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Abstract A nanotwinned 304 steel with a favorable combination of strength and ductility has been made in this work through the dynamic plastic deformation (DPD) treatment followed by a proper heat treatment. The DPD, DPD-annealed and coarse grain (CG) 304 samples were tensile tested with slow strain rate under electrochemical hydrogen charging condition. Compared with CG sample, DPD-annealed sample with a mixed structure of nanotwins (41% volume fraction), recrystallization grain (RXG, 32% volume fraction) and dislocation structures (27% volume fraction) possesses a high HE resistance. Hydrogen causes a 12.5% reduction in yield strength and 5.4% reduction in tensile strength of DPD-annealed sample because hydrogen enhances dislocations/twin boundaries (TBs) interactions and the subsequent dislocation mobility during the TB-mediated slip transfer. Hydrogen charging also causes a 41% reduction in strain induced martensite of DPD-annealed sample. Strain localization is supposed to be mitigated by the bundle form of TBs and the micro-sized RXGs, accounting for the high HE resistance of DPD-annealed alloy. Our findings suggest a new strategy to design high strength alloys with low HE susceptibility. Highlights • The nanotwinned austenitic steel shows a high hydrogen embrittlement resistance. • H-charging suppresses the martensite transformation of DPD-annealed alloy. • Slip localization at twin boundary (TB) is mitigated by the bundle form of TB. [ABSTRACT FROM AUTHOR]

Up to now, the precise role of reversed austenite (RA) in hydrogen embrittlement (HE) of steel is still not fully understood. This work presents new observations and interpretation of fracture surface features immediately beneath the fracture surface for S41500 martensitic stainless steels (MSS) with aim to reveal the role of RA in HE resistance. The MSS were tensile tested with slow strain rate under electrochemical hydrogen charging condition. Steel containing more RA was found to have less hydrogen embrittlement susceptibility. Focused ion beam (FIB) was used to prepare sample for TEM observation of fracture path after HE fracture. It clearly shows that RA near the fracture surface has transformed to the newly formed martensite (NFM) and cracking occurs along both the tempered martensite/NFM boundaries and the lath boundaries. Three dimension atom probe (3DAP) confirms that RA is the H trapping site. Thus the beneficial role of RA is that it can act as a stable hydrogen trapping site which can increase the HE resistance by reducing hydrogen content at lath and grain boundaries. But its beneficial effect should not be overestimated since cracking along tempered martensite/NFM boundaries can occur after martensitic transformation as a result of hydrogen redistribution. [ABSTRACT FROM AUTHOR]

• A novel optimization algorithm called disturbance inspired equilibrium optimizer was developed. • New hybrid population initialization technique was proposed to generate high-quality initial population. • Novel update rules and adaptive position disturbance mechanism were proposed to improve exploitation and exploration. • Evaluation on representative mathematical benchmark functions and three real-world engineering problems. • Comparisons verified efficiency and superiority of the proposed algorithm. This work proposes a novel adaptive global optimization algorithm called Disturbance Inspired Equilibrium Optimizer. The purpose of this study is to enhance the exploitation ability of the newly developed Equilibrium Optimizer, and to address the issue of getting trapped in local minima. The proposed algorithm is benefited from the novel disturbance-based hybrid initialization strategy, the new form of time factor, and the new update rule of particle's position. In addition, a novel boundary check strategy and an adaptive global position disturbance mechanism are proposed and installed into our algorithm. Based on the disturbance-inspired modifications, the exploration and exploitation ability of the standard Equilibrium Optimizer are significantly improved. The performance of the proposed algorithm is evaluated using representative different benchmark functions, consisting of three well-known mathematical benchmark functions, six complex composite functions, and four challenge functions proposed on 2017 IEEE Congress on Evolutionary Computation. Also, the proposed algorithm is conducted to optimize three engineering designs to examine its applicability in constrained real-world problems. In all experiments, the developed algorithm is compared with six other state-of-the-art metaheuristics. Experimental results and the average rank of Friedman test show that our algorithm provides promising results in solving mathematical problems and constrained real-world engineering optimization problems. Therefore, the proposed algorithm is competitive compared to the other state-of-the-art metaheuristic algorithms and is an effective solution to real-world engineering problems. [ABSTRACT FROM AUTHOR]

Abstract: The Laplacian spread of a graph is defined to be the difference between the largest eigenvalue and the second-smallest eigenvalue of the Laplacian matrix of the graph. Bao, Tan and Fan [Y.H. Bao, Y.Y. Tan,Y.Z. Fan, The Laplacian spread of unicyclic graphs, Appl. Math. Lett. 22 (2009) 1011–1015.] characterize the unique unicyclic graph with maximum Laplacian spread among all connected unicyclic graphs of fixed order. In this paper, we characterize the unique quasi-tree graph with maximum Laplacian spread among all quasi-tree graphs in the set with . [Copyright &y& Elsevier]

• The seriousness of the HTD problems in large-diameter tubes is highlighted experimentally. • SST k-ω model for S-CO 2 heat transfer prediction is experimentally validated within ±15% error range. • HTD mechanism is revealed by comprehensive comparisons between simulations and experiments. • Pseudo-nucleate boiling concept is proposed to further develop the pseudo-boiling theory for S-CO 2. • Conical strips insert can prevent HTD problems and achieve high PEC of 1.21-1.35. Heat transfer deterioration (HTD) problems can undermine the thermal safety of heating tubes for supercritical carbon dioxide (S-CO 2) flows. Relevant experimental evidence is highly desired, but most cases were investigated for small tube diameters (below 10 mm) and under limited operation parameters, far from the industry-scale applications. In the present work, large tubes (24 mm) are investigated experimentally with pressures of 7.5–15 MPa, mass flow rates of 100–1200 kg·m−2·s−1 and heat fluxes of 30–350 kW·m−2. The seriousness of HTD in large tubes was experimentally confirmed, and the mechanism is explored via a carefully-designed comprehensive comparison between present numerical simulations and experimental tests. Detailed analysis of vapor-like film development inspires us to mitigate the problem using structured-inner-surface, from the viewpoint of "supercritical pseudo-boiling". This inspiration is further evaluated numerically: five types of enhancement structures are proposed to interfere with the development of vapor-like film. It was found that the proposed methods can fully prevent HTD problems, and especially the conical strips can achieve a high heat transfer performance (with performance evaluation criteria PEC of 1.21–1.35) and are thus recommended for HTD prevention in vertical large-diameter tubes under near-critical conditions. Overall, the experimental tests and numerical explorations can deliver more evidence on the theory and applications of supercritical fluid flow and heat transfer. [ABSTRACT FROM AUTHOR]

The S–CO 2 Brayton cycle has been considered as a promising alternative in coal-fired power plants due to its high efficiency and high compactness. However, the high inlet temperature and low heat transfer coefficient of S–CO 2 in the cooling wall tubes arranged in the furnace could lead to thermal fragile in practical operation. In present study, thermal-hydraulic-structural evaluation on S–CO 2 cooling wall tubes is investigated. Firstly, it is found that the stress distribution is mainly dominated by the circumferential temperature gradient. Secondly, a new criterion, the thermal deviation factor (TDF), is proposed to evaluate the tube thermal stress. It indicates the plastic deformation if exceeding the critical value of TDF = 1, which is more efficient than conventional structural analysis. Thirdly, based on the TDF , two typical methods of inserting twisted tapes and unilaterally arranging dimples inside the tube are proposed to reduce the thermal stress. A novel configuration of elliptic dimple with rounded corners is finally proposed to reduce the local stress concentration efficiently. The results show that the elliptic dimples have the highest comprehensive performance, which is recommended as the heat transfer enhancement structures in S–CO 2 cooling wall tubes. • Thermal-hydraulic-structural performance of S–CO 2 cooling wall tube is investigated. • A new thermal deviation factor (TDF) is proposed to evaluate thermal stress. • Twisted tapes and unilaterally arranged dimples are used to reduce thermal stress. • A novel elliptic dimple can release local stress concentration and improve comprehensive performance. [ABSTRACT FROM AUTHOR]