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Homoclinic and Heteroclinic Flows in Global Attractor for Davey-stewartson Ⅱ Equation

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Acta Mathematicae Applicatae Sinica 2008年第4期24卷 599-612页

作者： Zheng-de Dai shao-lin li School of Mathematics and Statistics Yunnan University Kunming 650091 China Department of Information and Computing Science Guangxi Institute of Technology Liuzhou 545006 China Department of Mathematics Honghe College Mengzi 661100 China

By the variable transformation and generalized Hirota method, exact homoclinic and heteroclinic solutions for Davey-Stewartson II （DSII） equation are obtained. For perturbed DSII equation, the existence of a global attractor is proved. The persistence of homoclinic and heteroclinic flows is investigated, and the special homoclinic and heteroclinic structure in attractors is shown.

关键词： Persistence Homoclinic And Heteroclinic Flows Attractor Davey-Stewarson equation

Oxidation-induced damage of an uncoated and coated nickel-based superalloy under simulated gas environment

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Rare Metals 2018年第3期37卷 204-209页

作者： shao-lin li Hong-Yu Qi Xiao-Guang Yang School Of Energy and Power Engineering Beihang UniversityBeijing 100191 China Beijing Key Laboratory of Aero-Engine Structure and Strength Beijing 100191 China

Turbine blades and vans operated in an aggressive gas environment usually suffer from combined oxidation and cycle loading effects. The surface oxide layer will result in premature failure and lead to a significant reduction in the service lifetime. The effects of prior oxidation-induced damage under a simulated combustion-gas environment on the fatigue lifetime of the directionally solidified（DS） nickel-based superalloy DZ125 with and without an oxidation-resistant coating were presented. The fatigue lifetime of uncoated samples is adversely affected by prior oxidation exposure. The deterioration of fatigue lifetime in uncoated samples is associated with surface microstructural degradation, which occurs during prior exposure. However,the presence of MCrAlY coating is beneficial for the sample＇s lifetime under high stress. Further scanning electron microscopy（SEM） analysis demonstrates that the coating does not contribute to the initiation mode of fatigue cracks.

关键词： Gas environment Fatigue lifetime Coating Superalloy

Evaluation of service-induced microstructural damage for directionally solidified turbine blade of aircraft engine

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Rare Metals 2019年第2期38卷 157-164页

作者： Wei-Qing Huang Xiao-Guang Yang shao-lin li School of Energy and Power Engineering Beihang University Beijing Key Laboratory of Aero-Engine Structure and Strength

Turbine blades of gas turbine engines usually suffer from severe operational conditions characterized by high temperature and stress. Severe operational conditions during service cause microstructural changes in turbine blades and degrade their mechanical properties. In this study, service-induced microstructural damages in serviced turbine blades manufactured from a directionally solidified superalloy were evaluated. The observed microstructural damage of the turbine blade mainly involves the coarsening and rafting of γ' precipitates. The leading edge of 60% height of the turbine blades undergone most severe microstructural damage with significant microstructural evolution at this area. Microstructural damage affects the mechanical properties such as Vickers hardness, that is,Vickers hardness decreases as the equivalent diameter decreases. Microstructural damage shows great positiondependent feature as service temperature and radial stress on blade changes. With the aid of energy-dispersive spectrometer(EDS) analysis on carbide, the transformation of carbide does not exist. In addition, no topological closed-packed phase exists in the turbine blade.

关键词： Service-induced Microstructural damage Directionally solidified Turbine blade: Aircraft engine

Tensile properties and failure analysis of Ti–6Al–4V joints by electron beam welding

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Rare Metals 2016年第6期35卷 450-455页

作者： Xiao-Guang Yang shao-lin li Hong-Yu Qi School of Energy and Power Engineering Beijing University of Aeronautics and Astronautics

The microstructural and mechanical characterization of electron beam welded joints of forged Ti-6Al- 4V were investigated. Microhardness tests indicate that the hardness of the fusion zone （FZ） is higher than that of the heat-affected zone （HAZ） and base metal. The tensile results show that the mechanical properties of the welded joints are comparable with those of the base metal in terms of static strength and are in accordance with the relationship between microstructure and mechanical properties of welded joints. The ultimate tensile strength of the weld is equal to that of the hourglass joint, which indicates that the mechanical properties of the longitudinal FZ and those of the transverse FZ are the same. Macromechanical behavior and macrofracture and microfracture of the base material, joint, and weld specimens are observed. A comparison among the three types of specimen fracture phenomena reveals the following distinctive differences：（1） the fracture mode, （2） the micrograph of the dimple pattern at the central region, and （3） the size of the dimple at the central region and the transition region.

关键词： Electron beam welding Microstructure Mechanical properties Failure model

Low cyclic fatigue behavior of electron-beam-welded Ti–6Al–4V titanium joint

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Rare Metals 2016年第3期35卷 230-234页

作者： Hong-Yu Qi li-Qiang Ma shao-lin li Xiao-Guang Yang School of Energy and Power Engineering Beihang UniversityBeijing 100191China Collaborative Innovation Center of Advanced Aero-Engine Beijing 100191China

The low cycle fatigue（LCF） tests were carried out using symmetrical cyclic loading under total strain amplitude control *** present paper is devoted to investigating the cyclic deformation response of Ti–6Al–4V titanium and the electron-beam-welded（EBW） joint in the following aspects,i.e.,cyclic deformation behavior,fatigue life and fatigue fracture *** results show that the softening of the joint is significant at larger strain ranges,while not obvious at smaller strain *** joint shows shorter fatigue life at larger strain ranges and equivalent fatigue life at smaller strain ranges compared with Ti–6Al–4V base metal.A fatigue crack of the joint not only originates at the surface or subsurface,but also at defects in the fusion zone（FZ）.The crack propagation zone of Ti–6Al–4V base metal shows ductile fracture mechanism,while the joint shows brittle fracture *** all the fatigue fracture zones many dimples appear,showing the typical ductile fracture.

关键词： Ti–6Al–4V titanium alloy Electron beam welding Low cyclic fatigue Fracture analysis

Fatigue crack growth of titanium alloy joints by electron beam welding

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Rare Metals 2014年第5期33卷 516-521页

作者： Hong-Yu Qi Hao Shi shao-lin li Xiao-Guang Yang School of Jet Propulsion Beijing University of Aeronautics and Astronautics

Electron beam welding（EBW） has been widely used in the manufacture of titanium alloy welded blisk for aircraft engines. Based on fatigue crack growth tests on titanium alloy electron beam welding（EBW） joints, mechanism of fracture was investigated under scanning electron microscope（SEM）. The results show that fatigue crack growth rate increases as the experimental load increases under the same stress ratio and stress intensity factor range. At the beginning of crack growth, the extension mechanism of fatigue crack is the typical mechanism of cleavage fracture. In the steady extention stage, crack extends along the weld seam ***, crack growth direction changes to extend along the base metal. The extension mechanism of fatigue crack in the weld seam is the main mechanism of cleavage fracture and the extension mechanism of fatigue crack in the base metal is the main extension mechanism of fatigue band. In the instantaneous fracture stage, the extension mechanism of fatigue crack is the typical dimple-type static fracture *** growth was simulated by conventional finite element method and extended finite element method.

关键词： Microforming Ti–6Al–4V alloy Electron beam welding Fatigue crack growth Fracture analysis

High-temperature oxidation behavior of DZ125 Ni-based superalloy under tensile stress

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Rare Metals 2022年第12期41卷 4188-4193页

作者： Hong-Yu Qi Xiao-Bo liang shao-lin li Xiao-Guang Yang School of Energy and Power Engineering Beihang UniversityBeijing 100191China Collaborative Innovation Center of Advanced Aero-Engine Beijing 100191China

The key in antioxidant capacity of aero-engine hot components is the protective oxide *** factors can affect the performance of oxide *** effect of tensile stress on high-temperature oxidation behavior of directionally solidified DZ125 Ni-based superalloy was investigated by thermogravimetric analysis(TGA).Tensile samples were subjected to stress of 100 MPa,and the oxidation behavior was studied at 980℃in *** surface and cross-sectional morphologies of the oxidized coating were analyzed by scanning electron microscope(SEM).The influence of tensile stress on the oxidation behavior of directionally solidified DZ125 Ni-based superalloy at high temperature was *** show that an applied tensile stress generally leads to larger Cr/Ni mass ratio in the oxide scales,greater overall chromium depletion values directly under the oxide scales,lower specific weight gain values and thinner oxide *** factors are attributed to the formation of fast diffusion paths for Cr atoms to diffuse to the surface under tensile stress,thus causing a reduction in the duration of the less protective transient oxidation period and promoting a faster formation of the protective Cr_(2)O_(3)layer.

关键词： DZ125 Ni-based superalloy High-temperature oxidation Tensile stress Diffusion

Low-cycle fatigue behavior of a directionally solidified Ni-based superalloy subjected to gas hot corrosion pre-exposure

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Rare Metals 2019年第3期38卷 227-232页

作者： Hong-Yu Qi Ji-Shen Yang Xiao-Guang Yang shao-lin li School of Energy and Power Engineering Beihang University Collaborative Innovation Center of Advanced Aero-Engine

The influence of gas high-temperature hot corrosion(HTHC) pre-exposure on low-cycle fatigue(LCF)behavior was characterized for the directionally solidified(DS) Ni-based superalloy DZ125. Fatigue tests were carried out at 850 ℃ in the pre-exposed and unexposed specimens for 2, 15 and 25 h. Experimental results show that the porous corrosion scale and γ′-depleted layer formed in gas hot corrosion condition alter the crack initiation mechanisms of the superalloy. Fatigue cracks of the pre-exposed specimens originate from multiple surface locations where spalling of the corrosion products occur,while nucleation of unexposed specimen begins in the defects close to the surface. There is a significant reduction in LCF behavior for pre-exposed specimens in comparison with unexposed specimens.

关键词： Low-cycle fatigue Hot corrosion Preexposure Burner rig Ni-based superalloy

Fatigue behavior of uncoated and MCrAlY-coated DS nickelbased superalloys pre-exposed in hot corrosion condition

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Rare Metals 2018年第11期37卷 936-941页

作者： Hong-Yu Qi Ji-Shen Yang Xiao-Guang Yang shao-lin li li-Qiang Ma School of Energy and Power Engineering Beihang UniversityBeijing 100191China Collaborative Innovation Center for Advanced Aero-Engine Beijing 100191China Beijing Key Laboratory of Aero-Engine Structure and Strength Beijing 100191China Shenyang Aeroengine Research Institute Shenyang 110015China

The effects of a MCrAlY coating on low-cycle fatigue(LCF) behavior of directionally solidified(DS)nickle-based superalloy DZ125 were investigated. Before the fatigue testings, the specimens were pre-exposed in high-temperature hot corrosion(HTHC) environment generating by a burner rig at 850 ℃. The results show that the coating in hot corrosion condition has beneficial effects on the fatigue resistance of superalloy. Under corrosion condition, the MCrAlY-coated specimens tested have higher fatigue lives than the uncoated specimens at the same stress level. The coating failure results from fatigue process and numerous fatigue cracks were nucleated at the specimen surface, only one main crack propagates inward and the secondary cracks away from the fracture surface are perpendicular to the loading orientation.

关键词： MCrAlY coating Low-cycle fatigue Directionally solidified Ni-based superalloy Hot corrosion Prior exposure