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Effective operation strategies in the solid oxide fuel cell (SOFC) can adjust the spatial distribution of temperature gradient favoring the long-term stability. To investigate the effects of different operating conditions on the thermal behavior inside SOFC, a three-dimensional model is developed in this study. The model is verified by comparing it with the experimental data. The heat generation rate and its variation under different operating conditions are analyzed. The combined effects of operating voltage and gas temperature are considered to be the key factor influencing the temperature gradient. Compared to the original case, the temperature of SOFC decreases by 21.4 K when the fuel velocity reaches 5 m/s. But the maximum temperature gradient increases by 21.2%. Meanwhile, higher fuel velocities can eliminate about 32% of the area with higher temperature gradient. And when the oxidant velocity reaches 7.5 m/s, the peak temperature gradient effectively decreases by 16.59%. Simultaneous adjustment of the oxidant and fuel velocities can effectively reduce the peak temperature gradient and increase the safety zone. The effects of operation conditions on the temperature gradient of the cell are clarified in this study, which can be a reference for further research on the reliability of SOFCs. [ABSTRACT FROM AUTHOR]

The crystal plasticity finite element method (CPFEM) is widely used to explore the microscopic mechanical behavior of materials and understand the deformation mechanism at the grain-level. However, few CPFEM simulation studies have been carried out to analyze the nanoindentation deformation mechanism of polycrystalline materials at the microscale level. In this study, a three-dimensional CPFEM-based nanoindentation simulation is performed on an Inconel 718 polycrystalline material to examine the influence of different crystallographic parameters on nanoindentation behavior. A representative volume element model is developed to calibrate the crystal plastic constitutive parameters by comparing the stress-strain data with the experimental results. The indentation force-displacement curves, stress distributions, and pile-up patterns are obtained by CPFEM simulation. The results show that the crystallographic orientation and grain boundary have little influence on the force-displacement curves of the nanoindentation, but significantly influence the local stress distributions and shape of the pile-up patterns. As the difference in crystallographic orientation between grains increases, changes in the pile-up patterns and stress distributions caused by this effect become more significant. In addition, the simulation results reveal that the existence of grain boundaries affects the continuity of the stress distribution. The obstruction on the continuity of stress distribution increases as the grain boundary angle increases. This research demonstrates that the proposed CPFEM model can well describe the microscopic compressive deformation behaviors of Inconel 718 under nanoindentation. [ABSTRACT FROM AUTHOR]

The creep life of an aeroengine recuperator is investigated in terms of continuum damage mechanics by using finite element simulations. The effects of the manifold wall thickness and creep properties of brazing filler metal on the operating life of the recuperator are analyzed. Results show that the crack initiates from the brazing filler metal located on the outer surface of the manifold with the wall thickness of 2 mm and propagates throughout the whole region of the brazing filler metal when the creep time reaches 34900 h. The creep life of the recuperator meets the requirement of 40000 h continuous operation when the wall thickness increases to 3.5 mm, but its total weight increases by 15%. Decreasing the minimum creep strain rate with the enhancement of the creep strength of the brazing filler metal presents an obvious effect on the creep life of the recuperator. At the same stress level, the creep rupture time of the recuperator is enhanced by 13 times if the mismatch between the minimum creep rate of the filler and base metal is reduced by 20%. [ABSTRACT FROM AUTHOR]

The improved understandings of the mechanical properties as well as deformation mechanisms at cryogenic temperatures are the prerequisite for realizing the application of any new engineering materials to cryogenic industries. Here, a (CoCrNi)94Al3Ti3 medium entropy alloy (MEA) with nanoscale L12 coherent precipitates and heterogeneous grain structures was prepared by co-doping Al and Ti elements with subsequent cold rolling and heat treatment processes. The mechanical properties were evaluated at the temperature range of 293–113 K. The ultimate strength of the MEA increases almost linearly from 1326 to 1695 MPa as the temperature decreases from 293 to 113 K, while the total elongation remains approximately constant of ∼35%. The underlying deformation and strengthening mechanisms were investigated using various characterization techniques. Due to the effect of co-doped Al/Ti on channel width of the matrix and the increasing critical twinning stress induced by heterogeneous ultrafine grain size, the formation of deformation twins is inhibited at all temperatures. Consequently, only a slight increase of the deformation twins and stacking faults in the deformed specimens with a decreasing temperature, which leads to the relative temperature-independence of the ductility. The dislocation cutting mechanism of L12 coherent precipitates and the heterodeformation induced (HDI) hardening both significantly contribute to the strain hardening so that an excellent combination of strength and ductility is obtained. Additionally, the evolution of lattice friction stress with deformation temperature is determined by quantitative analysis, indicating an approximately linear relationship between the lattice friction and temperature. The present work provides new insights into the strategy of achieving outstanding strength-ductility synergy of the MEA under the wide temperature range by coupling heterogeneous ultrafine-grained structure and coherent precipitation strategy. [ABSTRACT FROM AUTHOR]

Traditional metallic materials usually face a dilemma between high strength and poor strain hardening capacity. However, heterogeneous structured metallic materials have been found to obviously overcome the trade-off. Herein, gradient lamellar structure was fabricated through ultrasound-aided deep rolling technique in pure Ni with high stacking fault energy after heat treatment. The gradient lamellar Ni was successively divided into the four regions. In-situ micropillar compression tests were conducted in different regions to reveal the corresponding microscopic mechanical properties. Microscopic characterization techniques were performed to explore underlying deformation mechanisms and the effects of microstructural parameters on deformation behaviors. This work demonstrates that the micropillar with near nanoscale lamellar thickness possesses excellent strength and plasticity. On one hand, the reason for high strength of near nanoscale micropillar is that the strength of micropillar increases with the decrease of lamellar thickness according to the Hall-Petch effect. On the other hand, numerous lamellar grain boundaries perpendicular to the loading direction is found to hinder the motion of slip bands, resulting in great strain hardening capacity in the near nanoscale lamellar micropillar. [ABSTRACT FROM AUTHOR]

At present, there are many studies on the residual stress field and plastic strain field introduced by surface strengthening, which can well hinder the initiation of early fatigue cracks and delay the propagation of fatigue cracks. However, there are few studies on the effects of these key factors on fretting wear. In the paper, shot-peening (SP) and ultrasonic surface rolling process (USRP) were performed on Ti-6Al-4V plate specimens. The surface hardness and residual stresses of the material were tested by vickers indenter and X-ray diffraction residual stress analyzer. Microhardness were measured by HXD-1000MC/CD micro Vickers hardness tester. The effects of different surface strengthening on its fretting fatigue properties were verified by fretting fatigue experiments. The fretting fatigue fracture surface and wear morphology of the specimens were studied and analyzed by means of microscopic observation, and the mechanism of improving fretting fatigue life by surface strengthening process was further explained. After USRP treatment, the surface roughness of Ti-6Al-4V is significantly improved. In addition, the microhardness of the specimen after SP reaches the maximum at 80 μm from the surface, which is about 123% higher than that of the AsR specimen. After USRP, it reaches the maximum at 150 μm from the surface, which is about 128% higher than that of AsR specimen. It is also found that the residual compressive stress of the specimens treated by USRP and SP increases first and then decreases with the depth direction, and the residual stress reaches the maximum on the sub surface. The USRP specimen reaches the maximum value at 0.18 mm, about − 550 MPa, while the SP specimen reaches the maximum value at 0.1 mm, about − 380 MPa. The fretting fatigue life of Ti-6Al-4V effectively improved after USRP and SP. The surface integrity of specimens after USRP is the best, which has deeper residual compressive stress layer and more refined grain. In this paper, a fretting wear device is designed to carry out fretting fatigue experiments on specimens with different surface strengthening. [ABSTRACT FROM AUTHOR]

Security and performance of aircraft engines are extremely affected by the blades' surface quality. Ultrasonic surface rolling process (USRP) can improve material surface properties effectively and increase the alloys' service life. However, applying USRP onto blades remains a challenge due to the free-form surface with thin-walled leading/trailing edges and the pre-load pressure on the blade during the USRP process. In this paper, a coordinated bilateral method of USRP on the blade is proposed. A machine tool is first designed and integrated with two USRP devices. A method of coordinated bilateral path generation is then proposed, so that the blade surface can be processed by the two USRP devices simultaneously. Finally, we perform experiments on a real aero-engine blade. The results demonstrate that by using the proposed method, USRP can be applied on the bilateral surfaces of the aero-engine blade coordinately. [ABSTRACT FROM AUTHOR]

In this paper, the local fracture properties in a Alloy52M dissimilar metal welded joint (DMWJ) between A508 ferritic steel and 316 L stainless steel in nuclear power plants were investigated by using the single-edge notched bend (SENB) specimens, and their use in integrity assessment of DMWJ structures was analyzed. The results show that the local fracture resistance in the DMWJ is determined by local fracture mechanism, and which is mainly related to the microstructures and local strength mismatches of materials at the crack locations. The initial cracks always grow towards the materials with lower strength, and the crack path deviation is mainly controlled by the local strength mismatch. If the local fracture properties could not be used for cracks in the heat affected zones (HAZs), interface and near interface zones, the use of the fracture properties ( J-resistance curves) of base metals or weld metals following present codes will unavoidably produce non-conservative (unsafe) or excessive conservative assessment results. In most cases, the assessment results will be potentially unsafe. Therefore, it is recommended to obtain and use local mechanical and fracture properties in the integrity assessment of DMWJs. [ABSTRACT FROM AUTHOR]

In recent years, the need for high temperature heat exchangers to improve the efficiency of power and chemical conversion systems has been growing. However, the creep design of the high temperature compact heat exchangers has been a primary concern because the working temperature can be well above the creep limit of the materials. To establish the high temperature design criterion for compact heat exchangers, creep behavior of the plate-fin structures and brazed joints are investigated in this paper. The time-dependent deformation and bending stress of the plate-fin structures are obtained analytically by simplifying the fins to elastic springs. The creep damage evolution inside the brazed joint is studied by coupling the finite element method with a damage constitutive equation. The significant effect of creep property mismatch in the brazed joint on the creep strength is demonstrated. [ABSTRACT FROM AUTHOR]

Structural integrity technology has been widely used with great success for the design, manufacture and failure prevention of modern constructions such as chemical and petrochemical plants, power generation and energy conversion systems, as well as space and oceanic exploration. The modern needs of structural integrity technology are largely attributed to the increase of service temperature of the structures that results in the efficiency improvement in energy conversion and chemical processing technologies. Besides the needs arising from large-scale high-temperature plants, the high tech developments, such as micro chemo-mechanical systems and high-power electronics, provide new challenges to structural integrity technology. The present paper summarizes the recent technical progresses in large process plants and the aviation industry, micro chemo-mechanical systems, fuel cells, high-temperature electronics, and packaging and coating technologies. The state-of-the-art of structural integrity technology for high temperature applications is reviewed. Suggestions are provided for the improvement of current design and assessment methods. [ABSTRACT FROM AUTHOR]

One form of energy generation that is expected to be on the rise in the next several decades is thermoelectric power generation (TEPG) which converts heat directly to electricity. Compared with other methods, TEPG possesses the salient features of being compact, light-weighted, noiseless in operation, highly reliable, free of carbon dioxide emission and radioactive substances. Low current conversion efficiency and high cost, however, are some of the disadvantages. Use of TEPG is therefore justified to hightech applications associated with aerospace, military operation, tel-communication and navigation, instrumentation of unmanned vehicles monitored from remote locations. More- over, TEPG does not contribute to the depletion of natural resource and pollution of the environment such as climate warming that has been a concern in recent times. This work is concerned with providing an overview of the state of the art of TEPG with emphases placed on assessing its current and potential application. Pointed out are the ways to fabricate high performance thermoelectric material, a hurdle to overcome for the enhancement of TEPG device efficiency. [ABSTRACT FROM AUTHOR]