Coherent Boundaries Research Articles

Superconducting Boride Nb2IrB2 with Variable and Tunable Stacking Behaviors of Two-Dimensional [Nb-Ir-Nb] Triangular Lattices.

In structures with special geometry lattices, variations in stacking sequences are ubiquitous, yielding many novel structures and functionalities. Despite a wealth of intriguing properties and wide-ranging applications, there remains a considerable gap in understanding the correlation between special geometry lattices and functionalities in borides. Here, we design and synthesize a new superconducting boride Nb2IrB2, with a body-centered orthorhombic structure, consisting of alternating two-dimensional [Nb-Ir-Nb] triple-triangular-lattice-layers and B fragment layers. Advanced aberration-corrected scanning transmission electron microscopy observations show variable stacking configurations between [Nb-Ir-Nb] triple-triangular-lattice layers that can be tuned through synthesis conditions. Density functional theory calculations reveal that the coherent low-energy boundary interface plane of {101} between [11̅1] and [010] domains is responsible for the variable stacking behaviors. Energetically favorable structures are thereby reasonably proposed, based on nanoscale imperfect structure units. These findings provide valuable insights for designing and exploring new structures and functionalities within boride systems involving special geometry lattices.

Fine-Grain High-Performance Densified Oxide Fibers Produced by Open Ultrafast High-Temperature Sintering.

The considerable grain growth occurring during the long-term high-temperature sintering of polycrystalline oxide fibers negatively affects their mechanical properties, which highlights the need for alternative sintering methods. Herein, open ultrafast high-temperature sintering (OUHS) in air, characterized by rapid heating/cooling (>10000 K min-1) and a short high-temperature holding time (<10 s), is used to produce 3 mol% yttria-stabilized zirconia continuous fibers with coherent boundaries forming robust connections between fine grains. The tensile strength of these fibers (2.33GPa on average, sintering temperature = 1673 K) notably exceeds that of their counterparts produced by traditional sintering (1.17GPa). The effects of pores on fiber mechanicalproperties are analyzed using experimental and theoretical methods. For a versatility demonstration, OUHS is applied to several types of polycrystalline oxide fibers (HfO2, Al2O3, TiO2, Y2O3, and La2Zr2O7), considerably improving their mechanical properties and enabling crystalline phase control, which demonstrates the suitability of this procedure for the development of high-performance materials.

Coherent growth of high-Miller-index facets enhances perovskite solar cells.

Obtaining micron-thick perovskite films of high quality is key to realizing efficient and stable positive (p)-intrinsic (i)-negative (n) perovskite solar cells1,2, but it remains a critical challenge. Here, we report an effective method for producing high-quality, micron-thick formamidinium-based perovskite films by forming coherent grain boundaries, where high-Miller-index-oriented grains grow on the low-Miller-index-oriented grains in a stabilized atmosphere. The resulting micron-thick perovskite films, with enhanced grain boundaries and grains, showed stable material properties and outstanding optoelectronic performances. The small-area solar cells achieved efficiencies of 26.1%. The 1-square-centimeter devices and 5 cm × 5 cm minimodules delivered efficiencies of 24.3% and 21.4%, respectively. The devices processed in a stabilized atmosphere presented a high reproducibility across all four seasons. The encapsulated devices exhibited superior long-term stability under both light and thermal stressors in ambient air.

Selectivity as discrimination: lessons from the case law on fiscal measures for identifying State aid

Abstract Article 107(1) of the Treaty on the Functioning of the European Union was originally designed to regulate the grant of subsidies by Member States, but it has been applied increasingly in recent years to regulate tax measures. Attempts to apply the State aid rules in this way have been met with controversy as they have been applied in unpredictable ways that have introduced significant changes to the existing tests in the case law for identifying State aid. These changes include a broader interpretation of the concept of selectivity, one of the criteria used to identify aid, which has been reoriented around the discrimination standard, whereby a measure is selective if it differentiates between categories of undertakings without being capable of justification by some legitimate public policy objective. Most controversially, this has facilitated the enforcement of the State aid rules against a wider range of State interventions. This article argues that the discrimination standard represents the most plausible and effective way of understanding selectivity among the alternative tests which have been proposed in the case law and academic literature. However, it goes on to identify a significant deficiency in the discrimination standard in the manner in which it is currently applied by the Union courts. The test inevitably requires certain value judgments to be made, but these are not made explicit by the existing case law. This article then proceeds to give further reflections on options for the refinement of the framework for assessing selectivity based on the discrimination standard and the existing logic of the case law and the objectives of the State aid control regime, which indicates how principled and coherent boundaries to the notion of State aid might be assessed in an individual case.

Nd isotope mapping in the Go Home region of Georgian Bay: a misunderstood key to the geology of the Central Gneiss Belt, Grenville Province of Ontario

The Georgian Bay transect forms an important geological section across the deeply exhumed Central Gneiss Belt of the Ontario Grenville Province. Its geological mapping was one of Nick Culshaw’s major research contributions, but the tectonic structure of the southern part of this transect in the Go Home region was never properly established. New Nd isotope data show that the classically recognized Go Home domain should be divided into three structural “decks” with distinct TDM (depleted mantle) Nd model ages. The upper deck consists of the Pine Island and Honey Harbour domains, with TDM ages averaging 1.56 and 1.57 Ga, respectively, which form two isolated units of the Grenvillian allochthonous belt. These are underlain by the newly-recognized Six Mile Lake domain, whose average TDM age of 1.71 Ga is representative of the tectonic duplex entrained elsewhere onto the base of the allochthon. Finally, the lowermost deck yields an average TDM age of 1.87 Ga, which identifies it as part of the Grenvillian parautochthon, exposed here in a tectonic window. The presence of a large retrogressed eclogite body within the allochthonous Honey Harbour domain confirms the exhumation of this unit from the deep crust on a major structural ramp. The coherent boundaries of these domains are consistent with the ramp–flat thrusting model of the Ottawan (Grenvillian) orogeny and limit the action of later extensional tectonics in the southwest Grenville Province.

Study on tensile behavior at various temperatures of the Mg-7Gd-5Y-1Nd-2Zn-0.5Zr alloy

Mg-rare earth (RE)-Zn alloys with long-period stacking ordered (LPSO) structure show excellent room temperature, elevated temperature mechanical properties. In this work, microstructure and mechanical properties of Mg-7Gd-5Y-1Nd-2Zn-0.5Zr (wt%) alloy after tensile deformation at various temperatures (room temperature (RT), 250 °C and 300 °C) were investigated. The pre-precipitated alloy exhibited tensile yield strength (TYS) of 149 MPa at room temperature, while it maintained relatively high TYS at 250 °C (154 MPa) and 300 ℃ (123 MPa). The microstructural characterizations revealed that such outstanding heat resistance is mainly attributed to the blocky LPSO structure near the grain boundary; basal plate-like precipitates, twins and kink bands inside the grain. The blocky LPSO structure exhibits a coherent boundary with one α-Mg grain, while being incoherent with other α-Mg grains or the LPSO structure. Cracks primarily occured along the incoherent interface. Basal plate-like precipitates formed during elevated temperature tensile deformation included GP zones, γ', metastable LPSO building blocks, and fine 14H-LPSO structure. A probable γ-precipitation sequence is supersaturated solid solution → GP zones (ABAB-type) → single LPSO building block (γ')→ various metastable LPSO building block clusters→ 14H-LPSO (γ). The deformation modes (sliding, twinning, kinking, grain boundary sliding, etc) modified with the tensile temperature increase. <c+a> dislocations were observed at elevated temperatures. As the temperature of tensile test increases, twins become less. Grain boundary sliding occurs at elevated temperatures.

Concurrent dramatic enhancement of high-temperature strength and ductility in a high-entropy alloy via chain-like dual-carbides at grain boundaries

Grain boundaries (GBs) are often known as intergranular cracking sources in alloys at high temperatures, resulting in limited high-temperature strength and ductility. Here, we propose a GB-dual-carbide (denoted as GB-DC) strengthening strategy and have developed a high-performance (NiCoFeCr)99Nb0.5C0.5 high-entropy alloy (HEA) with exceptional strength-ductility synergy at 1073 K. Chain-like coherent M23C6 carbides have been successfully introduced at GBs and remain a cube parallel crystallographic orientation with the face-centered cubic (FCC) matrix during deformation. Nano-scale NbC particles are distributed alternatively between M23C6 carbides and inhibit their coarsening. Both strength and ductility of the GB-DC HEA increase dramatically at strain rates ranging from 10−4 to 10−2 s−1 at 1073 K, compared with those of the single-phase NiCoFeCr HEA. Specifically, yield strength of 142 MPa, ultimate tensile strength of 283 MPa, and elongation of 34 % were obtained, which are twice that of the reference NiCoFeCr HEA (82 MPa, 172 MPa, and 18 %, respectively). EBSD investigations demonstrated that chain-like carbides enhance the GB cohesion at high temperature, and TEM analysis revealed that dislocations can go through the coherent phase boundaries (CPBs) and activate dipoles inner M23C6 carbides, which weakened the stress concentration in GBs. This substantially reduces the critical stress for dislocation generation and transmission to a stress level lower than that required for intergranular fracture. Theoretical estimation suggests that carbides result in a much higher activation energy (∼510 kJ/mol) for GB sliding and a rather low interface energy (∼101 mJ/m2) compared with the GB energy (1000 mJ/m2), which rationalizes the enhanced GB cohesion by carbides.

Synergistic Performance of Thermoelectric and Mechanical in Nanotwinned High-Entropy Semiconductors AgMnGePbSbTe5.

Introducing nanotwins in thermoelectric materials represents a promising approach to achieving such a synergistic combination of thermoelectric properties and mechanical properties. By increasing configurational entropy, a sharply reduced stacking fault energy in a new nanotwinned high-entropy semiconductor AgMnGePbSbTe5 is reached. Dense coherent nanotwin boundaries in this system provide an efficient phonon scattering barrier, leading to a high figure of merit ZT of ≈2.46 at 750K and a high average ZT of ≈1.54 (300-823K) with the presence of Ag2Te nanoprecipitate in the sample. More importantly, owing to the dislocation pinning caused by coherent nanotwin boundaries and the chemical short-range disorder caused by the high configurational entropy effect, AgMnGePbSbTe5 also exhibits robust mechanical properties, with flexural strength of 82MPa and Vickers hardness of 210HV.

Determination of five-parameter grain boundary characteristics in nanocrystalline Ni-W by scanning precession electron diffraction tomography

Determining the full five-parameter grain boundary characteristics from experiments is essential for understanding grain boundaries impact on material properties, improving related models, and designing advanced alloys. However, achieving this is generally challenging, in particular at nanoscale, due to their 3D nature. In our study, we successfully determined the grain boundary characteristics of an annealed nickel-tungsten alloy (NiW) nanocrystalline needle-shaped specimen (tip) containing twins using Scanning Precession Electron Diffraction (SPED) Tomography. The presence of annealing twins in this face-centered cubic (fcc) material gives rise to common reflections in the SPED diffraction patterns, which challenges the reconstruction of orientation-specific virtual dark field (VDF) images required for tomographic reconstruction of the 3D grain shapes. To address this, an automated post-processing step identifies and deselects these shared reflections prior to the reconstruction of the VDF images. Combined with appropriate intensity normalization and projection alignment procedures, this approach enables high-fidelity 3D reconstruction of the individual grains contained in the needle-shaped sample volume. To probe the accuracy of the resulting boundary characteristics, the twin boundary surface normal directions were extracted from the 3D voxelated grain boundary map using a 3D Hough transform. For the sub-set of coherent Σ3 boundaries, the expected {111} grain boundary plane normals were obtained with an angular error of <3° for boundary sizes down to 400 nm². This work advances our ability to precisely characterize and understand the complex grain boundaries that govern material properties.

Screen printing Ag2Se/carbon nanocomposite films for flexible thermoelectric applications

Flexible thermoelectric generators offer the possibility of harvesting waste heat from the human body. However, intrinsic brittleness and the high production cost of bulk thermoelectric materials limit their applications in flexible thermoelectric generators. Herein, flexible Ag2Se/carbon nanocomposite films on polyimide substrates are prepared by scalable screen-printing followed by facial heat treatment. An optimal film shows a maximum power factor of 1617 μW m−1 K−2 at room temperature and good flexibility. The film consists of mainly Ag2Se grains with a preferred (00Ɩ)-orientation and a small amount of carbon. Microstructure observations reveal that the Ag2Se grains with sizes of nano to micrometers have coherent or continuous grain boundaries, and the carbon is mainly amorphous with slight graphitization. A flexible thermoelectric device assembled with the optimal film exhibits a power density of 29.1 W/m2 at a temperature gradient of 35.4 K. This work demonstrates a cost-effective route to high-performance flexible thermoelectric films.

The Effect of Refined Coherent Grain Boundaries on High-Temperature Oxidation Behavior of TiAl-Based Alloys through Cyclic Heat Treatment

The grain size of the full lamellae TiAl-based alloy changes from ~400 μm to ~40 μm through the precipitation of metastable structures by cyclic heat treatment. Based on this, two kinds of variant selection processes—coherent metastable γ variants precipitated during the air-cooling process and αs variants precipitated during the holding at a single α phase region process—are identified to promote the formation of refined Type I and Type II coherent grain boundaries. The oxidation tests at 1000 °C for 100 h show that the formation of refined coherent grain boundaries can greatly improve oxidation resistance by inducing the continuous multi-layer protective barrier consisting of (Ti, (Nb, Ta))O2, TiN, and Al(Nb,Ta)2. This protective barrier inhibits the inward diffusion of oxygen and nitrogen.

Influence of Ti on enhancing the strength and ductility of Fe-rich medium-entropy alloys: A combined theoretical and experimental study

Developing body-centered-cubic (BCC) typed high/medium-entropy alloys (H/MEAs) with an exceptional combination of high strength and ductility remains a challenge for structural applications. Herein, by adjusting phase composition, an ambient ductile TixNi0.6CoFe1.4 (x = 0, 0.05 and 0.1) MEAs were prepared. The effects of Ti on the mechanical properties of TixNi0.6CoFe1.4 MEAs are revealed by combination of experiments and first principles calculations. The addition of Ti not only promotes the formation of (Co, Ti)-rich face-centered cubic (FCC) and (Ni, Ti)-rich R phases in BCC matrix, but also refines the microstructure of the TixNi0.6CoFe1.4 alloys. The mechanical results show that a proper Ti content can significantly improve the ductility of the alloys with sustaining a high strength. Specifically, the yield strength, tensile strength and fracture elongation of Ti0.1Ni0.6CoFe1.4 alloy are 733.0 ± 2.5 MPa, 1003.9 ± 14.9 MPa and 20.1 ± 0.4 %, respectively, which are increased by 22.4 %, 33.3 % and 286.5 % compared to single-phase Ni0.6CoFe1.4 alloy. The impressive mechanical properties are mainly attributed to the synergetic effect of the grain refinement, precipitation strengthening and the coherent boundaries between the precipitate-matrix. Furthermore, the first principles calculation results suggest that the charge enrichment of the Ti site and its induced sparseness of the surrounding charge are responsible for the strength-ductility equilibrium.

Coherent Phase Change in Interstitial Solutions: A Hierarchy of Instabilities.

Metal hydrides or lithium ion battery electrodes can take the form of interstitial solid solutions with a miscibility gap. This work discusses theory approaches for locating, in temperature-composition space, coherent phase transformations during the charging/discharging of such systems and for identifying the associated transformation mechanisms. The focus is on the simplest scenario, where instabilities derive from the thermodynamics of the bulk phase alone, considering strain energy as the foremost consequence of coherency and admitting for stress relaxation at free surfaces. The extension of the approach to include capillarity is demonstrated by an example. The analysis rests on constrained equilibrium phase diagrams that are informed by geometry- and dimensionality-specific mechanical boundary conditions and on elastic instabilities-again geometry-specific-as implied by the theory of open-system elasticity. It is demonstrated that some scenarios afford the analysis of chemical stability to be based entirely on a linear stability analysis of the mechanical equilibrium, which provides closed-form solutions in a straightforward manner. Attention is on the impact of the system geometry (infinitely extended or of finite size) and on the chemical (closed or open system) and mechanical (incoherent or coherent) boundary conditions. Transformation mechanism maps are suggested for documenting the findings. The maps reveal a hierarchy of instabilities, which depend strongly on each of the above characteristics. Specifically, realistic, finite-sized systems differ qualitatively from idealized systems of infinite extension. Among the transformation mechanisms exposed by the analysis are a uniform switchover to the other phase when the open system reaches its chemical spinodal, practical coherent nucleation, as well as chemo-elastically coupled spontaneous buckling modes, which may take the form of either, single-phase or dual-phasestates.

Evolution of helium bubbles in FeCoNiCr-based high-entropy alloys containing γ′ nanoprecipitates

Abstract A series of high-entropy alloys (HEAs) containing nanoprecipitates of varying sizes is successfully prepared by a non-consuming vacuum arc melting method. In order to study the irradiation evolution of helium bubbles in the FeCoNiCr-based HEAs with γ′ precipitates, these samples are irradiated by 100-keV helium ions with a fluence of 5×1020 ions/m2 at 293 K and 673 K, respectively. And the samples irradiated at room temperature are annealed at different temperatures to examine the diffusion behavior of helium bubbles. Transmission electron microscope (TEM) is employed to characterize the structural morphology of precipitated nanoparticles and the evolution of helium bubbles. Experimental results reveal that nanosized, spherical, dispersed, coherent, and ordered L12-type Ni3Ti γ′ precipitations are introduced into FeCoNiCr(Ni3Ti)0.1 HEAs by means of ageing treatments at temperatures between 1073 K and 1123 K. Under the ageing treatment conditions adopted in this work, γ′ nanoparticles are precipitated in FeCoNiCr(Ni3Ti)0.1 HEAs, with average diameters of 15.80 nm, 37.09 nm, and 62.50 nm, respectively. The average sizes of helium bubbles observed in samples after 673-K irradiation are 1.46 nm, 1.65 nm, and 1.58 nm, respectively. The improvement in the irradiation resistance of FeCoNiCr(Ni3Ti)0.1 HEAs is evidenced by the diminution in bubbles size. Furthermore, the FeCoNiCr(Ni3Ti)0.1 HEAs containing γ′ precipitates of 15.8 nm exhibits the minimum size and density of helium bubbles, which can be ascribed to the considerable helium trapping effects of heterogeneous coherent phase boundaries. Subsequently, annealing experiments conducted after 293-K irradiation indicate that HEAs containing precipitated phases exhibits smaller apparent activation energy (E a) for helium bubbles, resulting in larger helium bubble size. This study provides guidance for improving the irradiation resistance of L12-strengthened high-entropy alloy.

High thermoelectric properties in polycrystalline SnSe materials realized by rare earth halide Co-doping

SnSe materials have garnered significant interest due to the intrinsic low thermal conductivity. However, its limited electrical conductivity hinders their practical implementation. This study achieved a high ZT value of 1.40 at 773 K in the samples of n-type polycrystalline SnSe0.95 + x wt% SmCl3 + y wt% ErCl3 (x = 0, 0.75, 1.0, 1.25, and 1.5; y = 0, 0.0675, 0.125, and 0.25) since (SmCl3, ErCl3) co-doping decoupled the electrical-thermal transport properties. First, the released electron carrier concentration due to SmCl3 doping enhanced the electrical transport properties. The SnSe0.95 + 1.25 wt% SmCl3 sample exhibited a high power factor of 563 μWm−1K−2 at 773 K. Then, the lattice thermal conductivity markedly decreased with further ErCl3 doping due to the high-density dislocations and coherent boundary, resulting in effective the phonon scattering. Additionally, the special microtopography was conducive to the decreased lattice thermal conductivity. Therefore, the SnSe0.95 + 1.25 wt% SmCl3 + 0.125 wt% ErCl3 sample achieved a low lattice thermal conductivity value of 0.28 Wm−1K−1 at 773 K. Simultaneously, the (SmCl3, ErCl3) co-doped samples maintained the electrical transport properties than the SmCl3-doped samples. Consequently, the SnSe0.95 + 1.25 wt% SmCl3 + 0.125 wt% ErCl3 sample obtained a peak ZT value of 1.40 at 773 K and a competitive average ZT value of 0.37 from 323 to 773 K, and a theoretically calculated conversion efficiency reached 7.2%, which can be applied in the deep space for power generation. This strategy can be utilized to optimize the thermoelectric performance of other thermoelectrical material systems.

Voltage angle‐based coherency identification in power system

AbstractCoherency detection in power system is vital step for controlled‐islanding. Model‐based slow‐coherency method is the traditional method of detecting coherent areas, which suffers from computational burdens. With the implementation of a wide‐area measurement system (WAMS) in power systems, coherent areas can be detected early after a disturbance that helps to ensure quick separation or remedial action. In this study, a novel Synchrophasor‐based coherency identification method has been proposed with bus voltage angle measurement. A new algorithm is proposed based on the first and second derivatives of the bus voltage phase angle using hierarchical clustering method for coherency detection. The proposed approach leverages hierarchical clustering method and the dynamics reflected in the bus voltage phase angle behavior to achieve automatic and accurate coherency identification. Real time simulation results demonstrate the ability of the proposed algorithm to detect the coherency with three consecutive PMU data from all the system buses and showcase coherent area boundaries. Case studies show robustness of the proposed method and is least affected by the latency in communication network compared to conventional methods. The proposed algorithm has been mathematically formulated and evaluated with different benchmark systems in OPAL‐RT real‐time simulation environment with HYPERSIM platform.

A Spatial Optimization Model for Delineating Metropolitan Areas

A metropolitan area comprises a collection of cities and counties bound by strong socioeconomic ties. Despite the pivotal role that metropolitan areas play in regional economics, their delineation remains a challenging task for researchers and urban planners. Current threshold-based delineation methods select counties based on their connection strength with prespecified core counties. Such an approach often neglects potential interactions among outlying counties and fails to identify polycentric urban structures. The delineation of a metropolitan area is fundamentally a spatial optimization problem, whose objective is to identify a set of counties with high interconnectivity while also meeting specific constraints, such as area, contiguity, and shape. In this study, we present a novel spatial optimization model designed for metropolitan area delineation. This model aims to maximize intercounty connection strength in terms of both industry and daily life. This approach ensures a more accurate representation of the multicore structure that is commonly seen in developed metropolitan areas. Additionally, our model avoids the possibility of holes in metropolitan area delineation, leading to more coherent and logical metropolitan boundaries. We provide a mixed-integer programming formulation for the proposed model. Its efficacy is demonstrated by delineating the boundaries of the Nanjing and Lhasa metropolitan areas. This study also delves into discussions and policy implications pertinent to both of these metropolitan areas.

Microstructure evolution and properties of dissimilar YIG/MTC joint fabricated with bismuth-based glass ceramic

The paper uncovers microstructural evolution and bonding mechanism of yttrium iron garnet ferrite (YIG) and magnesium titanate ceramic (MTC) brazed by bismuth-based glass ceramic. The resulting mechanical and dielectric properties of the proposed YIG/MTC joint were investigated in detail. The reaction mechanism was thoroughly studied. Micro and nano CoFe2O4 were generated in the glass seam. Besides, a CoFe2O4 reaction layer was formed at the interface of MTC/glass ceramic. The atoms stacked along the plane (220), and a complete coherent boundary was emerged between the CoFe2O4 reaction layer and MTC ceramic subsequently. YBO3 clusters were formed at YIG/glass interface and Bi9Ti3Fe5O27 stripe phases were generated in the glass matrix. The joint shear strength increased with the brazing temperature increasing, and reached a maximum value of 72 MPa at 800 °C. The dielectric constant of YIG/MTC joint varied from 16.5 to 17.9, and the dielectric loss tangent was nearly 0.001 at higher frequency.

Enhancing Strength-Ductility Synergy of CoCrNi-Based Medium-Entropy Alloy Through Coherent L12 Nanoprecipitates and Grain Boundary Precipitates

The L12-strengthened Co34Cr32Ni27Al4Ti3 medium-entropy alloy (MEA) with precipitations of grain boundaries has been developed through selective laser melting (SLM) followed by cold rolling and annealing, exhibiting excellent strength-ductility synergy. The as-printed alloy exhibits low yield strength (YS) of ~ 384 MPa, ultimate tensile strength (UTS) of ~ 453 MPa, and uniform elongation (UE) of 1.5% due to the existence of the SLM-induced defects. After cold rolling and annealing, the YS, UTS, and UE are significantly increased to ~ 739 MPa, ~ 1230 MPa, and ~ 47%, respectively. This enhancement primarily originates from the refined grain structure induced by cold rolling and annealing. The presence of coherent spherical γ' precipitates (L12 phases) and Al/Ti-rich precipitates at the grain boundaries, coupled with increased lattice defects such as dislocations, stacking faults, and ultrafine deformation twins, further contribute to the property’s improvement. Our study highlights the potential of SLM in producing high-strength and ductile MEA with coherent L12 nanoprecipitates, which can be further optimized through subsequent rolling and annealing processes. These findings offer valuable insights for the development of high-performance alloys for future engineering applications.

Insight into the effect of Ti substitutions on the static oxidation behavior of (Hf,Ti)C at 2500 ​°C

Hf-based carbides are highly desirable candidate materials for oxidizing environments above 2000 ​°C. However, the static oxidation behavior at their potential service temperatures remains unclear. To fill this gap, the static oxidation behavior of (Hf, Ti)C and the effect of Ti substitutions were investigated in air at 2500 ​°C under an oxygen partial pressure of 4.2 ​kPa. After oxidation for 2000 ​s, the thickness of the oxide layer on the surface of (Hf, Ti)C bulk ceramic is reduced by 62.29 ​% compared with that on the HfC monocarbide surface. The dramatic improvement in oxidation resistance is attributed to the unique oxide layer structure consisting of various crystalline oxycarbides, HfO2, and carbon. The Ti-rich oxycarbide ((Ti, Hf)CxOy) dispersed within HfO2 formed the major structure of the oxide layer. A coherent boundary with lattice distortion existed at the HfO2/(Ti, Hf)CxOy interface along the (111) crystal plane direction, which served as an effective oxygen diffusion barrier. The Hf-rich oxycarbide ((Hf, Ti)CxOy) together with (Ti, Hf)CxOy, HfO2, and precipitated carbon constituted a dense transition layer, ensuring favorable bonding between the oxide layer and the matrix. The Ti content affects the oxidation resistance of (Hf, Ti)C by determining the oxide layer's phase distribution and integrity.