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Now showing items 1 - 16 of 33

  • Making Saints in Modern China. Edited by David Ownby, Vincent Goossaert, and Ji Zhe

    Valussi Elena  

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  • The non-destructive estimation of the superficial mechanical properties of components in the INCONEL 600 alloy by X-ray diffraction peak width

    Vincent Ji   Yong-Gang Zhang   Chang-Qi Chen  

    INCONEL 600 alloy is largely used as nuclear power center components. The non-destructive determination of the superficial mechanical proprieties is indispensable for the qualification of the fabrication process and for the service duration estimation in aggressive environment. X-ray diffraction (XRD) technique is non-destructive and can be used to determine microscopic internal stresses from the broadening of the XRD peaks. The average magnitude of XRD peak broadening is related directly to the hardness, the elastic limit or degree of cold work of the material which can be used as a means of quantifying the surface mechanical properties. With an empirical relationship between the {311} XRD peak width and the conventional mechanical proprieties (hardness, elastic limit, hard work degree) for INCONEL 600 alloy, the superficial mechanical characteristics can be estimated from diffraction peak width for just finished parts or for maintenance pieces. Several TEM observations are also realized for microstructure study
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  • Making Saints in Modern China. Edited by David Ownby, Vincent Goossaert, and Ji Zhe. Oxford: Oxford University Press, 2017. Pp. xii?+?509. Cloth, $99.00.

    Nichols   Brian J.  

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  • Characterization on surface mechanical properties of Ti–6Al–4V after shot peening

    Lechun Xie   Yan Wen   Ke Zhan   Liqiang Wang   Chuanhai Jiang   Vincent Ji  

    Abstract The surface mechanical properties of Ti–6Al–4V after shot peening (SP) were investigated in this work. Firstly, the surface yield strength was obtained from the in-situ X-ray stress analysis method combined with the tensile test technique, and the value was increased to 1080 MPa, which was improved about 27% compared with the unpeened bulk material. It was ascribed to the surface work-hardening during the process of SP. Secondly, after SP, the hardness increased obviously and the compressive residual stress was introduced in the surface layer, which were beneficial to the surface properties of Ti–6Al–4V. Even so, the compressive residual stress was relaxed under cyclic loading. So thirdly, the relaxation of residual stress on the peened surface was studied under cyclic loading. The fast relaxation took place in the first few cycles then became stable gradually in further cycles. When the applied tensile stress was approaching to the yield strength, the relaxation was drastic and distinct. The behaviors of residual stress relaxation under different applied stresses were quantified by a function. The results indicated that the relaxation behavior was mainly influenced by the original magnitude, the applied stress and the cycling numbers. In view of all analysis, it can be found that the method of combining in-situ X-ray stress analysis and the tensile test is an effective method to characterize the surface mechanical properties of metal materials after SP treatment. Graphical abstract Relationship between two surface principal stress σ 1 , σ 2 and applied tensile strain ε a for Ti–6Al–4V. Highlights • In-situ X-ray stress analysis method is used to study the surface yield strength. • Experiment is improved by combining X-ray stress analyzer and the tensile tester. • Surface yield strength is 1080 MPa, increasing 27% compared with original sample. • Under load of 800 MPa, the stress relaxation rate is fastest during first few cycles. • Stress relaxation behavior under different loading can be quantified by a function.
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  • Hydrogen adsorption and storage on palladium-decorated graphene with boron dopants and vacancy defects: A first-principles study

    Ling Ma   Jian-Min Zhang   Ke-Wei Xu   Vincent Ji  

    Abstract The geometric stability and hydrogen capacity of Pd-decorated graphene with experimentally realizable boron dopants and various vacancy defects including single carbon vacancy (SV), “585”-type double carbon vacancy (585 DCV) and “555-777”-type double carbon vacancy (555-777 DCV) are investigated using the first-principles calculations based on density functional theory (DFT). It is found that among the four types of defective structures, Pd′s binding energies on SV and 585 DCV defect graphene sheets exceed the cohesive energy of the Pd metal bulk, thus Pd atoms are well dispersed above defective graphene sheets and effectively prevent Pd clustering. Up to three H 2 molecules can bind to Pd atom on graphene with B dopants, SV and 555-777 DCV defects. For the cases of Pd-decorated graphene with B dopants and 555-777 DCV defect, a single H 2 or two H 2 are molecularly chemisorbed to Pd atom in the form of Pd–H 2 Kubas complex, where the stretched H–H bond is relaxed but not dissociated. Out of two adsorbed H 2 , the third H 2 binds to Pd atom by small van der Waals (vdW) forces and the nature of bonding is very weak physisorption. Different from above two cases, three H 2 are all molecularly chemisorbed to Pd atom with stretched H–H bond for Pd-decorated SV defect graphene, the hybridization of the Pd-4 d orbitals with the H 2 -σ orbitals and the electrostatic interaction between the Pd cation and the induced H 2 dipole both contribute to the H 2 molecules binding, and the binding energies of 0.25–0.41 eV/H 2 is in the range that can permit H 2 molecules recycling at ambient conditions. Graphical abstract Optimized atomic geometries for Pd-decorated B-doped, SV, 585 DCV and 555-777 DCV defective graphenes with one to three H 2 molecules. Green, pink, blue and white balls represent C, B, Pd and H atoms, respectively. Highlights • Pd atom can be stabilized on graphene with SV and 585 DCV defects. • Pd-decorated B-doped, SV and 555-777 DCV defect graphene can bind three H 2 molecules. • Pd-decorated 585 DCV defect graphene can bind two H 2 molecules. • Pd-decorated SV defect graphene is the best for hydrogen storage.
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  • Residual stress and microstructure evolution of shot peened Ni-Al bronze at elevated temperatures

    Chengxi Wang   Chuanhai Jiang   Ming Chen   Lianbo Wang   Huabing Liu   Vincent Ji  

    Abstract In this work, the relaxation of residual stress and strain hardening effect along with the evolution of microstructure of Ni-Al bronze at elevated temperatures were investigated. The samples were peened and then isothermal annealed at temperatures ranging from 200 to 400 °C for different time. The residual stress in the surface layer were determined by X-ray stress analyzer. Variations of domain size and micro-strain were estimated by X-ray line profile analysis in which the modified Williamson-Hall method with uniform deformation energy density model was employed. Experimental results showed that both residual stress and strain hardening effect relaxed dramatically in the initial stages and then gradually reached steady state. Although partially recrystallization occurred at higher temperatures, only part of compressive residual stress relaxed at each fixed temperature. The micro-strain and the dislocation density also declined with the heating temperature and exposure time increasing, which were presumably related to the vacancy transportation and dislocation rearrangement. Meanwhile, as demonstrated by XRD and TEM, the domain size increased significantly. The thermal relaxation activation enthalpies of residual stress and X-ray full width at half maximum were evaluated via Zener-Wert-Avrami relationship. The latter value was larger than that of residual stress, which indicated that the strain hardening effect was more difficult to fade at elevated temperatures. Hardness also reduced continuously during annealing process, which was mainly ascribed to the increased domain size and the weakening strain hardening effect. No phase transition occurred and the crystallinity was improved after annealing treatment. In the present study, the residual stress did not release completely even after annealing at the highest aging temperature for 2 h, which meant that shot peening could enable Ni-Al bronze alloys to keep excellent thermal stability of beneficial compressive residual stress.
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  • The detailed crystal and electronic structures of the cotunnite-type ZrO<sub>2</sub>

    Yan Zhang   Li Duan   Vincent Ji  

    Abstract The detailed crystal and orbital-decomposed electronic structures of cotunnite-type ZrO 2 have been investigated by using the first-principles projector augmented wave (PAW) potential within the generalized gradient approximation as well as taking into account on-site Coulomb repulsive interaction (GGA+U). The optimized structure shows that the O I and O II anions are surrounded by an arbitrary tetrahedron of four Zr cations and an arbitrary pentahedron of five Zr cations, respectively, in turn, the Zr cation is surrounded by an arbitrary tetrakaidecahedron formed by nine oxygen ligands. Although one more Zr cation is coordinated to O II , the larger bond lengths between O II and its adjacent five Zr cations ( d O II − Zr ) than those between O I and its adjacent four Zr cations ( d O I − Zr ) makes density of states (DOS) of s and three p ( px , py and p z ) states of the O II anion driving down in lower energy region and driving up in higher energy region. No crystal-field splitting is observed between three p ( px , py and p z ) states of anions O I and O II (between three p ( px , py and p z ) states and five d ( dxy , dyz , dxz , dz 2 and d x 2 - y 2 ) states of cation Zr) is resulted from the arrangements of the surrounding cations (anions) do not have any symmetry. The additional covalent character upon Zr–O ionic bonds is attributed to the hybridization of itinerant Zr(5s) and less filled Zr(4d) states to the separated O(2s) and O(2p) states.
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  • Surface layer characteristics of CNT/Al–Mg–Si alloy composites treated by stress peening

    Kaiyuan Zhu   Chuanhai Jiang   Vincent Ji  

    Abstract To determine the effect of carbon nanotubes (CNTs) on the characteristics of CNT/Al composites treated by stress peening, a 1.5 wt% CNT/Al–Mg–Si alloy composite and an Al–Mg–Si alloy were fabricated via flake powder metallurgy process. The evolution of residual stress fields, surface topography, and microhardness of peened layers were explored before and after the peening treatments. The differences in microstructure, including domain size, microstrain, and dislocation density, were determined via Rietveld refinement. After stress peening, the maximum residual stress values of CNT/Al–Mg–Si composite and Al–Mg–Si alloy were improved by 101 MPa, 83 MPa, respectively, compared with traditional shot peening. Meanwhile, the magnitude of the residual stress of CNT/Al–Mg–Si composite was generally higher than that of Al–Mg–Si alloy. Results also reveal that pre-stress and CNTs addition could improve the compressive residual stress fields treated by stress peening. Nevertheless, in the peened layers, the domain sizes were further refined (12%) and the microstrain was increased by applying prestress and CNTs. The difference in microhardness of peened layers was consistent with the change law of surface layer characteristics. Highlights • The prestress during SP treatment could improve the residual stress of the CNT/Al–Mg–Si composites. • The increment of CRS treated by prestress during SP could be further improved by the CNTs addition. • The improvement of CRS of one point is different in different directions. • Microstructure evolution of CNT/Al–Mg–Si composites along with prestress were measured investigated by TEM and XRD analysis.
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  • Effects of Co contents on the microstructures and properties of electrodeposited NiCo–Al composite coatings

    Fei Cai   Chuanhai Jiang   Peng Fu   Vincent Ji  

    Abstract In this work, the NiCo–Al composite coatings with different Co contents were prepared by electrodeposition from the modified Watt baths containing different Co 2+ concentrations. The effects of Co contents on the composition, crystal structure, texture, grain size, microstrain, surface morphology, microhardness, residual stress and corrosion resistance of the NiCo–Al composite coatings were investigated in detail. The composite coatings exhibited the solid solution NiCo crystal structure with the Co contents in the range of 18.3 wt% to 43 wt%. Further increasing the Co content to 60.5 wt% resulted in the formation of hexagonal (hcp) Co. As the Co contents increased, the NiCo–Al composite coatings exhibited the texture evolution from the (2 0 0) preferred orientations to the random orientations or slight (1 1 1) preferred orientations. The grain size decreased, while the microstrain and residual stress increased with increasing Co contents. The hardness of the composite coating attained the maximum at the Co content of 43 wt%. However, the increasing Co contents decreased the corrosion resistance of the NiCo–Al composite coatings.
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  • Effects of Co contents on the microstructures and properties of the electrodeposited NiCo–Zr composite coatings

    Fei Cai   Chuanhai Jiang   Yuantao Zhao   Vincent Ji  

    Graphical abstract Highlights • The novel NiCo–Zr coatings were prepared by electro-deposition. • Surface morphology, crystal structure, grain size and microstrain were examined. • Texture, residual stress and corrosion resistance were investigated. • Addition of Co increased the hardness and corrosion resistance of the coatings. Abstract In this study, the NiCo–Zr composite coatings were prepared from the electrolytes with different Co 2+ concentrations by electrodeposition method. The effects of Co contents on the crystal structure, surface morphology, grain size, microstrain and residual stress were examined by X-ray diffractometer (XRD), field emission scanning electron microscopy (FESEM), Energy dispersive X-ray spectroscopy (EDX) and atomic force microscope (AFM). The corrosion resistance of the composite coatings was also examined by the potentiodynamic polarization and electrochemical impedance (EIS) measurements. The results revealed that the crystal structures of the coatings were dependent on the Co contents and addition of Co content of 58 wt% resulted in the formation of hexagonal (hcp) Co. The increasing Co contents in the NiCo–Zr composite coatings resulted in the smoother and more compact surface, decreased the grain size and increased the microstrain. The micro-hardness and residual stress also increased with increasing Co contents. The addition of Co increased the corrosion resistance of the NiCo–Zr composite coatings compared with the Ni–Zr coating while the corrosion resistance of the NiCo–Zr composite coatings decreased as the Co contents increased.
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  • Microstructure evolution and hot corrosion mechanisms of Ba<sub>2</sub>REAlO<sub>5</sub> (RE = Yb, Er, Dy) exposed to V<sub>2</sub>O<sub>5</sub> + Na<sub>2</sub>SO<sub>4</sub> molten salt

    Lei Guo   Chenglong Zhang   Qing He   Jianxing Yu   Zheng Yan   Fuxing Ye   Chengyi Dan   Vincent Ji  

    Abstract Ba 2 REAlO 5 (RE = Dy, Er, Yb) powders were synthesized by a solid state reaction method, followed by cold pressing and sintering to produce pellets for hot corrosion tests. When exposed to V 2 O 5  + Na 2 SO 4 molten salt at 900 °C and 1000 °C for 4 h and 20 h, REVO 4 , Ba 3 (VO 4 ) 2 and BaAl 2 O 4 formed as corrosion products due to chemical interactions between the ceramics and the molten salt, which were temperature and time independent. After the hot corrosion tests at 1000 °C, continuous, dense reaction layers with a thickness of ∼80 μm formed on the sample surfaces, which had an effective function on suppressing further penetration of the molten salt. The hot corrosion mechanisms of Ba 2 REAlO 5 are proposed based on Lewis acid-base rule, phase diagrams and thermodynamics. From a thermodynamics perspective, the molten salt directly reacting with Ba 2 REAlO 5 is difficult compared with Yb 2 O 3 , Al 2 O 3 and BaO.
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  • Residual stress and microstructure evolutions of SAF 2507 duplex stainless steel after shot peening

    Ming Chen   Huabing Liu   Liaobo Wang   Zhou Xu   Vincent Ji   Chuanhai Jiang  

    Highlights • SAF 2507 duplex stainless steel was subjected to shot peening (SP) treatment. • High compressive residual stresses were induced on surface layer of the SAF 2507 alloy. • SP introduced satisfied surface nanocrystallization and microstructure refinement into the material. • Martensitic transformation sequence was considered to be: γ → austenite twins → α′-martensite. Abstract In this study, the SAF 2507 duplex stainless steel was subjected to surface shot peening (SP) treatment. The characteristics of processed samples such as surface morphology, residual stress distribution and microstructure were analyzed by scanning electron microscopy, X-ray diffractometry and transmission electron microscopy techniques. Results showed that SP had deleterious side effect on the surface roughness. After SP, high compressive residual stresses were induced on surface layer of the SAF 2507 alloy. The maximum stresses located at subsurface were approximately −930 and −715 MPa in γ and α phase. SP introduced satisfied surface nanocrystallization and microstructure refinement into the material. The minimum grain size located at the top surface was about 25 nm, the thickness of nanocrystalline layer with crystallites smaller than 100 nm reached to ∼125 μm. Moreover, a mass of dislocations and dislocation cells was formed in γ phase, but was rarely observed in α phase. The quantified dislocation density at surface was about 2.29 × 10 15  m −2 and 1.28 × 10 15  m −2 in γ and α phase, respectively. SP leaded to the martensitic transformation, and the transformation sequence was considered to be: γ → austenite twins → α′-martensite. SP treatments resulted in work hardening and increased the microhardness of the near surface material, which was mainly ascribed to the grain refinement effects and the formation of the strain-induced α′-martensite.
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  • Evaluation of the residual stress and microstructure character in SAF 2507 duplex stainless steel after multiple shot peening process

    Ming Chen   Huabing Liu   Lianbo Wang   Chengxi Wang   Kaiyuan Zhu   Zhou Xu   Chuanhai Jiang   Vincent Ji  

    Abstract In this study, multiple shot peening treatment was implemented to the SAF 2507 duplex stainless steel. The residual stress, microstructure and strain-induced transformation in the shot-peened specimens were evaluated by X-ray diffraction line profile analysis. Results pointed out that multiple shot peening was helpful to develop deep layer of high compressive residual stress with moderate work hardening. After triple shot peening treatment, a maximum compressive residual stress of −1070.5 MPa (at the surface for austenite) and −910.5 MPa (at 10 μm depth for ferrite) were obtained. The multiple shot peening is more effective in producing the microstructure refinement than single shot peening. The depth of the refinement layer with domain size smaller than 100 nm reached to 150 μm in the austenite and 100 μm in the ferrite phase of the dually shot-peened specimen, the maximum magnitudes of which after single shot peening were 100 μm and 75 μm, correspondingly. The quantitative measurement of stained-induced martensite in the shot-peened samples by X-ray phase analysis revealed that larger amount of α′-martensite formed after multiple shot peening compared to that of single shot peening, nearly 31.6% α′-martensite was formed at the impacted surface of triply peened specimen. The results also demonstrated that multiple shot peening could significantly decrease the surface roughness and help to improve the surface quality. Highlights • Multiple shot peening treatments were implemented to DSS SAF 2507. • Residual stress and microstructure character were evaluated by XRD line profile analysis. • More strain-induced α′-martensite was produced in the multiply SP-processed specimen. • Multiple SP produced a thicker nanocrystalline layer with greater compressive stresses than single SP.
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  • Residual stresses in oxide scale formed on Fe–17Cr stainless steel

    Ning Li   Ji Xiao   Nathalie Prud’homme   Zhe Chen   Vincent Ji  

    Highlights • Determine the residual stresses in different oxide layers formed on Fe–17Cr. • Oxide growth stress plays an important role in residual stress at room temperature. • RS evolutions in two layers are different according to the oxidation temperature. • The stresses in the two oxide layers are interactional. Abstract The purpose of this work was to investigate residual stresses in the oxide scale formed on ferritic stainless steel, which is proposed to be used as interconnector in the planar solid oxide fuel cells (SOFCs). The oxidation of the alloy has been conducted at 700 °C, 800 °C and 900 °C for 12–96 h by thermal gravimetric analysis (TGA) system. The oxide surface morphology, cross-section microstructure and the chemical composition of the oxide scale were studied after oxidation, and the residual stresses distribution of the oxide scale were determined at room temperature. It has been found that the oxide scale composed of an inner Cr 2 O 3 layer and an outer Mn 1.5 Cr 1.5 O 4 spinel layer, the residual stresses in both oxide layers are compressive and the growth stresses plays an important role. The competition of the stresses generation and relaxation during oxidation and cooling affects the residual stresses level. The evolution of residual stresses in the two layers is different according to the oxidation temperature, and the stresses in the two layers are interactional.
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  • Estimation of microstructure and corrosion properties of peened nickel aluminum bronze

    Chengxi Wang   Chuanhai Jiang   Ze Chai   Ming Chen   Lianbo Wang   Vincent Ji  

    Abstract In this study, the effects of shot peening on the residual stress, microstructure and corrosion properties of nickel aluminum bronze were evaluated by using X-ray stress analyzer, X-ray diffraction profile analysis, transmission electron microscopy and electrochemical tests. Shot peening improved the hardness and compressive residual stress of the surface layer. A thin layer of nanometer-scaled domain formed at the top surface and the domain size raised constantly with depth until it reached nearly the value of matrix. The micro-strain and dislocation density dropped gradually with the increasing distance toward interior. Moreover, the samples treated by shot peening with different intensities all showed superior corrosion resistance than that of unpeened ones. However, further improving the peening density resulted in a poor corrosion performance, which was mostly ascribed to the increased surface roughness due to the raised deformation. Highlights • Shot peening improved hardness and introduced compressive residual stress. • Nano-scaled layer formed at top surface and domain size raised with the depth. • Increased micro-strain and dislocation density dropped with the depth. • Corrosion resistance controlled by residual stress, domain size and roughness.
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  • Magnetic properties and possible martensitic transformation in Mn<sub>2</sub>NiSi and Ni<sub>2</sub>MnSi Heusler alloys

    Qing-Long Fang   Jian-Min Zhang   Xu-Mei Zhao   Ke-Wei Xu   Vincent Ji  

    Abstract The magnetic properties and electronic structure of Mn 2 NiSi and Ni 2 MnSi Heusler alloys have been studied by using the first-principles. The possible non-modulated martensitic transformations in Mn 2 NiSi and Ni 2 MnSi alloys have been investigated. In both austenitic and martensitic phases, Mn 2 NiSi is a ferrimagnet. The antiparallel alignment of Mn (A) and Mn (B) moments is found. In martensitic Mn 2 NiSi, the global energy minimum occurs at c / a =1.37 with 1% decrease of the martensitic cell volume. The energy difference Δ E between the austenitic and martensitic phases is 400 meV/cell, so it is more likely to realize martensitic transformation for Mn 2 NiSi alloy. Unlike Mn 2 NiSi alloy, the global energy minimum occurs at c / a =1.02 without cell volume change in martensitic Ni 2 MnSi. But the energy difference Δ E between the austenitic and martensitic phases is only 0.3 meV/cell. Meanwhile, around c / a =1 an anomaly is observed in the E tot – c / a curve which is related to a very slightly martensitic distortion trend in Ni 2 MnSi. Highlights • The global energy minimum occurs at c / a =1.37 in martensitic Mn 2 NiSi. • It is more likely to realize martensitic transformation in Mn 2 NiSi. • The energy difference Δ E between the austenitic and martensitic phases is only 0.3 meV/cell. • A very slightly martensitic distortion trend in Ni 2 MnSi.
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