facile approach to overcome this problem by introducing a layered distribution of key dopants (Li and Sb) in a monolithic ceramic, featuring stepwise varied polymorphic phase transition (PPT) temperatures along the thickness direction. The dopant‐graded ceramic exhibits an outstanding d33 of 508 pC/N and a very large piezoelectric strain (Suni, of 0.18%). More importantly, an excellent temperature stability (d33 variation within 13% over the temperature range of 25–150 °C) is achieved, which is superior to that of most state‐of‐art KNN counterparts. These are attributed to the construction of spatially diffused PPT in combination with enhanced polarization, permittivity, and piezoresponse through interfacial effect, including the Maxwell–Wagner effect and intergranular stress by gradient doping. The results offer an alternative strategy for designing high‐performance piezoelectric materials with desirable temperature reliability.

The field of multi-principal element or (single-phase) high-entropy (HE) alloys has recently seen exponential growth as these systems represent a paradigm shift in alloy development, in some cases exhibiting unexpected structures and superior mechanical properties. However, the identification of promising HE alloys presents a daunting challenge given the associated vastness of the chemistry/composition space. We describe here a supervised learning strategy for the efficient screening of HE alloys that combines two complementary tools, namely: (1) a multiple regression analysis and its generalization, a canonical-correlation analysis (CCA) and (2) a genetic algorithm (GA) with a CCA-inspired fitness function. These tools permit the identification of promising multi-principal element alloys. We implement this procedure using a database for which mechanical property information exists and highlight new alloys having high hardnesses. Our methodology is validated by comparing predicted hardnesses with alloys fabricated by arc-melting, identifying alloys having very high measured hardnesses.

ABSTRACTThe benefits of using the pseudo-binary and pseudo-ternary diffusion couple methods in multicomponent inhomogeneous systems are demonstrated by estimating different types of composition-dependent diffusion coefficients. These are important for understanding the basic atomic mechanism of diffusion and complex compositional evolutions. These were otherwise considered impossible during the last many decades. Without any options previously, sometimes the average values over a composition range of random choice were estimated, which are not the material constants but depend on the composition range and also the end member compositions. The steps and analyses for utilising the pseudo-binary and pseudo-ternary methods are first described in the Ni-Co-Fe-Mo system by producing the ideal diffusion profiles fulfilling the concepts behind these methods. Following, the discussion is extended to the systems related to medium (Ni-Co-Cr) and high (Ni-Co-Fe-Mn-Al) entropy alloys. In fact, this is the first report showing a correct experimental method that should be followed for the estimation of the interdiffusion and intrinsic diffusion coefficients in inhomogeneous high entropy alloys. In the end, the limitations of following these methods because of the generation of non-ideal diffusion profiles are discussed based on experimental results. The steps are also suggested to avoid such complications. These methods are easy to adopt for research engineers. Most importantly, these give an opportunity to validate the data estimated following newly proposed numerical methods by different groups with experimentally estimated diffusion coefficients, which were not possible earlier.

BackgroundLaser metal deposition (LMD) is a widely used additive manufacturing technique for producing complex high entropy alloys with special properties for several applications. The AlCoCrFeNiCu HEAs compositional design has six elements with a configurational entropy of 1.79 R and atomic concentrations between 5 and 35%, so the HEA system is thermodynamically favorable according to Boltzmann’s theory, attributed to the core effects. However, the high-entropy alloy has dominant Body-Centered Cubic structures which may be too brittle to be examined in tension experimentally. Preheating the substrate before and during layer deposition could be a potential solution that is currently under development since tensile loading necessitates an understanding of a material's behavior under tension through an analysis of its yield and ultimate tensile strength. A computer-aided design (CAD) solid model was used to generate the near-net dog-bone form of the alloy with moderately complicated geometrical characteristics using laser metal deposition (LMD) technology. This study investigates a straightforward and effective computational model for simulating material properties, using COMSOL Multiphysics 5.4 software for laser-deposited high entropy alloys that are excessively brittle to be tested in tension. The AlCoCrFeNiCu high-entropy alloy "dog bone" test sample was modeled using COMSOL Multiphysics for tensile loading. The first principal stresses and longitudinal strain under axial loading conditions were measured using a three-dimensional (3D) structural mechanics’ model.ResultsThe results showed the ultimate tensile strength is 8.47 N/m2, attributed to the high entropy effect and the dominant phase structure of the alloy.ConclusionNumerical models in this paper demonstrate the effect of stresses on the tensile behavior of the AlCoCrFeNiCu high-entropy alloy. The model optimizes the LMD process by analyzing residual stresses and predicting tensile strength, thus, providing insights that show the potential of high entropy alloys for structural integrity in aerospace applications.

The photocatalytic performance of high entropy alloy (HEA) and its oxide form (high entropy oxide, HEO) have been evaluated in this study. The HEA, composed of Nb, Ta, Zr, Mo, and W powders, was synthesized through a mechanical alloying process for 120 h. This process was carried out under a high-purity Ar atmosphere to prevent oxidation. Subsequently, the HEA was converted into HEO via a mechano-thermal oxidation method 900 °C for 12 h. The photocatalytic activity of both oxidized and non-oxidized samples was systematically evaluated by degradation of methyl blue (MB) under UV irradiation. Electron dispersive spectroscopy (EDS), X-ray diffraction (XRD), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS) were used to investigate the structural and chemical features of the alloys. Furthermore, BET surface area analysis and Zeta potential measurements were conducted to understand the alloys’ surface properties and stability in aqueous solutions. The results showed that the HEA has a remarkable photodegradation performance of around 58.77% which is two times higher than that observed with the HEO.Graphical abstract

In this study the effect of three different nickel concentration on the microstructure, hardness and corrosion properties of high entropy alloys (HEAs) from AlCrFeCoNi system as an alternative material for medical instruments fabrication was investigated. The analyzed HEAs were AlCrFeCoNix obtained by vacuum arc remelting from high purity raw materials and having nickel atomic ratio x = 1.0, 1.4 and 1.8. The microscopy examination revealed the dendritic morphology for the reference alloy (AlCrFeCoNi) and that the extent of the interdendritic areas increased with the concentration of nickel while Cr was more segregated in the interdendritic areas than in dendrites. Hardness values decreased as the percentage of nickel increased due to the dissolution of the precipitates in a nickel-rich matrix and consequently the formation of continuous solid solutions. The corrosion properties of the synthesized HEAs were evaluated using a potentiodynamic polarization method. The alloys were immersed in Simulated Body Fluid during one week and the corrosion parameters were recorded. The low corrosion rates, low corrosion currents and high polarization resistance attest the good stability of these HEAs in simulated biological environment indicating their possible use for surgical and dental instruments.

A review on fundamental of high entropy alloys with promising high–temperature properties

High-entropy alloys (HEAs), defined as solid solution alloys which have at least 5 principal elements but no more than 13 elements, with concentrations of each principal element ranging from 5% to 35% in atomic fraction, are emerging as one of the hot research frontiers in the metallic materials field. The significance of HEAs originates from their various combinations of high strength, good thermal stability and excellent resistance to corrosion, wear and oxidation. HEAs exhibit simple solid solutions with bcc and/or fcc structure(s) due to the effect of high mixing entropy in the solid solution state of HEAs, which may make the HEAs with improved mechanical and physical properties. However, a small quantity of intermetallic compounds can also form in certain HEAs, indicating that the formation of simple solid solutions cannot solely depend on the high mixing entropy. Then, the theory of HEAs based on the concept of entropy-enthalpy competition to judge whether or not simple phases will form was proposed. However, even if an alloy meets these criterions, it can still contain intermetallic phases. Why *国家自然科学基金项目51271126和沈阳市科技计划项目F16-032-0-00资助 收到初稿日期: 2016-01-02,收到修改稿日期: 2016-02-20 作者简介:吴臣亮,男, 1989年生,博士生 DOI: 10.11900/0412.1961.2016.00004

Microstructure and strengthening mechanisms in an FCC structured single-phase nanocrystalline Co25Ni25Fe25Al7.5Cu17.5 high-entropy alloy