High Al Content Research Articles

Early Devonian Slab Melting of the Proto-Tethys Ocean: Insights from Adakitic Granitoids in the Jiayuguan Area, Hexi Corridor (NW China)

Abstract The tectonics of the Proto-Tethys Ocean during the Early Devonian are still heavily debated in the North Qilian orogen. In order to further constrain this issue, we explore geology, chronology, geochemistry, and isotopes of three newly discovered Early Devonian adakitic granitoids of the Jiayuguan complex in the North Qilian orogen, NW China. The granitoids exhibit typical adakitic geochemical signatures with high SiO2 (>56%), Na, Al, and Sr contents, depleted in Yb and Y, and high Sr/Y (82–277) and (La/Yb)N (7.65–15.16) values. Additionally, their high Mg# (62–68) and εHf(t) (+6.5–+11.9) values indicate partial melting of slab genesis. However, they have comparatively low εNd(t) (−2.3–+0.13). Their incompatible Hf and Nd isotopes could be caused by contamination between the source magma and continental crust during the emplacement processes. The three adakitic granitoids yield zircon U–Pb ages of 415–403 Ma, implying that the northern Qilian Ocean was subducting until the Early Devonian (403 Ma) and that young/hot/ridge subduction formed the Jiayuguan adakitic granitoids. Combined with regional data, we propose that the Proto-Tethys Ocean was subducting until 403 Ma in the Qilian area.

Heterostructural interface engineering for ultrawide-gap nitrides from first principles: TaC / AlN and TaC / GaN rocksalt-wurtzite interfaces

Epitaxial lattice matching is an important condition for the formation of coherent interfaces with low defect densities. However, lattice-matched substrates with the same crystal structure as the active layer are often not available, suggesting opportunities for utilizing heterostructural interfaces. For example, at high Al contents that are interesting for ultrawide-gap applications in power electronics, AlxGa1−xN semiconductor alloys in the (0001) orientation of the wurtzite (wz) structure become lattice-matched to (111)-oriented rocksalt (rs) TaC substrates. To predict the expected interface atomic structures under different synthesis conditions, we perform high-throughput density-functional-theory calculations, using an algorithm for systematic sampling of the possible stacking sequences of the atomic layers on the in-plane hexagonal lattice. The approach considers octahedral, tetrahedral, and prismatic coordination motifs, and is generally applicable for the modeling of commensurate rs/wz heterostructural interfaces. Our results provide guidance for synthesis control of substrate-film bonding and the polarity of ultrawide-gap AlxGa1−xN alloys on TaC substrates. Published by the American Physical Society 2024

Effects of Al Substitution for Fe in Na₅FeSi₄O₁₂ (5.1.8) Glasses: Structure and Crystallization

In this study, the effects of substituting Al for Fe in 5Na2O∙(Al2O3)x∙(Fe2O3)1-x∙8SiO2 glass, x=0 to 1, and Na5AlxFe1-xSi4O12 (5.1.8) crystal, were investigated using thermal analysis, Fe K-edge X-ray absorption, X-ray diffraction, Raman spectroscopy, and Electron Probe Microanalysis. In both glass and crystallized glass, nearly all the Fe was tetrahedrally coordinated Fe3+, as expected from the high concentration of Na2O. The substitution of Al for Fe in the glasses caused the glass transition temperature to increase as polymerization increased, as evidenced by Raman, likely due to both field strength differences of Al vs Fe and a small amount of Fe2+ network modifier present with Fe. After heat treatment at 700 °C for 24 hours, the glasses had crystallized, forming Na2SiO3 and NaAlSiO4 in compositions with high Al concentrations and the 5.1.8 crystal in compositions with high Fe concentrations. Through electron microprobe, it was determined that <0.04 formula unit Al incorporated into the 5.1.8 crystal, i.e. Na5Fe0.96Al0.04Si4O12. The 5.1.8 crystal only formed when Fe concentration was higher than Al in the starting glass.

Excellent Wavelength Selectivity of p-AlGaN/n-Ga2O3 Solar-Blind UVC PD.

Due to the advantages of ultrawide bandgap, chemical stability, self-powered, and low cost, gallium oxide (Ga2O3) based photodetectors (PDs) are considered as one of the most promising solar-blind ultraviolet PDs, having garnered significant attention in fields such as missile warning and flame arc detection. High selective ratios and excellent responsivity are important to reduce the false alarm rate in solar-blind detection. However, due to the lack of p-type Ga2O3, existing Ga2O3-based PN PDs utilized heterostructures with narrower bandgap p-type semiconductors (e.g., p-GaN, p-SiC, etc.), inducing poor selective ratios with visible light and ultraviolet-A/B (UVA/B). Therefore, a high Al content AlGaN was used in this study to form a p-AlGaN/n-Ga2O3 solar-blind ultraviolet-C (UVC) PD. Since the bandgap of p-AlGaN aligned with the UVC region, the device exhibited exceptionally high selective ratios with R247nm/R360nm = 3.71 × 105 and R247nm/R300nm = 1.47 × 105 at 0 V, which surpassed the reported Ga2O3-based PN PDs. The formation of the type-II heterojunction facilitated the effective separation and transport of photogenerated carriers, resulting in high responsivity (0.36 A/W) under zero bias. Overall, the high selective ratios and high responsivity of the p-AlGaN/n-Ga2O3 PD in this paper provided a reliable pathway for self-powered solar-blind PDs with a low false alarm rate.

Aqueous carbonation of steel slags: A comparative study on mechanisms

This study investigated the aqueous carbonation mechanisms of three typical steel slags: ladle metallurgy furnace (LMF) slag containing high Al content, electric arc furnace (EAF) slag featuring high Si content and relatively low Al content, and ladle-arc fusion (LAF) slag with medium-Al content. It was found that the carbonation kinetics of the three slags were similar and followed the surface coverage model within the first 6 h of carbonation. Initially, the carbonation process was primarily governed by the reaction product precipitation. After 3 h of carbonation, the process was dominated by mineral dissolution, controlled by the uncovered reactive sites. The carbonation-reactive Ca-bearing minerals in the slags, including silicates (CRSis) and aluminates (CRAls), sequestered CO2 to form calcite during carbonation, accompanied by the formation of silica gel and alumina gel, respectively. CRSi, mainly larnite, was present in EAF and LAF slags, showing high reactivity, whereas mayenite (i.e., C12A7), a CRAl mineral present across all slags, exhibited high reactivity in LMF slag but lower reactivity in EAF and LAF slags. Furthermore, AFm phases and katoite (i.e., C3AH6) were detected in LMF slag as CRAls along with mayenite, and their carbonation reactivity decreased in the order of AFm>mayenite>katoite. As a result, low-Al steel slag tends to have higher carbonation reactivity, as manifested by the high carbonation degree of EAF slag throughout the reaction period, notably achieving a 40 % carbonation degree within 30 min and 71 % after 24 h under the studied conditions.

Investigation of the Dissolution Behavior of AlN in CaO–Al2O3–MgO Refining Slag and CaO–Al2O3–F–Li2O–BaO Mold Flux

Low‐density steels have garnered widespread attention as novel lightweight materials. Owing to its high Mn and Al content, brittle AlN inclusions, detrimental to both steelmaking and steel properties, can precipitate directly from the molten steel. Employing slag absorption effectively removes these floating AlN inclusions from the molten steel. Herein, a series of analytical methods are utilized to investigate the dissolution behavior of AlN in CaO–Al2O3–MgO refining slag and CaO–Al2O3–F–Li2O–BaO mold flux. The results indicate that the initial dissolution temperatures of AlN in the slag and mold flux are 1361 and 1080 °C, respectively. The dissolution in the refining slag is attributed to the high‐temperature self‐decomposition of AlN, producing N2(g). In the mold flux, the dissolution reaction involves both the high‐temperature self‐decomposition of AlN and its reaction with Li2O in mold flux, generating N2(g) and Li(g). No reaction interface is observed at the AlN‐slag/mold flux boundary. As the dissolution mass of AlN in the slag/mold flux increases, the Al2O3 content in the slag/mold flux rises. The main phases in slag transition from Ca9(Al6O18) to Ca12Al14O33 and Ca3Al2O6. In the mold flux, the LiBaF3 phase disappears, and the 11CaO·7Al2O3·CaF2 phase, which can degrade the flux's physicochemical properties, becomes the dominant phase.

Geochronological and geochemical constraints on the multistage magmatic processes of the Lianhuashan batholith, South China: Implications for the petrogenesis and polymetallic mineralization

The Lianhuashan batholith, which is a composite pluton located in the middle part of a world-class giant tungsten (W) polymetallic belt of South China, hosts a huge reserve of more than 50000 t of W. Despite this great economic significance, the petrogenesis of each phases of the Lianhuashan batholith, as well as its temporal and genetic relationships to the W polymetallic mineralization, is still unclear. To address these questions, we conducted a comprehensive study for the Lianhuashan batholith, including LA-ICP-MS zircon U-Pb dating, whole-rock geochemical and in-situ mineral trace element analyses. Different from previous studies, four main granitic phases (G1 to G4) were identified. The biotite granite (G1) formed from 168.6 to 165.3 Ma, has the lowest Si content and high Al and Fe contents, and is significantly depleted in Ba, Sr, Ti, P, and Nb whereas enriched in U, Hf, Zr, and Y contents. In contrast, the two-mica granite (G2) and fine-grained muscovite granite (G3), formed from 162.8 to 160.5 Ma, exhibit similar characteristics including enrichment in La, Y, Hf, Th, U, and depletion in Ba, Sr, and P. The porphyritic granite (G4) is the latest magmatic phase and formed at 158.3 Ma. It is characterized by high K, Si, Th, U, Zr, Hf contents but low Al, Fe, Sr, P, and Ti contents. These features support that four main phases of the Lianhuashan batholith belong to S-type granites that experienced significant fractional crystallization, and display reduced and low temperature features. Combined with previously published studies, we suggest that the Lianhuashan batholith formed in an intraplate extensional setting triggered by the high-angle rollback subduction of the paleo-Pacific plate. The three early phases (i.e., G1–G3) are in turn more oxidized and are responsible for the transition from Sn- to W-dominated mineralization. In contrast, the G4 granite, characterized by lower oxygen fugacity, is resposible for Pb-Zn-Ag-U polymetallic mineralization.

Surface modification of cotton fabric by kaolin-derived zeolite N to enhance efficient removal of particulate matter

Reducing the amount of fine particulate matter (PM) in the atmosphere is essential for preserving a secure and healthy environment. Here, we synthesized zeolite N (potassium ion form, KN) from kaolin clay minerals, coated it on cotton fabric using a simple refluxing method and then removed PM from the air. The following conclusion can be drawn. The average cation exchange capacity (CEC) of KN was 512 meq/100 g, and a CEC value > 400 meq/100 g indicated pure zeolite with a Si/Al ratio of approximately 1.0. The synthesized KN, containing a high proportion of Al, proved highly effective in removing particulates. For instance, KN/cotton (746 GSM (weight of fabric in g/m2) cotton, coated with approximately 1.46 wt% KN) demonstrated an 8-times higher removal efficiency (RE) for PM2.5 and PM1.0 than bare 746 GSM cotton, with only a 7 Pa increase in the pressure drop. PM may have been removed by electrostatic interactions between the charged PM and KN (with charge-balancing cations) owing to the charges and polarity of the outer layer of PM particles. According to the increases in RE and the quality factor, KN/cotton (746 GSM) was the most effective in removing PM from the air compared to other materials. The results suggested that KN, when applied to the surface of cotton fabric, is highly efficient due to its EDI framework structure and high Al content. Finally, biodegradation tests revealed that used filters disintegrated into pieces after being buried for one month in soil, indicating their environmental friendliness.

Significantly Enhanced Interfacial Thermal Conductance across GaN/Diamond Interfaces Utilizing AlxGa1-xN as a Phonon Bridge.

Improving the thermal conductance at the GaN/diamond interface is crucial for boosting GaN-based device performance and reliability. In this study, first-principles calculations and molecular dynamics simulations were employed to explore the interfacial thermal conductance of GaN/diamond interfaces with AlxGa1-xN transition layers. The AlxGa1-xN alloy exhibits a lower thermal conductivity than GaN, primarily due to enhanced anharmonic phonon scattering. However, for the interfacial thermal conductance at the GaN/diamond interface, we discovered that introducing an AlxGa1-xN with a high Al concentration (x > 0.5) as a phonon bridge between GaN and diamond can significantly enhance the interfacial thermal conductance. In particular, it increases from 4.79 MW·m-2 K-1 to a maximum of 158 MW·m-2 K-1 at x = 0.75, surpassing the 152 MW·m-2 K-1 achieved by AlN. The AlxGa1-xN alloy has been confirmed computationally as a more efficient phonon bridge, which can provide a valuable theoretical reference for experimentally investigating the thermal management and thermal design of high-power electronic devices based on the GaN/diamond interface.

Influence of NH3 flow rate on the photoelectric properties of high Al content p-AlGaN.

High Al content (60%) p-AlGaN with different NH3 flow rates was grown using metal-organic chemical vapor deposition (MOCVD), and changes in its photoelectric properties were studied using the Hall effect tester (Hall) and cathodoluminescence (CL) spectrometer. The results show that the film resistivity increases from 3.8 Ω·cm to 46.5 Ω·cm with increasing NH3 flow rate. The impurity peak intensity of p-AlGaN grown under high NH3 flow conditions is particularly high, indicating numerous point defects. The results of high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) show a large number of Ga interstitial atoms (Gai) at the interface. As Gai acts as a donor, this may be the main reason for the increase in resistivity. And under high NH3 flow conditions, a lattice distortion and a high density of dislocation occur between p-AlGaN and p-GaN, which can lead to enhanced carrier scattering and decreased mobility. Additional validation via LED growth experiments indicates that the luminescence intensity of samples with low ammonia concentration increased by more than 13000 times.

Structural and Electrochemical Properties of Binary ZnO:Al Nanocomposites as Anode for Lithium-ion Batteries

In conventional lithium-ion batteries (LIBs), carbon compounds are commonly utilised as the anode owing to their great performance, low cost, and abundance. However, due to the limited storage capability of pure carbon materials that restrict further improvement of LIBs, zinc oxide (ZnO) has been one of the promising anode materials to be used as an alternative to strengthen the electrochemical performance of LIBs due to its high theoretical capacity of 987 mAh g-1. This study aims to synthesise ZnO:Al nanowires using the hot-tube thermal evaporation method. Three types of samples are made using this method by varying the concentration of 0 wt% (S1), 3wt% (S2), and 6 wt% (S3) of aluminium (Al) during the Al deposition process. The EDX findings indicated that the sample has a high proportion of zinc (Zn) and oxygen (O), with the S3 sample having the highest Al concentration after being deposited. The most substantial diffraction peak for XRD of all samples was found at (101), exhibiting a single crystalline hexagonal structure with optimum growth direction on the c-axis. For EIS analysis, the S3 sample has the lowest bulk resistance and maximum ionic conductivity. In conclusion, the ZnO sample with 3 wt% of Al as a dopant was selected as the optimum result to synthesise a homogenous surface of ZnO:Al with good crystallinity by using a hot-tube thermal evaporation process and giving the best conductivity in electrochemical performance.

Changes in composition and concentration of differential metabolites in root exudates are associated with aluminum-tolerance of Ricinus communis under a high CO2 environment

Root exudates are the most direct performance for plants responding to adverse environments, and are also important media for materials exchange, energy transmission and information communication between the roots and rhizosphere. However, how plant roots and exudates respond to aluminum (Al) stress under elevated CO2 concentration (eCO2) is still unclear. Ricinus communis is a famous oilseed crop throughout the world, which has strong tolerance to metal contaminated soil. In the present study, root physiological changes and the exudates of this species under aluminum stress and eCO2 based on metabolomic were investigated. The results showed that high Al concentration stress significantly increased aluminum, MDA, proline, and soluble sugar contents, and decreased the dry weights and soluble protein concentration. Furthermore, eCO2 alleviated the inhibition of root growth under high Al stress by increasing the dry weights, antioxidant enzyme activities and decreasing the MDA content. Collectively, a total of 511 metabolites were detected in the castor exudates of which lipids, organic acids, and organic oxygen compounds occupied 40.82%, 17.78% and 12.54%, respectively. There were 83, 15, and 100 differential metabolites for high Al stress, eCO2 and the interaction of the two factors compared with the control. 12 differential metabolites were found under eCO2 and Al stress compared with Al stress alone. Under Al stress, TCA cycle, organic acids, and lipids metabolisms were inhibited; coumarins and carbohydrates conjugates were significantly up-regulated, which may help castor adapt to aluminum-contaminated conditions. Moreover, eCO2 increased the secretion of organic acids, fatty acyls, and carbohydrates to enhance the antioxidant capacity and root growth of castor under Al stress; eCO2 enhanced the TCA cycle, organic acids accumulation, lipids metabolism, biosynthesis of amino acids, pentose and glucuronate interconversions, and inhibited DNA oxidative stress of castor roots under Al stress. The present study provides new insights into the crucial role of root exudates in improving Al-tolerance of castor under eCO2.

Microstructure evolution and high-temperature erosion behaviors of laser cladded AlxCoCrFeNi2.1 HEA coatings

High entropy alloys (HEAs) are anticipated to be candidates for high-temperature applications. In this work, high-speed laser cladding has prepared AlxCoCrFeNi2.1 (1≤x≤1.4) HEA coatings with high Al content. The effects of Al content on the microstructure, microhardness, and high-temperature erosion behaviors have been systematically investigated. With the increase of Al content, the coating’s microstructure transforms from the dual-phase structure of FCC and BCC to a single-phase BCC structure. Correspondingly, the microhardness of HEA coating gradually increases at room temperature due to the increment of BCC proportion. The Al1.4 HEA coating, consisting of a single BCC phase, exhibits the highest resistance to solid-particle erosion at 600℃, while Al1.0 coating with FCC and BCC dual-phase structure takes the second place. Ploughing, fracture, and spalling occurred during the high-temperature erosion process of AlxCoCrFeNi2.1 HEA coatings. The synergistic interaction between the FCC and BCC phases in dual-phase coatings primarily influences erosion behaviors. However, oxidation resistance is critical to erosion resistance for single-phase BCC coatings.

Designing TaC Virtual Substrates for Vertical AlxGa1−xN Power Electronics Devices

Power electronics are critical for a sustainable energy future, playing a key role in electrification and integration of renewable energy sources into the grid. Advances in ultrawide band gap materials are needed to handle higher powers in smaller form factors while reducing electrical and thermal losses. High Al content AlxGa1−xN is theoretically capable of meeting these demands, but its impact in power electronics has been severely restricted by a lack of substrates that can satisfy conductivity, lattice matching, and/or thermal expansion requirements. We demonstrate that electrically conductive TaC can be used as a virtual substrate for AlxGa1−xN heteroepitaxy. Scaleably sputtered TaC grown on Al2O3, followed by high-temperature face-to-face annealing, produces a thin film TaC template with an effective hexagonal lattice constant matched to Al0.70Ga0.30N. Annealing of the TaC promotes recrystallization, significantly improving crystallinity and reducing crystalline defects from as-deposited columnar grains to a step-and-terrace surface morphology, enabling the subsequent growth of high-quality Al0.70Ga0.30N by molecular beam epitaxy. X-ray diffraction and scanning transmission electron microscopy confirm that the AlxGa1−xN layer is heteroepitaxially aligned, strain-free, and lattice-matched, transitioning abruptly from TaC to AlxGa1−xN without intermediate phases. These results demonstrate TaC virtual substrates as electrically conductive, lattice-matched, and thermally compatible templates for vertical AlxGa1−xN devices that can meet the growing power needs of a sustainable energy future. Published by the American Physical Society 2024

Impact of Al profile in high-Al content AlGaN/GaN HEMTs on the 2DEG properties

Ultra-thin high-Al content barrier layers can enable improved gate control and high-frequency operation of AlGaN/GaN high electron mobility transistors (HEMTs) but the precise composition control is very challenging. In this work, we investigate the compositional profiles of AlxGa1−xN/GaN HEMT structures with targeted Al content in the barrier layer, x = 0.50, 0.70, and 1, and thickness in the sub-10 nm range in correlation with the two-dimensional electron gas (2DEG) properties. The HEMT structures are grown by metal-organic chemical vapor deposition on SiC. The maximum Al content in the barrier layer, experimentally determined by scanning transmission electron microscopy combined with energy-dispersive x-ray spectroscopy, is found to be lower than that intended and the deviations from the designed structures increase progressively with increasing x. Compositionally sharp interface between GaN and Al0.46Ga0.54N and box-like Al profile is achieved for intended x∼0.50 while pronounced Al grading is found in the samples with intended x of 0.70 and 1, with a maximum Al content of 0.78 reached for the HEMT structure with intended AlN barrier layer. The impact of the experimentally determined Al profiles on the 2DEG properties, obtained by contactless and electrical Hall effect measurements and coupled with self-consistent solution of the Poisson–Schrödinger equation, is evaluated and discussed. It is shown that the observed deviations from the intended Al profiles have a negative effect on the 2DEG confinement and result in reduced mobility parameters, which have significant implications for the implementation of high-Al content AlGaN/GaN structures in high-frequency devices.

The effects of Ni and Al elements on microstructure and mechanical properties of low carbon CoCrMo alloy coatings

Low-carbon cobalt-based alloy coatings with high ductility and low thermal fatigue crack propagation rates were prepared using laser cladding (LC) technology. The microstructure of the coatings was characterized using SEM, EBSD, and TEM, while the mechanical properties were tested and analyzed. The CoCrMo alloy primarily consists of γ-Co and a minor amount of ε-Co. The addition of Ni increased the stacking fault energy (SFE), resulting in the retention of γ-Co in the alloy. However, the increase of Al content reduced the SFE, leading to the precipitation of Al3Ni intermetallic compound along with a eutectic region with Al3Ni, Cr23C6, and ε-Co. More ε-Co phase appeared after tensile deformation due to the stress-induced transformation induced plasticity (TRIP) effect. Ni addition increased the content of face-centered cubic (fcc) γ-Co, enhancing the coating's ductility by approximately 85 % compared to CoCrMo. With higher Al content, the ε-Co phase increased, and the dispersion of Al3Ni improved the coatings' strength. The jagged structure of the eutectic region increased the resistance to thermal fatigue crack propagation, causing the crack propagated along the grain boundary. Optimal mechanical properties were achieved with the addition of 13 wt% Ni and 7 wt% Al, which achieved a 53 % increase in ductility and a 40 % reduction in the thermal fatigue crack propagation rate compared to CoCrMo.

Phase evolution at interface between magnesia refractories and low-density steel/slag by Mg/Ca mass transfer

This study investigated the interfacial phase evolution between magnesia refractory and low-density molten steel with different Al contents (0.5 % and 10 %) during the steel-slag reaction. Simulated experiments involving a refractory-steel-slag coupling reaction were conducted under laboratory conditions. The thickness and compositional distribution of the interface layers formed after the refractories were exposed to molten steel and slag were analyzed. The results indicate that, in the absence of slag, a MgAl2O4 layer is formed at the interface between the refractory and molten steel due to the reaction between [Al] and Al2O3 inclusions in the molten steel with the refractory. Upon the addition of slag, CaO in the slag is reduced to Ca and transferred into the molten steel, modifying the solid MgAl2O4 layer at the interface into a primarily liquid CaO·MgO·Al2O3 layer. The liquid interface intensified the erosion of the refractory by the molten steel, resulting in a thickened interface layer. The higher the Al content in the steel, the greater the degree of modification of the CaO·MgO·Al2O3 layer by Ca. The Mg/Ca mass transfer process during the steel-slag reaction was simulated using the FactSage Macros calculation, and the differences in mass transfer between low and high Al contents were compared. The results indicate that under high Al conditions, the supply of Mg/Ca increases significantly. The simulation results were compared with numerous findings in the literature, confirming the accuracy of the simulation.

Near-α titanium alloy dwell load-induced deformation twinning to coordinate the deformation mechanism associated with crack initiation

In this study, we identified a specific phenomenon of coordinated deformation of twins in near-α titanium alloys during dwell fatigue (DF). The main crack source regions and internal cracks in the micro-texture region (MTR) and no-MTR samples with inconsistent orientation characteristics were characterized. The results demonstrate that the main cracks in all specimens are aligned with the (0001) basal plane, which is associated with basal slip. Notably, twins are found and confirmed to be involved in the DF crack initiation process in high Al content near-α Ti60 alloys where deformation twins are rare, and tend to nucleate at the DF basal cracks. Further in-situ dwell investigation reveals and proposes that the dislocation pile-up and prismatic-basal (PB) interfacial features between soft/hard grains lead to deformation twin nucleation and growth from hard grain boundaries. Concurrently, the pyramidal 〈c+a〉 dislocation slip is observed to be in concurrent operation with the induced twins. These findings suggest that deformation twins not only coordinate basal grain deformation but also hinder the initiation and propagation of basal cracks caused by basal 〈a〉 slip. Moreover, a high density of pyramidal 〈c+a〉 dislocations within the twin and their dissociation to form basal stacking faults (SFTs) with a specific nanometric spacing make the twinning process accompanied by significant lattice distortions inside the twin. These newly formed nanotwin boundaries and SFTs act as barriers to dislocation motion, enhancing the strength and DF lifetime of near-α Ti60 alloys and effectively reducing the dwell sensitivity of no-MTR alloys. Our findings extend the understanding of the coordinated roles of dislocation slip, twin nucleation and formation of basal SFTs in near-α titanium alloys in dwell fatigue.

High Temperature Oxidation Behaviours of NiCoCrAlY alloys with High Al Content

Abstract MCrAlY coatings are of paramount importance in the protection of hot-end components from thermal oxidation and corrosion. The Al content is a crucial factor in enhancing the oxidation resistance properties of the coatings. This paper studies the oxidation behaviors of Ni-27.5Co-17Cr-xAl-0.5Y (x = 12 and 16 wt.%) alloys with high Al content. Following an oxidation process at 1100 °C over a period of 100 h, both alloys exhibit the oxide scales primarily consist of Al2O3 with minor amount of Co(Ni)Al2O4 spinels. The increased Al content results in a higher proportion of β phase, thereby promoting the development of an oxide scale in response to thermal exposure. This study provides insights into the fabrication of high Al content MCrAlY coatings with potential applications in next-generation thermal barrier coatings.

Influence of anionic silica forms in clear sodium silicate precursors on metakaolin geopolymerisation via 29Si and 27Al MAS-NMR and microstructural studies

A number of synthesis parameters directly influence the degree of reticulation/geopolymerisation of metakaolin exposed to alkaline solutions of sodium hydroxide and/or sodium silicate. In the latter case, a sodium silicate solution can be depolymerised by the introduction of an appropriate amount of NaOH. The effects of the ageing of the activator solution on the reticulation of metakaolin-based geopolymers are quantified for the first time in this work. We studied the anionic species of the sodium silicate solution with the addition of NaOH made just before the preparation of the paste, 24 h or 7 days before. These three ageing periods cause a significant difference in the Si-bearing species in solution, as demonstrated by nuclear magnetic resonance on 29Si. The effect of these anionic species on the reticulation/polymerisation of metakaolin at room temperature was demonstrated by solid-state 27Al and 29Si MAS-NMR, the chemical stability in various solutions (deionised water, HCl, HNO3, H2SO4), and X-ray diffraction on geopolymer powders before and after immersion in acids. Compressive strength before and after the immersion in acidic media was an additional measurement to assess the overall structural stability of the 3D polymerised network of the final dense ceramic-like product. Ageing of the activator solution affected the chemical stability of the hardened geopolymers accompanied by a slight to severe reduction in strength after leaching in HNO3 or HCl and in H2SO4, respectively. The quantitative MAS-NMR description of the Si and Al coordination in the geopolymers was correlated with the chemical stability where the formulations with the higher number of Q4(0Al) and Q4(1Al) for the silicon species were more resistant (lower number of Na+ compensating for Al+3 to be exchanged with H+). The formulations with higher Al content in the structure, i.e. higher number of Q4(3Al) silicon species showed higher mechanical stability. These results show that the timing of the preparation of the alkaline activator is essential for a correct mix design.