Mikrostruktur und mechanische Eigenschaften der reibgeschweißten γ‐TiAl‐Feingusslegierung Ti‐47Al‐3.5(Mn+Cr+Nb)‐0.8(B+Si)

Microstructure and mechanical properties of friction welded γ‐TiAl based alloy Ti‐47Al‐3.5(Mn+Cr+Nb)‐0.8(B+Si) in investment cast condition. This paper describes properties of joints produced by friction welding of the intermetallic γ‐TiAl based alloy Ti‐47Al‐3.5(Mn+Cr+Nb)‐0.8(B+Si) in investment cast and hot‐isostatically pressed condition. The effect of friction welding parameters on microstructure and local properties are examined and discussed. It is found that the properties of the joint are essentially affected by properties of as‐cast Ti‐47Al‐3.5(Mn+Cr+Nb)‐0.8(B+Si) base material, both at room temperature and 700 °C.     

Oxidation protective coatings for γ‐TiAl – recent trends

Engine designers show continued interest in γ‐TiAl based titanium aluminides as light–weight structural materials to be used at moderately elevated temperatures. Although alloy development has made significant progress in terms of mechanical properties and environmental resistance, protective coatings have been developed that help to extend the lifetime of these alloys significantly. The major challenge of coating development is to prevent the formation of fast growing titania. Furthermore, changes of coating chemistries at high temperatures have to be considered in order to avoid rapid degradation of the coatings due to interdiffusion between substrate and coating. The paper describes recent work of the authors on different coatings produced by means of magnetron sputter technique. Thin ceramic Ti‐Al‐Cr‐Y‐N layers tested at 900 °C exhibited poor oxidation resistance. In contrast, intermetallic Ti‐Al‐Cr, Si‐based and aluminum rich Ti‐Al coatings were tested at exposure temperatures up to 950 °C for 1000h resulting in reasonable and partially excellent oxidation behaviour.     

Formation of Ultrafine‐Grained Microstructure of Ti–6Al–4V Alloy by Hot Deformation of α′ Martensite Starting Microstructure

We have presented a formation of ultrafine‐grained microstructure (d_α ≈ 0.2 µm) of industrial Ti–6Al–4V alloy produced by the hot compression of a sample with the acicular α′ martensite starting microstructure. The hot‐deformation behavior was different from the case of the conventional (α + β) starting microstructure, that is, the phase transformation of α′/(α + β) during hot working enhanced the microstructural conversion, especially under the conditions of a low temperature and a high‐strain rate.     

Magnetic Metal–Organic Frameworks: γ‐Fe2O3@MOFs via Confined In Situ Pyrolysis Method for Drug Delivery

A general one‐step in situ pyrolysis route for the construction of metal–organic frameworks encapsulating superparamagnetic γ‐Fe₂O₃ NPs dispersed in the confined cavities of MOFs homogeneously is described. The integration of γ‐Fe₂O₃ NPs or clusters into MOFs can endow these porous materials with superparamagnetic element. By the combination of the thermal stability of MOFs and pyrolysis of metal triacetylacetonate complex at matched conditions, the porous structure of MOFs are well maintained while the size‐induced superparamagnetic property of nano γ‐Fe₂O₃ is obtained. As a proof of concept, both the γ‐ Fe₂O₃@ZIF‐8 and γ‐Fe₂O₃@MIL‐53(Al) were successfully prepared, and the latter was chosen to demonstrate its potential drug delivery as a magnetic MOF.     

Creep Behavior of High‐Nb TiAl Alloy at 800–900 °C by Directional Solidification

Cold crucible directional solidification Ti44Al6Nb1.0Cr alloy is crept at 800–900 °C. Experimental results show that creep lifetime significantly decreases with the increasing creep temperature. When creeping at 900 °C under 130 MPa, the T_Q twinning is activated in lamellar structures. The T_Q twinning shows a strong dependency on temperature during creep under low creep‐stress and it can overcome α₂ lamellae and transfer into adjacent γ lamellae. The hardening by mechanical twinning and the softening by α₂ lamellar dissolution take place at different zones in lamellar structures and the strain incompatibility between hardening zone and softening zone promotes the microcracks to form in lamellar structures. The deformation characteristic of hard and soft lamellae is studied. Moreover, recrystallization γ phase formed in lamellar structures near colony boundary during creep at 900 °C accelerates the creep failure.

     

Facile Synthesis of γ‐In2Se3 Nanoflowers toward High Performance Self‐Powered Broadband γ‐In2Se3/Si Heterojunction Photodiode

An effective colloidal process involving the hot‐injection method is developed to synthesize uniform nanoflowers consisting of 2D γ‐In₂Se₃ nanosheets. By exploiting the narrow direct bandgap and high absorption coefficient in the visible light range of In₂Se₃, a high‐quality γ‐In₂Se₃/Si heterojunction photodiode is fabricated. This photodiode shows a high photoresponse under light illumination, short response/recovery times, and long‐term durability. In addition, the γ‐In₂Se₃/Si heterojunction photodiode is self‐powered and displays a broadband spectral response ranging from UV to IR with a high responsivity and detectivity. These excellent performances make the γ‐In₂Se₃/Si heterojunction very interesting as highly efficient photodetectors.     

Microstructure and oxidation behaviour of TiAl(Nb)/Ti<Subscript>2</Subscript>AlC composites fabricated by mechanical alloying and hot pressing

TiAl-based intermetallic matrix composites with dispersed Ti₂AlC particles and different amounts of Nb were successfully synthesized by mechanical alloying and hot pressing. The phase evolution of Ti–48 at%. Al elemental powder mixture milled for different times with hexane as a process control agent was investigated. It was found that after milling for 25 h, a Ti(Al) solid solution was formed; also with increase in the milling time to 50 h, an amorphous phase was detected. Formation of a supersaturated Ti(Al) solid solution after 75 h milling was achieved by crystallization of amorphous phase. Addition of Nb to system also exhibited a supersaturated Ti(Al,Nb) solid solution after milling for 75 h, implying that the Al and Nb elements were dissolved in the Ti lattice in a non-equilibrium state. Annealing of 75 h milled powders resulted in the formation of equilibrium TiAl intermetallic with Ti₂AlC phases that showed the carbon that originates from hexane, participated in the reaction to form Ti₂AlC during heating. Consolidation of milled powder with different amounts of Nb was performed by hot pressing at 1000°C for 1 h. Only the presence of γ-TiAl and Ti₂AlC was detected and no secondary phases were observed on the base of Nb. Displacement of γ-TiAl peaks with Nb addition implied that the Nb element was dissolved into TiAl matrix in the form of solid solution, causing the lattice tetragonality of TiAl to increase slightly. The values for density and porosity of samples indicated that condition of hot pressing process with temperature and pressure was adequate to consolidate almost fully densified samples. The isothermal oxidation test was carried out at 1000°C in air to assess the effect of Nb addition on the oxidation behaviour of TiAl/Ti₂AlC composites. The oxidation resistance of composites was improved with the increase in the Nb content due to the suppression of TiO₂ growth, the formation and stabilization of nitride in the oxide scale and better scale spallation resistance.     

Multilayer Amorphous‐Si‐B‐C‐N/γ‐Al2O3/α‐Al2O3 Membranes for Hydrogen Purification

The hydrogen and carbon monoxide separation is an important step in the hydrogen production process. If H₂ can be selectively removed from the product side during hydrogen production in membrane reactors, then it would be possible to achieve complete CO conversion in a single‐step under high temperature conditions. In the present work, the multilayer amorphous‐Si‐B‐C‐N/γ‐Al₂O₃/α‐Al₂O₃ membranes with gradient porosity have been realized and assessed with respect to the thermal stability, geometry of pore space and H₂/CO permeance. The α‐Al₂O₃ support has a bimodal pore‐size distribution of about 0.64 and 0.045 µm being macroporous and the intermediate γ‐Al₂O₃ layer—deposited from boehmite colloidal dispersion—has an average pore‐size of 8 nm being mesoporous. The results obtained by the N₂‐adsorption method indicate a decrease in the volume of micropores—0.35 vs. 0.75 cm³ g^?1—and a smaller pore size ?6.8 vs. 7.4 Å—in membranes with the intermediate mesoporous γ‐Al₂O₃ layer if compared to those without. The three times Si‐B‐C‐N coated multilayer membranes show higher H₂/CO permselectivities of about 10.5 and the H₂ permeance of about 1.05 × 10^?8 mol m^?2 s^?1 Pa^?1. If compared to the state of the art of microporous membranes, the multilayer Si‐B‐C‐N/γ‐Al₂O₃/α‐Al₂O₃ membranes are appeared to be interesting candidates for hydrogen separation because of their tunable nature and high‐temperature and high‐pressure stability.     

Investigation of In Situ and Conventional Post‐Weld Heat Treatments on Dual‐Laser‐Beam‐Welded γ‐TiAl‐Based Alloy

This paper describes a way to improve the microstructure and mechanical properties of welding seams by in situ and conventional post‐weld heat treatments for laser beam welding of the Ti–45Al–5Nb–0.2C–0.2B alloy. The seams are crack‐free with reduced longitudinal residual stress and higher elongation to fraction after post‐weld heat treatment. The welding zone consists of α₂ after welding, transforms to a massive γ during in situ post‐weld heat treatment, and finally forms a convoluted microstructure after conventional heating. The phase composition across the welding zone is discussed.     

Metal Injection Moulding Using Intermetallic γ‐TiAl Alloy Powder

Argon gas atomized γ‐TiAl alloy powder of high purity has been used for metal injection moulding. In order to keep the pick‐up of impurities as low as possible most of the process steps were done under inert gas or high vacuum. The binder system used had been especially developed for titanium alloys. In the sintered structures the nitrogen and carbon levels are low, oxygen scatters over a wide range. The porosity of ∼4 % after sintering could be reduced to ∼0.4 % by additional hot isostatic pressing. Tensile tests at room temperature indicate promising properties of σ_0.2 = 409 MPa, UTS = 433 MPa and ϵ_pl = 0.6 %.     

Microstructure and Texture Formation During Near Conventional Forging of an Intermetallic Ti–45Al–5Nb Alloy

Texture formation was studied in an intermetallic Ti‐45at%Al‐5at%Nb alloy after uniaxial compression and near conventional forging. Depending on the deformation conditions the texture of the γ‐TiAl phase is formed by pure deformation components, components related to dynamic recrystallization, or transformation components. This changing corresponds with microstructural observations. The α₂‐Ti₃Al and the α‐Ti(Al) phase show a similar texture as it is known for Ti and Ti‐base alloys after compressive deformation at elevated temperatures. In contrast to the γ texture, no significant change of the α/α₂ texture was observed in the temperature range between 800 °C and just below the α‐transus temperature (T_α = 1295 °C).     

Self‐Assembled Pd@CeO2/γ‐Al2O3 Catalysts with Enhanced Activity for Catalytic Methane Combustion

Pd@CeO₂/Al₂O₃ catalysts are of great importance for real applications, such as three‐way catalysis, CO oxidation, and methane combustion. In this article, the Pd@CeO₂ core@shell nanospheres are prepared via the autoredox reaction in aqueous phase. Three kinds of methods are then employed, that is, electrostatic interaction, supramolecular self‐assembly, and physical mixing, to support the as‐prepared Pd@CeO₂ nanospheres on γ‐Al₂O₃. A model reaction of catalytic methane‐combustion is employed here to evaluate the three Pd@CeO₂/γ‐Al₂O₃ samples. As a result, the sample Pd@CeO₂‐S‐850 prepared via supramolecular self‐assembly and calcined at 850 °C exhibits superior catalytic performance to the others, which has a far lower light‐off temperature (T₅₀ of about 364 °C). Moreover, almost no deterioration of Pd@CeO₂‐S‐850 is observed after five sequent catalytic cycles. The analysis of H₂‐TPR curves concludes that there exists hydrogen spillover related to the strong metal–support interaction between Pd species and oxides. The strong metal–support interaction and the specific surface areas might be responsible for the catalytic performance of the Pd@CeO₂ samples toward catalytic methane combustion.     

Dipolar‐Distribution Cavity γ‐Fe2O3@C@α‐MnO2 Nanospindle with Broadened Microwave Absorption Bandwidth by Chemically Etching

Developing microwave absorption materials with ultrawide bandwidth and low density still remains a challenge, which restricts their actual application in electromagnetic signal anticontamination and defense stealth technology. Here a series of olive‐like γ‐Fe₂O₃@C core–shell spindles with different shell thickness and γ‐Fe₂O₃@C@α‐MnO₂ spindles with different volumes of dipolar‐distribution cavities were successfully prepared. Both series of absorbers exhibit excellent absorption properties. The γ‐Fe₂O₃@C@α‐MnO₂ spindle with controllable cavity volume exhibits an effective absorption (2O₃@C spindle reaches as high as ?45 dB because of the optimized electromagnetic impedance balance between polymer shell and γ‐Fe₂O₃ core. Intrinsic ferromagnetism of the anisotropy spindle is confirmed by electron holography. Strong coupling of magnetic flux stray lines between spindles is directly imaged. This unique morphology and facile etching technique might facilitate the study of core–shell type microwave absorbers.     

Enhanced Grain Refinement Through Deformation Induced α Precipitation in Hot Working of α + β Titanium Alloy

This study reports a novel forging process to fabricate bulk fine‐grained (grain size ≈ 1 µm) Ti–6Al–4V alloy, in which temperatures near the β transus (T_β) and strain rates around 0.15 s^?1 are used for the deformation. The formation of fine‐grained microstructure is mainly result from the deformation‐induced precipitation of α grains from the β matrix.     

Giant Magnetic Heat Induction of Magnesium‐Doped γ‐Fe2O3 Superparamagnetic Nanoparticles for Completely Killing Tumors

Magnetic fluid hyperthermia has been recently considered as a Renaissance of cancer treatment modality due to its remarkably low side effects and high treatment efficacy compared to conventional chemotheraphy or radiotheraphy. However, insufficient AC induction heating power at a biological safe range of AC magnetic field (H_appl·f_appl < 3.0–5.0 × 10⁹ A m^?1 s^?1), and highly required biocompatibility of superparamagnetic nanoparticle (SPNP) hyperthermia agents are still remained as critical challenges for successful clinical hyperthermia applications. Here, newly developed highly biocompatible magnesium shallow doped γ‐Fe₂O₃ (Mg_0.13‐γFe₂O₃) SPNPs with exceptionally high intrinsic loss power (ILP) in a range of 14 nH m² kg^?1, which is an ≈100 times higher than that of commercial Fe₃O₄ (Feridex, ILP = 0.15 nH m² kg^?1) at H_appl·f_appl = 1.23 × 10⁹ A m^?1 s^?1 are reported. The significantly enhanced heat induction characteristics of Mg_0.13‐γFe₂O₃ are primarily due to the dramatically enhanced out‐of‐phase magnetic susceptibility and magnetically tailored AC/DC magnetic softness resulted from the systematically controlled Mg²⁺ cations distribution and concentrations in octahedral site Fe vacancies of γ‐Fe₂O₃ instead of well‐known Fe₃O₄ SPNPs. In vitro and in vivo magnetic hyperthermia studies using Mg_0.13‐γFe₂O₃ nanofluids are conducted to estimate bioavailability and biofeasibility. Mg_0.13‐γFe₂O₃ nanofluids show promising hyperthermia effects to completely kill the tumors.     

Einfluß des Gefügezustandes auf die mechanischen Eigenschaften von (α + β)- Titanlegierungen

Influence of Microstructure on Mechanical Properties of (α + β)-Ti-Alloys The amount of α- and β-phase in (α + β)-Titanium alloys for example in the alloy TiAl6 V4 may be altered to a large extend by annealing procedures. In the alloy TiAl6 V4 each temperature corresponds to a certain amount of each of these phases, the composition of which is also dependent on annealing temperature. Above about 970 °C the homogeneous β-phase appears. Additions of Iron, Oxygen, Nitrogen and Hydrogen have a great influence on the transition temperature and the distribution of the α- and β-phase. There is a great influence of the distribution of the phases on the mechanical properties. The β-phase and the α-solid solution can be quenched from high temperature and decomposed at a lower temperature, so that intermediate phases as ω from β and α₂ from α may be precipitated. The α-solid solution is anisotropic so that texturized samples have different properties in various directions. Based on these fundamental aspects it is shown to what extend these microstructures may influence the mechanical properties especially the fatigue behaviour of (α + β)-Ti-alloys.     

Auswirkungen von statischen Langzeitglühungen auf Gefüge und mechanische Eigenschaften einer γ‐TiAl Basislegierung

Influence of long‐term ageing on microstructure and mechanical properties of a γ‐TiAl based alloy This work deals with the results of static annealing treatments (700 °C and 800 °C in air, up to 10.000 hours) conducted on a powder metallurgical processed Ti‐46.5at.%‐4at.%(Cr,Nb,Ta,B) alloy with a designed fully lamellar (DFL) microstructure. It is the aim of this work to discuss possible changes in microstructure and related changes in mechanical properties as a function of annealing time and temperature. After long‐term exposure it can be expected that amount, type and composition of constitutional phases have reached their thermodynamic equilibrium values. These experimental results were compared with theoretical considerations based on thermodynamic equilibrium calculations which provide baseline information upon amount, type and composition of constituting phases.     

Mechanochemical Synthesis of γ‐Graphyne with Enhanced Lithium Storage Performance

γ‐Graphyne is a new nanostructured carbon material with large theoretical Li⁺ storage due to its unique large conjugate rings, which makes it a potential anode for high‐capacity lithium‐ion batteries (LIBs). In this work, γ‐graphyne‐based high‐capacity LIBs are demonstrated experimentally. γ‐Graphyne is synthesized through mechanochemical and calcination processes by using CaC₂ and C₆Br₆. Brunauer–Emmett–Teller, atomic force microscopy, X‐ray photoelectron spectroscopy, solid‐state ¹³C NMR and Raman spectra are conducted to confirm its morphology and chemical structure. The sample presents 2D mesoporous structure and is exactly composed of sp and sp²‐hybridized carbon atoms as the γ‐graphyne structure. The electrode shows high Li⁺ storage (1104.5 mAh g^?1 at 100 mA g^?1) and rate capability (435.1 mAh g^?1 at 5 A g^?1). The capacity retention can be up to 948.6 (200 mA g^?1 for 350 cycles) and 730.4 mAh g^?1 (1 A g^?1 for 600 cycles), respectively. These excellent electrochemical performances are ascribed to the mesoporous architecture, large conjugate rings, enlarged interplanar distance, and high structural integrity for fast Li⁺ diffusion and improved cycling stability in γ‐graphyne. This work provides an environmentally benign and cost‐effective mechanochemical method to synthesize γ‐graphyne and demonstrates its superior Li⁺ storage experimentally.     

Aligned Single‐Crystalline β‐Si3N4 Whiskers Prepared with SHS Process

Aligned single‐crystalline β‐Si₃N₄ whiskers with high aspect ratio were first prepared via a Self‐propagating high temperature‐synthesis (SHS) process, by using tungsten powders as catalysts. The as‐synthesized Si₃N₄ whiskers typically have uniform diameters of 400 nm, length about 200 µm, and exhibit smooth and straight surfaces. Above all, the products possesses a perfect aligned structure, which is quite different from the reported β‐Si₃N₄ whiskers. Elastic bending modulus of individual whiskers was measured by in‐situ TEM process, the average value of elastic bending modulus of individual as‐synthesized whiskers was 488 GPa. Results revealed that tungsten powders plays an significant effects on the morphology of Si₃N₄ whiskers.     

Newly observed prismatic Mg2Sn laths in a Mg–Sn–Zn–Mn alloy

Lath-shaped Mg₂Sn precipitates with their habit planes parallel to the prismatic planes of the Mg matrix are characterized in a Mg–Sn–Zn–Mn alloy. The orientation relationships (ORs) between these β-Mg₂Sn precipitates and α-Mg matrix are [0 1 1]_β//[0 1 ?1 0]_α and (0 1 ?1)_β deviating 0.36° to 1.20° from (0 0 0 1)_α, in which the deviation angle of 0.39° is most frequently observed. Although the ORs vary, the laths always exhibit four side facets bearing fixed relationships with g vectors in reciprocal space. Their major side facets incline to the basal plane of Mg matrix from 4.3° to 14.3°.