For proton exchange membrane fuel cells (PEMFCs) powered vehicles using on-board H2 production by methanol steam reforming (SRM), how to enhance the anti-oxidation and anti-sintering abilities of SRM ...catalysts are two challenges. Herein, to address the two challenges, a PdZn@ZnO catalyst has been synthesized by in-situ transformation of Pd@ZnO core-shell structure under thermal treatment and H2 reduction, which is a facile and green method. The PdZn@ZnO catalyst exhibited an excellent anti-oxidation ability without obvious change in catalytic activity before and after oxidation treatment. Density functional theory calculations revealed that the abundant PdZn/ZnO interface sites of PdZn@ZnO catalyst could suppress O2 dissociation and subsequent oxidation of the PdZn catalyst. In addition, ZnO shell of PdZn@ZnO prevented the encapsulated PdZn alloys from sintering, resulting in its superior stability to those of the reported catalysts. The PdZn@ZnO catalyst showed great potential for the on-board production of hydrogen for PEMFC-powered vehicles.
•PdZn@ZnO was prepared by self -assembly and in-situ transformation methods for SRM.•PdZn@ZnO exhibited superior anti-oxidation ability and stability.•Dissociation of O2 on ZnO/PdZn interface site is suppressed obviously.
•A modeling approach by combining crystal plasticity with creep cavity model is developed.•Transition mechanism from transgranular to intergranular crack with increasing hold time is ...revealed.•Predicted crack initiation life agrees with the experimental data within an error band of ±2.
In this study, a dual-scale numerical procedure is developed to reveal the creep-fatigue damage mechanisms and estimate the crack initiation life for notched structures made of Inconel 718 superalloy at 650 °C. The macro-scale simulation solves the creep-fatigue deformation behavior with viscoplastic constitutive models, and the local deformation histories are supplied to the micro-scale simulation as boundary conditions. In the micro-scale simulation, the local damage evolutions are solved based on crystal plasticity combined with grain boundary cavity model. The creep damage is calculated by a special formulation in the form of cavity nucleation, growth and coalescence. The fatigue damage is represented by accumulated energy dissipation originated from crystal plasticity finite element simulation. Experimentally, the creep-fatigue tests of notched structures are carried out for Inconel 718 superalloy at 650 °C to validate the feasibility and robustness of the proposed numerical procedure. Moreover, the crack initiation behavior, including transgranular cracks under fatigue loading and intergranular cracks under creep-fatigue loading, is explained through different types of microstructure observations together with a dual-scale numerical procedure. In detail, the crack initiation sites transferred from the grain interior at notched surface to the grain boundaries at notched subsurface with an increase in hold times can be well predicted by the proposed numerical procedure. In addition, the simulated life based on the developed life prediction approach agrees well with the experimental data within an error band of ±2. Parametric studies show that the creep damage is more sensitive to grain boundary diffusion than to the external conditions of strain level and hold time.
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For nuclear power generation as a carbon‐neutral energy source, in‐vessel retention (IVR) must be implemented to maintain the structural integrity of nuclear reactor pressure vessel (RPV) for more ...than 72 h under severe accidental conditions. This technology requires accurate prediction of creep deformation and life of RPV material being operated under pressure and extremely high‐temperature gradient. The current work develops a simplified deformation‐mechanism‐based true‐stress (DMTS) model for creep behavior/life‐prediction of SA508 Gr.3 steel, a typical RPV material, above the phase transformation temperatures (800–1000°C). This model is used to evaluate the time to specific creep strain (t3% and t5%) and rupture (tr), in comparison with popular empirical methods such as Orr–Sherby–Dorn (OSD) and Larson–Miller (LM). The simplified DMTS model achieves an excellent agreement with the experimental observations. The controlling deformation mechanisms are also discussed by metallurgical examinations, which provide the physical premise for the model development and application.
Highlights
A simplified deformation‐mechanism‐based true‐stress (DMTS) creep model was proposed.
This model predicted the creep‐strain time and life of SA508 Gr. 3 at 800–1000°C.
The prediction accuracy of simplified DMTS was compared with LM and OSD methods.
The dominant creep deformation mechanism was dislocation climb‐plus‐glide.
In order to improve product design efficiency and guarantee the high temperature structural integrity during the long-term creep of solid oxide fuel cell (SOFC), the creep strength design method is ...studied by using the finite element method (FEM) and the response surface method (RSM) with considering the interaction between the geometric parameters. A multi-regression model representing the correlation between the sealant failure probability and the geometric parameters is established for rapid estimation of creep strength and optimization design of geometric dimensions. The sealant failure probability is decreased from 0.994 to 0.015 by the optimization of SOFC geometrical size. And the error between results predicted by the FEM and results predicted by the multi-regression model is less than 10%. Therefore, the multi-regression model is proven to be an excellent tool for creep failure prediction and structural design optimization, reducing research costs and time, and improving design efficiency.
•Correlation between creep failure probability and geometric parameter is established.•Interaction between anode thickness Ha, sealant thickness Hs and frame thickness Hf is considered.•The optimal geometric parameters is obtained through regression equation analysis.•50,000 h creep life is met by optimizing the thicknesses of the components.
In this study, the weld residual stresses (RS) in a 25mm thick ferrite steel plate with newly developed low-temperature transformation (LTT) welding wire were investigated by finite element method ...and neutron diffraction (ND) measurement. A thermo-elastic–plastic finite element model coupled with solid-state phase transformation (SSPT) was developed to investigate the distribution and formation mechanism of RS, which has been verified by ND measurement. The results demonstrate that the developed LTT alloy can significantly reduce the RS and even generate compressive RS in the weld zone, due to the interrupted cooling shrinkage caused by the LTT. The higher inter-pass temperatures related to the microstructure evolution result in an increased region of compressive stress within the weldment. Moreover, the longitudinal RS in the weld zone gradually changes to tension as the initial temperature of martensitic transformation increases. Notably, the relaxation effect of transformation-induced plasticity on RS and its influence on model accuracy were discussed.
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•The effectiveness of the LTT weld wire in reducing tensile RS are validated.•Numerical model in couple with solid-state phase transformation is developed.•Prediction model is validated by using neutron diffraction technique.•Thermo-mechanical-metallurgical behavior for LTT alloy is predicted.
•Submerged micro-abrasive waterjet peening is proposed to improve the metal surface integrity and fatigue performance.•The surface integrity state of TA19 titanium alloy after the SMA-WJP is ...systematically analyzed.•The high-cycle fatigue life of TA19 titanium alloy treated by SMA-WJP is increased by a maximum of 2.72 times.•The improved fatigue life is attributed to the plastic deformation layer and large CRS induced by SMA-WJP.
Waterjet peening has become a critical surface treatment technology due to its great potential for improving the surface integrity and fatigue performance of metallic materials. The present study aims to investigate the influence of submerged micro-abrasive waterjet peening (SMA-WJP) on the surface integrity and fatigue properties of TA19 titanium alloy. First, the SMA-WJP with different water pressure (P = 70, 100, and 130 MPa) was conducted on the TA19 specimen. The surface integrity of the specimen before and after SMA-WJP treatment was studied, including the microstructure, surface roughness, microscopic morphology, microhardness, and residual stress. Results showed that the SMA-WJP treated specimen with different water pressure formed a plastic deformation layer with a depth of 24–44 μm, the minimum surface roughness of Ra = 0.363 μm and Sa = 0.95 μm. The depth of the work-hardened layer and compressive residual stress (CRS) layer was approximately 100–150 μm and 160–290 μm. The microstructure evolution on the top surface and sub-surface of the as-received and SMA-WJP treated specimens were characterized by transmission electron microscopy (TEM), which showed that nanocrystals with an average size of 12 nm and high density of dislocations formed on the top surface of the SMA-WJP treated specimen. Finally, stress-controlled high-cycle fatigue (HCF) tests were carried out to study the fatigue behavior of the TA19 titanium alloy before and after SMA-WJP treatment. The HCF life of the specimen is increased by a maximum of 2.72 times. The fatigue fracture surface was examined with scanning electron microscopy (SEM), which revealed that the existence of the plastic deformation layer and large CRS induced by SMA-WJP could effectively inhibit the initiation and propagation of cracks. This work enriches the waterjet peening process by investigating submerged abrasive waterjet peening and brings a new solution for improving the surface integrity and fatigue performance of TA19 titanium alloy.
A new type of waveguide transducer with a bidirectional-tapering structure is proposed to generate and receive high-frequency narrow-beam ultrasonic waves which can effectively improve the defect ...detection resolution of high temperature critical components in specific direction. The waveguide unit is composed of an excitation region with a tapering structure, a uniform-thickness strip region, and an incident region with a broadening structure. The tapering structure will contribute to generating the high frequency quasi-fundamental shear horizontal (SH0*) wave, and the uniform-thickness strip region is conducive to transmitting SH0* wave nondispersively, while the broadening structure can conduce to the incidence of the high-frequency narrow-beam ultrasonic wave. The critical criteria for SH0* wave propagation with nondispersive waveform and high signal-to-noise ratio in the bidirectional-tapering waveguide are established using simulation and experiments. Moreover, the heat dissipation performance and beam radiation of the bidirectional-tapering waveguide are verified. The results show that the proposed bidirectional-tapering waveguide transducer can generate pure high-frequency narrow-beam ultrasonic wave in the high temperature specimen.
The recently emerged multicomponent (or medium/high entropy) alloys have generated considerable excitement globally in the last 10 years because of their excellent mechanical and functional ...properties, particularly in terms of strength-ductility combinations that can surpass most other metallic materials. However, the achieved high strength level (above 1 GPa in many cases) fuels strong concerns about hydrogen embrittlement (HE). Detailed investigation in this field is still scarce, especially pertaining to the face-centered cubic medium entropy alloys (MEA) that are typically strengthened by ordered precipitates. Here, we unravel the effect of γ' (L12) ordered precipitates on H-induced damage behavior and the associated HE resistance in CoCrNi-based MEAs. Compared with the equi-molar CoCrNi MEA, the precipitation-hardened (CoCrNi)94Al3Ti3 MEA shows an enhanced HE resistance even at a higher strength level. Both alloys are fractured due to H-assisted intergranular cracking at the initial failure stage when loaded in the presence of H. The formation of intergranular cracks is primarily attributed to the H-induced decohesion at grain boundaries, where a high stress/strain concentration accompanied by a more intensive dislocation planar slip (or stacking fault formation) caused by H was observed. The presence of γ' precipitates serves to slow down the internal diffusion/migration of H due to the trapping effects. The precipitates with a relatively larger size (∼50 nm) also hinder dislocation planar slip thus decreasing the number of pile-up dislocations at grain boundaries. Both effects collectively reduce the tendency of H-induced intergranular cracking, leading to the improved HE resistance. The work reveals the positive role of ordered precipitates in H tolerance and thus provides some insights in further microstructure design of medium/high entropy alloys for applications in H-abundant environment.
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