Impact toughness and its low-temperature transformation are key indicators of high-speed railway axle materials, which are closely related to the microstructure of the material. The embrittlement degree of materials with different microstructures at low temperatures also varies. Aiming to find a way to optmize the strength-toughness relationship of the relevant steel, the effect of adjusting the tempering temperature on the strength-toughness relationship of DZ2 steel, which was newly designed and developed at home for high-speed railway axle, was studied via tensile tests, room- and low-temperature impact tests, as well as observation and analysis of the microstructure evolution of the steel. Then the performance changes of the steel during the tempering treatment process were explained. The results indicate that as the tempering temperature increases, the tensile strength gradually decreases from 1357 MPa to 761 MPa, the elongation after fracture increases from 11.7% to 28.4%, the room temperature impact energy gradually increases from 34.3 J to 98.7 J, and the low temperature impact energy gradually increases from 24 J to 90.3 J. However, the impact of temperature reduction on the impact energy gradually weakens, the minimum impact energy decrease percentage of low-temperature is 8%. The transformation trend of these properties is closely related to the micro-scale deformation mechanism changes caused by the gradual spheroidization of carbides. It can be concluded from the comprehensive analysis that, while meeting the requirements of room temperature mechanical performance standards, adjusting the tempering temperature can improve the low-temperature impact toughness of the axle steel, to meet the needs of further high-speed railway speed increase and safe operation in harsh environments.
The application of electrochromic devices (ECDs) in smart windows provides new solutions for energy conservation and environmental protection. WO3/Pt composite films for electrochromic cathodes were prepared through a combination of hydrothermal and electrodeposition methods. WO3/Pt composite films of different deposition times were characterized and their electrochromic properties were examined. Next, NiO thin films were prepared by electrodeposition as electrochromic anodes. WO3/Pt-NiO electrochromic devices were constructed with WO3/Pt composite films and NiO films as color changing cathodes and anodes, respectively. The WO3/Pt-NiO electrochromic device has a relatively fast response time (coloring/fading rate of 18.84/29.62 s) and a good optical modulation range (47% at 630 nm). Its designed ECD has the characteristics of large optical modulation, high coloring efficiency, and fast switching speed. These characteristics make WO3/Pt-NiO electrochromic devices a very promising candidate for smart windows in lighting control and energy-saving applications, and their use as color-changing smart windows has broad application prospects in fields such as architecture, aircraft, and automobiles.
As electrode material for supercapacitors, graphene-supported Zn-BTC metal-organic framework materials (Zn-BTC/rGO) with different morphologies were synthesized via a simple ultrasonic vibration method. The prepared Zn-BTC/rGO materials were characterized by means of TG, SEM, XRD, BET, Raman and electrochemical workstation in terms of the structure, morphology and electrochemical property. Their capacitance performance was examined by a three-electrode system. The results show that the synthesized one-dimensional rod-like Zn-BTC is uniformly anchored to the wrinkled layer of graphene nanosheets, and the material exhibits excellent capacitance performance, with a specific capacitance of 182.4 C·g-1 (1 A·g-1), which is better than the graphene-supported two-dimensional sheet-like Zn-BTC (139.3 C·g-1), graphene (97.9 C·g-1) and simple one-dimensional rod-like Zn-BTC (62.8 C·g-1). A symmetrical supercapacitor was assembled with the graphene supported one-dimensional rod-shaped Zn-BTC, which presents excellent performance: the specific capacity of 57.7 F·g-1 at a current density of 1 A·g-1, the maximum energy density of 1.99 Wh·kg-1 at a power density of 1390 W·kg-1, and the specific capacity retention rate of 90.3% after 2000 charge-discharge cycles.
The cation exchange membrane was prepared by co-deposition of Dopamine (DA) and Polyethyleneimine (PEI) based on mussel bionic binders, and its properties were characterized by infrared spectrometer, scanning electron microscope, and UV visible spectrophotometer. The effect of DA to PEI concentration ratio on the performance of the modified membrane was studied. The results showed that with the increase of PEI concentration, the selective permeability and membrane resistance increased first and then decreased, while the oxidation percentage of the modified membrane decreased first and then increased. When the concentration ratio of DA to PEI was 1:1, the modified membrane had lower oxidation percentage and membrane resistance. Accordingly, the water content and ion exchange capacity were 48.68% and 2.49 mmol/g, respectively, and the selective permeability was up to 97.8%, that is 8.08% superior to the original membrane.
The thermal cycling (1000~10000 cycles) testing of H13 steel between molten ADC12 Al-alloy (670~730oC) and spray cooling chamber was conducted in this paper. The microstructure evolution and hardness change of H13 steel were studied against the cycling process, and then, a quantitative model of hardness change was established based on the kinetics of solid phase transformations theory. The results indicate that H13 steel undergoes softening along with the thermal cycling. At the beginning of the cycling, the softening of the matrix is mainly due to the decrease of dislocation density. With the increase of the cycling times, the softening at the middle and late stages was mainly due to the coarsening of carbides, the broadening of martensite lath and the growth of sub-grains. During different cycling tests, increasing the temperature of the molten Al-alloy may accelerate the softening of the matrix. According to the kinetics equation of solid phase transformations, the calculated phase transformation activation energy of the H13 steel is 200.78 kJ/mol, which is similar to the diffusion activation energy of alloy elements Cr, V, and Mo in ferrite, indicating that the softening rate of H13 steel depends on the diffusion of these elements.
SiC / ZL102 Al-alloy composite foam was prepared via secondary foaming process in the temperature range of 660oC~700oC, by taking the prepared SiC/ZL102 Al-alloy composites with addition of appropriate foaming agent and various SiC amount as precursors. The influence of viscosity of melt composites on the stability of ZL102 Al-alloy foam was studied by revealing the relation between the SiC content with the density of precursor, the variation of porosity, pore number, average pore size and pore wall thickness of the prepared ZL102 Al-alloy foams at different temperatures. The acquired foams were characterized by means of EDS, SEM and super deep field microscope. The results show that with the increasing SiC content the density of the precursors is increased, whilst, the density, precursor with 6wt.%SiC is the highest. The suitable secondary foaming temperature is 680oC. With the increasing foaming temperature, the average pore size and pore wall thickness of low viscosity Al-alloy foam decrease, while the average pore size of high viscosity Al-alloy foam increases and the pore wall thickness decreases. The high viscosity Al-alloy foam has stable pore structure and higher porosity.
B4C ceramics has extremely high hardness, but its fracture toughness is low. In order to improve the fracture toughness of B4C ceramics, the effect of introducing the second phase Al2O3 on the fracture toughness of B4C ceramics is studied, and the toughening mechanism of B4C-Al2O3 composite ceramics is explored. The results indicate that the addition of Al2O3 as the second phase can improve the fracture toughness of B4C ceramics. Among others, the fracture toughness of B4C-Al2O3 composite ceramics with 40%Al2O3 reaches a maximum value of 4.96 MPa·m1/2. The toughening mechanism of B4C-Al2O3 composite ceramics is that Al2O3 grains experience cleavage cracking during the crack propagation, increasing the path of crack propagation; thus, part of crack propagation energy is consumed. Meanwhile, residual stress is generated between Al2O3 grains and B4C grains due to their thermal expansion mismatch. On the one hand, the compressive stress inside B4C grains is beneficial for inhibiting crack propagation. On the other hand, the tensile stress generated at the phase boundary between B4C grains and Al2O3 grains weakens the bonding of the phase boundary to some extent, leading to some cracks propagating along the phase boundary during the propagation process; therefore, some cracks are deflected, and so the fracture toughness of B4C-Al2O3 composite ceramics is improved.
The low cycle fatigue behavior of nickel based cast superalloy K4169 at 650oC was studied, while its microstructure variation before and after test was assessed by means of SEM and TEM. The results show that the fatigue life of the alloy gradually decreases with the increasing strain. When the strain is 0.5% and 0.6%, the alloy experiences strain hardening within the early 2~200 cycles, followed by cyclic stability and cyclic softening, respectively; When the strain is 0.8% and 1.0%, the alloy exhibits continuous cyclic softening behavior; Initial cyclic hardening is related to the hindering effect of γ″ strengthening phases on the movement of dislocations, while cyclic softening is attributed to dislocations shearing γ″phases repeatedly. The Coffin-Manson equation for the relationship between the plastic strain amplitude and the reverse number of fatigue failure of the alloy exhibits a bilinear relationship. Observation of the microstructure shows that the cyclic deformation mode of the alloy at high and low strains is all dislocations shearing γ″ phase and slip, and the fatigue failure of the alloy under different strains all exhibits transgranular fracture. Therefore, the reason for the bilinear behavior of the alloy may be the transformation of deformation uniformity, and the non-Masing characteristic exhibited by the alloy also demonstrate the transformation of deformation uniformity.
Aiming at the problem of low chloride-poisoning resistance capacity and low electrocatalytic activity for the catalytic material of oxygen reduction, hierarchical composite material was synthesized and its oxygen reduction process was studied in the natural seawater. Herein, A highly active material of hierarchical Pt@Co@-N-C composite coating on carbon cloth was prepared via a combined technique composed of in-situ growth method, high-temperature carbonization treatment and electrodeposition. Characterization results indicate that the as-synthesized composite displays a multi-layered core-shell encapsulation structure with carbon fibers serving as the core matrix, ZIF8/ZIF67-deriving microporous Co-N-C as the bottom coating and the electrodeposited Pt nanoclusters as the apparent catalytic coating. Whereinto, the Co-N-C coating provides lots of depositing sites for improving the dispersibility of Pt nanoparticles, expediting the uniform growth of Pt nanoclusters. Electrochemical results show that in comparison to the commercial Pt/C catalyst, Pt@Co-N-C@CC possesses better electrocatalytic oxygen reduction performance, i.e.which presents onset potential 0.075 V and half-wave potential -0.156 V all much more positive than those of the commercial ones -0.028 V and -0.401 V (vs. Ag/AgCl) respectively. The seawater battery assembling Pt@Co-N-C@CC and Mg shows higher cell voltage of above 0.8 V and maximum power density of 7.6 mW/cm2, in contrary, below 0.5 V and 3.9 mW/cm2 respectively for the assembling Pt/C and Mg. These prove that the high-efficiency recombination of ZIF8/ZIF67-deriving Co-N-C and Pt nanoclusters benefits to enhance the catalytic activity and improve the chloride-poisoning resistance.
