This study investigates the influence of tungsten (W) on the microstructure and mechanical properties of high-entropy superalloys, focusing specifically on the formation and evolution of the hierarchical microstructure. Transmission electron microscope analysis indicates that aging at 750 degrees C for 5-10 h induces the formation of nanoscale gamma particles (gamma(p)) within the gamma' precipitates, thereby constructing a hierarchical microstructure. Vickers hardness testing reveals a strong correlation between microstructural evolution and hardness. The formation of the hierarchical microstructure significantly increases hardness (similar to 40 HV). Furthermore, increasing the W content (from 0.40 to 0.60 to 1.00 at. %) delays the butterfly-shaped splitting of the gamma', resulting in the maximum achieved hardness. Atom probe tomography confirms that the supersaturation of gamma-forming element Cr within the gamma' drives the formation of the gamma(p). The emergence of Cr-rich clusters reduces the mixing enthalpy of the gamma' (similar to 0.60 kJ/mol), providing a thermodynamic driving force. After aging, the gamma matrix (gamma(m))/gamma' interface width is also observed to increase (similar to 0.36 nm). The partitioning behavior of most elements aligned with Thermo-Calc predictions, which only W exhibited a transition from preferential partitioning to the gamma matrix (in gamma(m)/gamma') to the gamma' precipitate (in gamma'/gamma(p)). Calculations based on Vegard's law show that an increased W content reduces the lattice parameter of the gamma(p) (similar to 0.002 angstrom), thereby increasing the lattice misfit between gamma(p) and gamma' (similar to 0.08 %). (c) 2025 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.

Reproducibility of hierarchical gamma/gamma' microstructures in Co-based superalloys significantly influences their reliability and performance at elevated temperatures. This study investigates the formation and stability of such microstructures in nominally identical Co-9Al-9W-8Cr alloys. Despite nearly identical composition and heat treatment, significant microstructural differences emerged, including the formation of a hierarchical gamma/gamma' + gamma architecture in one variant. Using scanning electron microscopy (SEM), atom probe tomography (APT), wavelength-dispersive spectroscopy (WDS), inductively coupled plasma optical emission spectroscopy (ICP-OES), and Thermo-Calc simulations, we identify subtle local Cr fluctuations and incomplete homogenization as key drivers of phase instability. High Cr levels alter partitioning behavior, promote the formation of the chi-Co3W phase, modify the gamma matrix composition, reducing supersaturation and affecting gamma particle formation inside gamma' precipitates. APT analyses across multiple locations reveal significant nanoscale compositional variability, underlining the need for statistical sampling. Our findings show that hierarchical microstructures can form under narrow local conditions, with gamma' size and local chemistry. The results highlight the sensitivity of the Co-Al-W-Cr system and the importance of precise process control to achieve reproducible, stable microstructures for advanced Co-based superalloys. (c) 2025 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.

Nanophase Separation Sintering (NPSS) has been shown to be a viable method for rapidly consolidating refractory alloys at temperatures and applied pressures lower than typically expected resulting in densified, ultra-fine-grained material. Using this technique, a W-5Cr binary alloy has been synthesized through high energy ball milling and direct current sintering and subjected to high temperature mechanical and oxidation testing. The results showed near 100 % densification of the alloy at a temperature of only 1150 degrees C using DCS for 1 h. The sintered components sustained compressive strengths above 500 MPa when tested at 1000 degrees C and exceeded 1 GPa below 900 degrees C. While the oxidation of tungsten alloys is often catastrophic, these chromium containing alloys were able to sustain protective chromia growth for short durations up to 1000 degrees C. In both these mechanical and oxidative tests, we have shown that NPSS W-Cr alloys may be a viable alternative to similar tungsten alloys in extreme environment applications. Significant carbide and contaminant inclusions are believed to have induced brittle behavior in the material at intermediate and room temperatures, and their reduction is the subject of future works.

In this study, we examine a high entropy superalloy (HESA-Y1: Ni49.37Co20Cr7Fe4Al11.6Ti6Re1Mo0.5W0.5Hf0.03 at %), focusing on hierarchical microstructure formation and its effects on mechanical properties. Thermodynamic modeling using Thermo-Calc predicts equilibrium phase fractions, compositions, and transition temperatures, which are validated by experimental data from differential scanning calorimetry (DSC). Transmission electron microscopy (TEM) reveals that secondary aging induces nanometer-sized gamma particles within gamma' precipitates, forming a hierarchical gamma/gamma' microstructure. Atom probe tomography (APT) confirms supersaturation of gamma' precipitates with gamma-forming elements (Co, Cr, Fe), driving gamma particle formation, and measures interfacial widths between gamma' and gamma phases. Partitioning coefficients derived from APT align with Thermo-Calc predictions for most elements. Vickers microhardness testing shows an increase of about 50 HV in the hierarchical microstructure compared to the conventional one. In situ synchrotron X-ray diffraction (XRD) from 25 to 750 degrees C determines a small, negative lattice misfit delta between gamma and gamma' phases, suggesting enhanced microstructural stability, consistent with Thermo-Calc calculations. Our methodological approach enables measurement of the unconstrained lattice parameter of phase-extracted gamma' in a single-crystal XRD setup. Due to their small size and low volume fraction, gamma particles do not produce distinct reflections in the X-ray diffractogram. Elucidating hierarchical microstructures across multiple scales, we establish that the presence of Re and Hf and controlled aging processes lead to enhanced mechanical properties, offering valuable insights for the design of advanced high entropy superalloys.

In high-entropy superalloys (HESAs), compositional and thermal processing strategies play a pivotal role in microstructure control and performance optimization. This study explores the impact of tungsten (W) content on segregation behavior, homogenization kinetics, microstructural evolution, and mechanical properties in a NiCoFeCr-based HESA system. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) reveal elemental partitioning between dendrite cores (DC) and interdendritic (ID) regions in the as-cast state, with W addition enhancing segregation. Diffusion analysis and ThermoCalc simulations are combined with differential scanning calorimetry (DSC) to establish a homogenization treatment (1500 K/44 h) that effectively reduces as-cast segregation. Upon aging at 1023 K, a hierarchical architecture forms, featuring fine gamma particles precipitating within gamma' precipitates. This hierarchical phase separation is promoted by W-induced chemical instability of the gamma' phase, as supported by thermodynamic modeling of the Gibbs free energy of mixing (G(mix)). Vickers hardness measurements show that the hierarchical microstructure contributes an additional similar to 20 HV in strength over the as-cast condition, with hardness increasing systematically with W content. These findings provide insights for alloy design strategies targeting thermally stable, hierarchically structured HESAs.

Ultra-high temperature ceramics (UHTCs), most notably transition metal carbides and borides, exhibit melting temperatures exceeding 3000 °C, making them appropriate candidates to withstand the extreme temperatures (∼2000 °C) expected to occur at the leading edges of hypersonic vehicles. However, their propensity to react rapidly with oxygen limits their sustained application. The high entropy paradigm enables the exploration of novel UHTC compositions that may improve on the oxidation resistance of conventional refractory mono-carbides and -diborides. The oxidation kinetics of candidate high entropy group IV + V (HfZrTiTaNb)C and (HfZrTiTaNb)B2 materials were evaluated at 1500–1800 °C using Joule heating in one atmosphere 0.1%–1% oxygen/argon gas mixtures for times up to 15 min. Possible mechanisms based on the resulting complex time, temperature, and oxygen partial pressure dependencies are discussed. The carbides formed porous and intergranular oxides. Oxidation resistance was improved upon a continuous external scale formation. The diborides formed dense external scales and exhibited better oxidation resistance compared to the carbides. This improvement was attributed to the formation of liquid boria. Both compositions showed an unexpected reduction in material consumption at 1800 °C for all times tested, compared to results at lower temperatures. An in-depth analysis of the composition and morphology of the oxide scale and sub-surface regions for specimens tested at 1800 °C revealed that the formation of denser group IV-rich (Hf, Zr, Ti) oxides mitigated the formation of the otherwise detrimental liquid-forming group V (Ta, Nb) oxides, leading to the improved oxidation resistance.

High-Entropy Alloys (HEAs) are an exciting area of research for metallurgists seeking to push through the temperature limits of the traditional nickel- and iron-based superalloys. The HEA design principal is to suppress the formation of low-symmetry phases and maintain a solid solution through the elevated entropy of mixing which is a result of alloying at least four principal elements. Despite early promise, this solid solution-strengthened subsection of HEAs is quite limited in improved operational capacity over dilute alloys with respect to temperature and mechanical behavior. High-Entropy Superalloys (HESAs) refer to those multiple-principal element materials which exhibit secondary strengthening phases such as γ′ which has historically allowed nickel-based superalloys to operate near their melting temperatures. As this alloy space is surveyed with arc-melted castings in a high-throughput fashion, potential alloy candidates may be prematurely dismissed based on oxidation behavior without consideration of microstructural uniformity. The Gleeble 3500D thermomechanical simulator was used to develop optimal working parameters for hot thermomechanical processing (HTMP) of the Ni47.5Fe13Co11Al12Cr10Ta4Ti2.5 HESA and create multiple thermogravimetric oxidation specimens. These HTMP samples were compared to solely homogenized specimens and were found to exhibit measurably slower oxidation rates as well as decreased variability in their final performance.

In this investigation, we explore the impact of the Nb-Al ratio on the microstructural and mechanical properties of high -entropy superalloys (HESAs), focusing on hierarchical microstructures. Utilizing a series of HESAs with varying Nb-Al ratios, our study employs advanced characterization techniques, including differential scanning calorimetry (DSC) for thermal analysis, electron probe micro -analyzer (EPMA) for compositional analysis for the design of a homogenization treatment at 1500 K/24 h. Transmission electron microscopy (TEM) reveals that the increasing Nb-Al ratio refines the gamma' precipitates and influences the size and volume fraction of embedded hierarchical gamma particles. ThermoCalc equilibrium phase analysis and Vegard's-law calculations reveal a minimal lattice misfit between these phases, highlighting the interplay between Nb-Al ratio and phase stability. The increasing Nb-Al ratio inhibits the formation of hierarchical gamma particles. We observe an enhancement in hardness from 433 HV to 492 HV with an increasing Nb-Al ratio. This study provides valuable insights into the role of Nb and the Nb-Al ratio in HESAs with hierarchical microstructures, demonstrating its significant influence on gamma particle formation within gamma' precipitates and mechanical strength. The findings advance our understanding of alloy design and pave the way for developing advanced HESAs for high -temperature applications.

High-entropy alloys (HEAs) are new alloy systems that leverage solid solution strengthening to develop high-strength structural materials. However, HEAs are typically cast alloys, which may suffer from large as-cast grains and entrapped porosity, allowing for opportunities to further refine the microstructure in a non-melting near-net shape solid-state additive manufacturing process, additive friction stir deposition (AFSD). The present research compares the microstructure and mechanical behavior of the as-deposited AFSD Al0.35CoCrFeNi to the cast heat-treated properties to assess its viability for structural applications for the first time. Scanning electron microscopy (SEM) revealed the development of fine particles along the layer interfaces of the deposit. Quasi-static and intermediate-rate compression testing of the deposited material revealed a significant strain-rate sensitivity with a difference in yield strength of ~400 MPa. Overall, the AFSD process greatly reduced the grain size for the Al0.35CoCrFeNi alloy and approximately doubled the strength at both quasi-static and intermediate strain rates.

The impacts of thermal treatment on the precipitate morphology and oxidation behavior of a dual-phase (FCC + L12) multi-principal element alloy (MPEA), Ni45Co17Cr14Fe12Al7Ti5, was studied at 1000 °C via isothermal and cyclic testing. Thermogravimetric analysis and subsequent characterization revealed that smaller precipitates had an increased capacity to form protective sub-surface oxide layers which mitigated total mass gain. The smaller-precipitate-containing samples exhibited a decrease in thickness of the primary Cr2O3 scale and parabolic growth rate. Mechanistically this behavior is believed to stem from the increased growth rate of initial Al2O3 nuclei and decreased inter-precipitate spacing which results in faster lateral diffusion and agglomeration.

Here, we study the homogenization behavior and microstructure of seven Ni-Al-Ti alloys with quaternary additions of gamma forming elements 4Cr, 4Co, 4Ru, 4Mo, 4Hf, 4 W and 2Re. To design a homogenization treatment, the as-cast microstructure is analyzed revealing the diffusion distances x between dendrite cores and interdendritic regions. The temperatures for homogenization are determined using differential scanning calorimetry (DSC) and Thermo-Calc simulations, to be between 1150 and 1275 degrees C. The time to achieve homogenization is modelled based on the residual segregation index delta utilizing diffusion distance, homogenization temperature and diffusion data. Electron probe micro analyzer (EPMA) measurements show that our predictions match for the 4Cr, 4Co, 4Ru, 4 W and 2Re alloys while the 4Hf alloy shows insufficient homogenization. Transmission electron microscopy (TEM) reveals a two-phase gamma/gamma' microstructure after 750 degrees C / 24 h, whereby the 4Co and 4Ru alloys form hierarchical microstructures. We observe gamma plates in the 4Co alloy and gamma spheres in the 4Ru alloy. Ru in the 4Ru alloy is involved in stabilizing the morphology of gamma spheres. We provide a straightforward method for the design of homogenization treatments of Ni-based superalloys and demonstrate an alloy design pathway for tailoring the phase stability of hierarchical microstructures.

Download This Paper Open PDF in Browser Add Paper to My Library Share: Permalink Using these links will ensure access to this page indefinitely Copy URL Copy DOI

Pure titanium and titanium alloys are essential materials for structural applications due to their high specific strength and corrosion resistance. However, at temperatures above 550-600 degrees C, an unprotective oxide layer forms, promoting oxygen diffusion and degradation of material properties. Enhancement of titanium's oxidation resistance lies in creating a stable and protective oxide layer on its surface which can be achieved through controlled manufacturing, microalloying, or post processing via surface engineering treatments. In this study, laser peening has been employed to improve the high-temperature oxidation resistance of pure titanium. Two laser-peened and two un-peened titanium specimens were subjected to oxidation in an environment consisting of 21 % oxygen and 79 % argon at 750 degrees C for 100 h using thermogravimetric analysis. Characterization techniques such as optical and electron microscopy, micro- and nano-indentation, and Raman spectroscopy were employed to analyze the alpha-case and oxide scale formed during exposure to oxygen at high temperature. The results reveal that laser peening modified the surface characteristics of pure titanium and improved its oxidation resistance. On average, laser peened specimens experienced a 22.3 % reduction in mass gain following oxidation. The study sheds light on mechanisms responsible for this phenomenon, providing valuable insights into the development of novel surface engineering techniques for titanium and its alloys.

Metallic powders are widely utilized as feedstock materials in metal additive manufacturing (MAM). However, only a limited number of alloys can currently be processed using these technologies, with most of them being casting alloys. The objective of this study is to investigate novel aluminum alloys produced via a close-coupled gas atomizer (CCGA) by adding an increasing amount of copper (4, 8, and 20 wt%) to an AlSi10Mg alloy. The obtained powders were fully characterized to evaluate the effect of copper, a well-established strengthener for aluminum alloys, in order to correlate the obtained hardness to the powder phase composition and microstructure. In particular, a dendritic microstructure was observed in all alloys, and, as the copper content was increased, the size of the secondary dendrite arm spacing (SDAS) decreased progressively. Consequently, the hardness measured on the powder cross-section linearly increased with the copper content, and the hardness value of 185 ± 13 HV of the AlCu20Si10Mg composition was found to be twice that of the AlSi10Mg alloy (88 ± 5 HV).

This paper reports the effects of annealing at 1050 degrees C in an inert atmosphere on the microstructures and high temperature oxidation behaviors of Alx(CoCrFeNi)1-x CCAs, where x = 8, 12, 15, 20, and 30 (at%) complex concentrated alloys. In all cases, annealing stabilized multi-phase microstructures consisting of a combination of BCC, B2, and/or FCC phases with the final phase constitution being dependent on the Al content in the alloy. All the alloys exhibited brief periods of transient oxidation, followed by various degrees of parabolic oxide growth. The as-cast alloys, which were observed to predominantly be Cr2O3 formers, oxidation resistance increased with increasing Al content. After annealing, all of the alloys transitioned into Al2O3 formers but with increased mass gain up to 20 at% Al. Comparison of the experimental results with thermodynamic models of alloy phase equilibria and oxidation products suggests that an increase in the volume fractions of Al-rich phases near the alloy surfaces as a result of annealing, increases oxidation resistance by providing Al reservoirs for the formation of Al2O3. The oxidation mechanisms are discussed relative to existing M-Cr-Al oxide formation models.(c) 2023 Elsevier B.V. All rights reserved.

Metallic powders are one of the most common feedstock materials for metal additive manufacturing (MAM). Nowadays, only few alloys can be processed by these technologies and most of them are casting alloys. This work is focused on the characterization of a novel aluminum alloy produced by a close coupled gas atomizer (CCGA) with composition AlSi10Mg + X Cu (X= 4, 8, 20 wt%). These compositions are very attractive because copper is a well-known strengthener for aluminum alloys. The produced powders were characterized in terms of morphology, flowability, particle size distribution (PSD) and density. Furthermore, the powders microstructures were analyzed to evaluate the composition and the morphology of the phases generated by the rapid solidification that characterized the gas-atomization process.

Antimony selenide (Sb 2 Se 3 ) has excellent directional optical and electronic behaviors due to its quasi‐1D nanoribbons structure. The photovoltaic performance of Sb 2 Se 3 solar cells largely depends on the orientation of the nanoribbons. It is desired to grow these Sb 2 Se 3 ribbons normal to the substrate to enhance photoexcited carrier transport. Therefore, it is necessary to develop a strategy for the vertical growth of Sb 2 Se 3 nanoribbons to achieve high‐efficiency solar cells. Since antimony sulfide (Sb 2 S 3 ) and Sb 2 Se 3 are from the same space group (Pbnm) and have the same crystal structure, herein an ultrathin layer (≈20 nm) of Sb 2 S 3 has been used to assist the vertical growth of Sb 2 Se 3 nanoribbons to improve the overall efficiency of Sb 2 Se 3 solar cell. The Sb 2 S 3 thin layer deposited by the hydrothermal process helps the Sb 2 Se 3 ribbons grow normal to the substrate and increases the efficiency from 5.65% to 7.44% through the improvement of all solar cell parameters. This work is expected to open a new direction to tailor the Sb 2 Se 3 grain growth and further develop the Sb 2 Se 3 solar cell in the future.

The pioneering work of Yeh et al. (2004) and Cantor et al. (2004) initiated an expansive research activity on high entropy alloys (HEAs), aiming at discovering novel materials in the newly opened compositional spaces. The original concept of entropically stabilized solid solution phases around equimolar alloy compositions with not less than 5 elements has driven the quest for single phase high entropy alloys, as described in a critical update (Streuer, 2020). Recent review articles provide critical assessments of the HEA concept while trying to structure and condense the ample and diverse research results and also provide guidance for future research (Miracle and Senkov, 2017; George et al., 2019; George et al., 2020; Li et al., 2021). Besides continuous research on “ad litteram” HEAs which closely follows the original definition, the scope was soon extended as to accommodate multicomponent alloys with lower configurational entropy of mixing, so-called medium entropy alloys, MEAs (i.e., Zhou et al., 2018). Special interest was further focused on alloys that provide pathways to dual-phase or multi-phase microstructures (these alloys are sometimes categorized as compositional complex alloys, CCAs, to differentiate with single phase HEAs/MEAs), thus enabling more options for microstructure engineering. The alloy systems Al-Cr-Fe-Ni and Al-Co-Cr-Fe-Ni are most attractive in this respect. Both systems host compositional ranges for the design of dual-phase materials composed of a face centered cubic (FCC-A1) and a body centered cubic (BCC-B2) phase following distinct phase transformation pathways. Examples are the alloys Al0.7CoCrFeNi and AlCrFe2Ni2 (Dong et al., 2016; DeJeer et al., 2017) which pass through a BCC-B2→FCC-A1 solid state phase transformation and the alloys AlCoCrFeNi2.1, Al0.9CrFeNi2.1 which display eutectic growth following Liquid → BCCB2+FCC-A1 (Lu et al., 2014; Jin et al., 2019). Importantly, in both alloy systems the Al-rich BCC-B2 phase is prone to spinodal decomposition, which may impact on the overall phase transformation cascade and certainly affect the mechanical properties. This “Research Topic” was initiated with main focus on dual-phase HEAs and MEAs from the above alloy systems, calling for contributions on a wide range of open issues, including phase transformation pathways, alloy processing by conventional and additive technologies, mechanical and functional properties, i.e., corrosion behavior, and other more. From the perspective of future applications knowledge on all these aspects is required in equal measure. For dual-phase MEAs Edited and reviewed by: John L. Provis, The University of Sheffield, United Kingdom

The relationship between precipitate evolution, martensitic transformation temperatures, hardness, and functional load-bias behavior has been analyzed for a Ni50.8Ti34.2Zr15 (at.%) alloy. In the solutionized condition, the alloy was fully austenitic and no transformation (at least to -90 degrees C) was observed. Upon aging at 550 degrees C, the onset of a martensitic transformation (Ms), as determined by differential scanning calorimetry, was observed at approximately -10 degrees C and 75 degrees C after 24 and 300 h, respectively. Electron diffraction identified the precipitation of the orthorhombic H-phase within the B2 matrix. When the inter-precipitate spacing was similar to 12 nm, a greater undercooling was necessary to initiate the martensitic transformation due to overlapping strain fields of the precipitates. As the precipitates coarsened with aging time, a corresponding increase in the inter-precipitate spacing occurred and the chemical partitioning effects between the matrix and precipitate, as determined by atom probe tomography, began to dominate the transformation behavior resulting in an increase in transformation temperatures. For selected aging conditions, the load-biased shape memory behavior was determined under compressive and tensile loading using uniaxial constant-force thermal cycling experiments. A tension compression asymmetry was noted with larger transformation strains in tension than compression at constant stresses up to 400 MPa. A recoverable transformation strain of 3% was observed in the sample aged for 4 h at 550 degrees C under a tensile stress of 400 MPa, which is the largest recoverable strain currently reported for a precipitation-strengthened NiTiZr alloy.