Microstructure and mechanical properties of Inconel 625 superalloy

The service-exposed (∼60 000 h/873 K) Alloy 625 ammonia cracker tubes showed higher strength and lower ductility compared to the virgin material in the solution annealed state. Precipitation of intermetallic γ″ and Ni₂(Cr,Mo) phases and the inter and intragranular carbides were found to be responsible for higher strength of the service-exposed alloy. Subjecting the service-exposed alloy to thermal aging treatments subsequently at 923 K and 1123 K (above the service temperature of the exposed alloy) led to the dissolution of the intermetallic phases that in turn increased the ductility of the alloy. Post-service aging of the alloy at 923 K for short durations resulted in the dissolution of the Ni₂(Cr,Mo)-phase. The dissolution of the Ni₂(Cr,Mo)-phase exhibited significant influence upon yield strength (YS) but negligible effect on ductility. Prolonged aging of the alloy for 500 h at 923 K resulted in the precipitation of intermetallic δ-phase. Post-service aging of the alloy at 1123 K promoted the dissolution of both Ni₂(Cr,Mo) and γ″ formed during service. Longer duration aging at the same temperature led to the precipitation of the δ-phase with an associated increase in strength and loss in ductility. Re-solution annealing of the service-exposed alloy at 1423 K caused the dissolution of the strengthening phases. When the re-solution annealed alloy was subjected to prolonged exposure at 923 K, the yield stress was found to increase rapidly with aging time with attendent loss in ductility due to the precipitation of γ″.     

Control of strip casting process: decentralization and optimal roll force control

In this paper, modeling and control of a twin-roll strip caster are investigated. The control objectives are to achieve a constant strip thickness and to maintain a constant roll separating force. Mathematical models are derived by analyzing five critical areas: molten steel level in the pool, solidification process, roll separating force and torque, roll gap dynamics, and roll drive dynamics. A two-level control strategy is proposed. At low level, three local controllers regulate three subsystems independently. They are a variable structure controller for the molten steel level of the pool, an adaptive predictive controller for the roll gap, which is directly related to the strip thickness, and a two-degree-of-freedom robust servo controller for the roll speed. At high level, an H₂ optimal controller governs the interaction dynamics among subsystems and generates a reference signal to the local roll speed controller in the fashion that a constant roll separating force is maintained. In designing the high level controller, the complex strip casting dynamics is linearized at an operating point and parameter estimation and uncertainty quantification methods are used. Simulation results are provided.     

Bulk amorphous FC20 (Fe–C–Si) alloys with small amounts of B and their crystallized structure and mechanical properties

The addition of a small amount (0.4 mass%) of B to a commercial FC20 cast iron was found to cause the formation of an amorphous phase in melt-spun ribbon and cast cylinders with a diameter of up to 0.5 mm. The structure of a melt-spun B-free FC20 alloy consisted of α-Fe, γ-Fe and Fe₃C. The effectiveness of additional B is presumably due to the generation of attractive bonding nature among the constituent elements. The amorphous alloy ribbon exhibits a high tensile strength of 3480 MPa and good bending ductility. The annealing causes the formation of an amorphous phase containing α-Fe particles with a size of about 30 nm. The mixed phase alloy exhibits an improved tensile strength of 3800 MPa without detriment to good ductility. With further increasing temperature, the mixed amorphous and α-Fe structure changes to α-Fe+Fe₃C+graphite through the metastable structure of α-Fe+Fe₃C. The structure after annealing for 900 s at 1200 K has fine grain sizes of about 0.5 μm for α-Fe, 0.3 μm for Fe₃C and 1 μm for graphite. The graphite-containing alloy exhibits high tensile strength of 1200–2000 MPa and large elongation of 5–13%. The high tensile strength and good ductility were also obtained for the 0.5 mm cylinder annealed at 1200 K. The good mechanical properties are due to the combination of fine subdivision of crack initiation sites by the homogeneous dispersion of small graphite particles and the dispersion strengthening of Fe₃C particles against the deformation of the α-Fe phase. The synthesis of the finely mixed α-Fe+Fe₃C+graphite alloys having good mechanical properties by crystallization of the new amorphous alloy in the melt-spun ribbon and cast cylinder forms is encouraging for the future development of a new Fe-based high-strength and high-ductility material.     

Operative slip systems and anomalous strengthening in Ni3Nb single crystals with the D0a structure

The plastic deformation behavior of Ni₃Nb single crystals was examined in tension and compression to determine the operative slip and twinning systems, and to explore the anomalous strengthening behavior. A strong temperature dependence of the CRSS for both the slip and twinning systems was observed, which was dependent on the sample orientation. Anomalous flow behavior was also observed in Ni₃Nb crystals deformed by (010)[100] and (001)[100] slip. The anomalous strengthening mechanism is discussed on the basis of both the anisotropy of APB energy and the formation of dragging atmosphere around moving dislocations.     

A thermodynamic assessment of the Cu-Si system

Experimental information on the thermochemistry and phase equilibria in the Cu-Si system have been combined to calculate a set of coefficients representing the thermodynamic properties of the pure components, stoichiometric phases and solution phases of the system considered. With these coefficients, the thermodynamic functions and the phase diagram have been calculated. The calculated results agree well with the experimental results, except for the chemical potential of Si in liquid Cu-Si at low Si contents.     

The effect of observed climatic conditions on the moisture equilibrium level of fibre-reinforced plastics

Calculations have been made to determine the moisture absorption behaviour of fibre-reinforced epoxy-matrix composites after exposure to prescribed outdoor climatic environments. A review of world-wide meteorological conditions has been made and six environments were chosen to represent a variety of regions. The results suggest that the level of moisture currently assumed to be absorbed by a composite during the service life of an aircraft is too low and that a much higher value should be used. Since the absorption kinetics of resin matrices differ widely and also change with physical ageing, the validity of specifying a moisture level to define the degree of environmental degradation in structural assessment is questioned. An alternative criterion, a constant relative humidity environment that will produce a representative moisture level in all parts of the structure and for all matrices currently in use, is proposed. Using this philosophy it is suggested that the world-wide worst environment might best be simulated by a constant humidity of 84%.     

滚挤压加工对颗粒增强金属基复合材料表面粗糙度的影响

将滚挤压加工工艺引入到颗粒增强铁基复合材料的精加工领域,得出了各工艺参数、增强相含量等因素对表面粗糙度的影响规律,分析了这种复合材料的滚挤压加工机理,并优化了工艺条件.     

Cryogenic 3D Printing of w/o Pickering Emulsions Containing Bifunctional Drugs for Producing Hierarchically Porous Bone Tissue Engineering Scaffolds with Antibacterial Capability

How to fabricate bone tissue engineering scaffolds with excellent antibacterial and bone regeneration ability has attracted increasing attention. Herein, we produced a hierarchical porous β-tricalcium phosphate (β-TCP)/poly(lactic-co-glycolic acid)-polycaprolactone composite bone tissue engineering scaffold containing tetracycline hydrochloride (TCH) through a micro-extrusion-based cryogenic 3D printing of Pickering emulsion inks, in which the hydrophobic silica (h-SiO₂) nanoparticles were used as emulsifiers to stabilize composite Pickering emulsion inks. Hierarchically porous scaffolds with desirable antibacterial properties and bone-forming ability were obtained. Grid scaffolds with a macroscopic pore size of 250.03 ± 75.88 μm and a large number of secondary micropores with a diameter of 24.70 ± 15.56 μm can be fabricated through cryogenic 3D printing, followed by freeze-drying treatment, whereas the grid structure of scaffolds printed or dried at room temperature was discontinuous, and fewer micropores could be observed on the strut surface. Moreover, the impartment of β-TCP in scaffolds changed the shape and density of the micropores but endowed the scaffold with better osteoconductivity. Scaffolds loaded with TCH had excellent antibacterial properties and could effectively promote the adhesion, expansion, proliferation, and osteogenic differentiation of rat bone marrow-derived mesenchymal stem cells afterward. The scaffolds loaded with TCH could realize the strategy to “kill bacteria first, then induce osteogenesis”. Such hierarchically porous scaffolds with abundant micropores, excellent antibacterial property, and improved bone-forming ability display great prospects in treating bone defects with infection.     

A highly active and stable Pt modified molybdenum carbide catalyst for steam reforming of dimethyl ether and the reaction pathway

To improve the stability of molybdenum carbide catalysts in dimethyl ether steam reforming (DSR), the inactivation mechanism and the performance of Pt modified catalyst has been investigated. The Mo₂C oxidation induced by H₂O is verified to be the main reason of catalytic deactivation. After modified with Pt, the H₂ production rate and selectivity are greatly enhanced, reaches 1605 μmol min⁻¹·g_cat⁻¹ at 350 °C, in comparison to that of the Mo₂C/Al₂O₃ catalyst. Moreover, the 2%Pt–Mo₂C/Al₂O₃ catalyst is more stable with only 20% activity loss after 50 h on stream compares to the 73% activity loss in 12 h with Mo₂C/Al₂O₃ catalyst. By means of in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), the enhancement brought by Pt is ascribed to the consumption acceleration of intermediate oxygen species on catalyst surface and the decline of onset temperature of DSR reaction. It is expected that these findings can lead us to more practical molybdenum carbide catalysts in DSR.     

Concentrated solar photocatalysis for hydrogen generation from water by titania-containing gold nanoparticles

Photocatalysis is an effective way to utilize solar energy to produce hydrogen from water. Au/TiO₂ nanoparticles (NPs) have a better performance in photocatalytic hydrogen generation because of the localized surface plasmon resonance (LSPR) effect of Au/TiO₂ NPs. In the photocatalytic hydrogen generation experiments, it was found that light intensity plays a key role in the photocatalytic reaction rate of Au/TiO₂ NPs. At a light intensity of 0–7 kW/m², the reaction rate has a super-linear law dependence on the light intensity (Rate ∝ Intensityⁿ, with n > 1). However, at a light intensity of 7–9 kW/m², the dependency becomes sub-linear (n < 1). This means that the increase rate of photocatalytic rate is smaller than that of light intensity when the light intensity exceeds 7 kW/m². In addition, the finite element method (FEM) was utilized to further elucidate the role of light intensity by calculating the absorption power and nearfield intensity mapping of a Au/TiO₂ nanoparticle. The variation trend of the calculated total absorption power agrees with the photocatalytic experimental results for different light intensities. These results shed light on the utilization of concentrated solar photocatalysis to increase the solar-to-hydrogen performance of Au/TiO₂ NPs.