Xiaolong Song

Effect of shot peening on hydrogen embrittlement of high strength steel

The effect of shot peening (SP) on hydrogen embrittlement of high strength steel was investigated by electrochemical hydrogen charging, slow strain rate tensile tests, and hydrogen permeation tests. Microstructure observation, microhardness, and X-ray diffraction residual stress studies were also conducted on the steel. The results show that the shot peening specimens exhibit a higher resistance to hydrogen embrittlement in comparison with the no shot peening (NSP) specimens under the same hydrogen-charging current density. In addition, SP treatment sharply decreases the apparent hydrogen diffusivity and increases the subsurface hydrogen concentration. These findings are attributed to the changes in microstructure and compressive residual stress in the surface layer by SP. Scanning electron microscope fractographs reveal that the fracture surface of the NSP specimen exhibits the intergranular and quasi-cleavage mixed fracture modes, whereas the SP specimen shows only the quasi-cleavage fractures under the same hydrogen charging conditions, implying that the SP treatment delays the onset of intergranular fracture.

Investigation of tensile deformation mechanisms at room temperature in a new Ni-based single crystal superalloy

Abstract The deformation microstructures of a new Ni-based single crystal superalloy, M 4706, have been characterized by transmission electron microscopy after interrupted tensile tests at room temperature. It is found that besides shearing of γ′ precipitates by strongly coupled dislocations, another unusual shearing process involving a single a/2 〈1 0 1〉 matrix dislocation as well as the formation of the isolated superlattice stacking fault and Shockley loop also operates actively during initial yielding. Based on experimental observations, occurrence of these different shearing processes is discussed.

Structural characterization of individual graphene sheets formed by arc discharge and their growth mechanisms

Graphene sheets formed by arc discharge are hard to characterize in detail due to their complex pristine states in raw soot, which always exhibit an overall morphology of overlapping aggregation, together with other carbonaceous by-products. Here we used an improved arc method and simple separation procedure to obtain a large number of individual graphene sheets with single- to few-layers, and further probed their structural details using optical microscopy, transmission electron microscopy (TEM), atomic force microscopy and Raman spectroscopy. By TEM characterization, two major types of graphene sheets are shown; one is observed with folded fringes and polycrystalline structure, whereas the other is with an even graphene plane and single crystalline structure. In contrast to that of supported graphene, the Raman spectra of these graphene sheets show some different characteristics such as opposite shift of G band frequency as the layers increase. With increasing layers, the frequencies of G and G′ bands and the full width at half maximum (FWHM) of the G′ band totally exhibit layer-dependence. According to the FWHM of G′ bands, the folding within graphene sheets is also discussed. In addition, the defect types for arc graphene are analysed based on the D and D′ bands. Our results suggest that the D bands of such graphene sheets result from edges, rather than topological defects or disorder. Based on the findings, a new growth mechanism of arc graphene is proposed rationally responsible for the difference of two types of graphene sheets.

Extraordinary plastic behaviour of the γ′ precipitate in a directionally solidified nickel-based superalloy

Abstract The deformation behaviour of the γ′ precipitate in a directionally solidified nickel-based superalloy is investigated using microscopic observations after tensile testing at room temperature. It is found that coarse γ′ precipitates (604 nm) are sheared by strongly coupled dislocations, and some γ′ precipitates are elongated to approximately 3–6 times of their original lengths. It reveals that, at room temperature, the γ′ precipitate within the experimental superalloy has a significant plastic deformation capacity in comparison with Ni3Al bulk alloys. Based on the experimental observations, the extraordinary plastic behaviour of the γ′ precipitate is analysed.

Nonlinear elastic behavior and failure mechanism of polyhedral graphite particles undergoing uniaxial compression

Load-displacement responses and ultimate strength of polyhedral graphite particles (PGPs) undergoing in situ nano-compression at ambient temperature have been studied. The dynamic responses of PGPs to uniaxial loads exhibit a typical nonlinear elastic behavior for graphitic nanomaterials. Based on the analysis of stress-strain relationship, the intrinsic strength is slightly larger than actual ultimate strength, indicating the mechanical properties influenced by the initial defects in PGPs. For a given case, compressive Youngs modulus E and third-order elastic modulus D achieve to 12.8 GPa and −13.9 GPa, respectively. Weibull probability analysis confirmed its broad range of structural defects inside PGPs and mechanical properties are sensitive to initial defects. The values of ultimate strength of tested PGPs with diameter of 150–400 nm fall within 2–4.5 GPa, which are in the range between shear elastic modulus C44 of turbo-g (minimum) and C44 of hex-g (maximum) in the literature. The deformation and fa...

Investigation on mechanical properties of laser welded joints for Ti-6Al-4V titanium alloy

Abstract Shear punch test was used to study the mechanical properties of Ti–6Al–4V titanium alloy laser welded joint. Meanwhile, microhardness test was used to further prove the results of the shear punch test. In order to get a better understanding of the reason why the highest mechanical properties are found in the coarse welded metal, the in situ method for viewing microvoid nucleation, growth and coalescence was applied. The in situ experiment showed that the crack in weld metal (WM) that was nucleated in the twin boundaries of the inner martensite (M) phase and the intersection of slip line and grain boundary grew rapidly once forming; in the heat affected zone (HAZ), there were three main cracks initiating, and only one propagated; microvoid was nucleated first along the interface between α and β phase in base metal (BM), the bluntness and expansion happening alternately. Except for a small amount of quasi-cleavage fracture in WM, the fracture appearance of the joint was all dimples. With the twin presented due to plastic deformation, the damage along the grain boundary area within HAZ and BM happened much more easily than in WM.

Plastic deformation mechanisms in a new Ni-base single crystal superalloy at room temperature

The evolution of dislocation configurations in a new Ni‐base single crystal superalloy, M4706, during tensile deformation at room temperature is characterised by transmission electron microscopy. Experimental results show that contrary to previous reports, numerous isolated superlattice stacking faults and extended stacking faults are formed in the slightly deformed specimens with and without tertiary γ′ precipitates. Meanwhile, it is also found that as the plastic deformation proceeds, the dominant deformation mechanism changes from stacking fault shearing to antiphase boundary shearing. Finally, based on experimental observations, the reasons for the formation of these faults and the transition in the deformation mechanism are discussed.

Loading rate effect of TiAl-Cr on fracture behavior at high temperature

Abstract A fracture test of TiAl-Cr was carried out at high temperatures and different loading rates. The result shows that the fracture energy decreases with increasing loading rate even at temperature 1050 °C at which the ductility of TiAl-Cr has been recovered totally. Increasing temperature makes the transition happen at a relatively high loading rate. Observation of fracture surface shows that the cleavage corresponds to the lower loading rate, i.e. a higher energy state, while fracture surface is composed of blocks and lamellar structures when the loading is high and fracture energy is low.

Carbon nanohorns under cold compression to 40 GPa: Raman scattering and X-ray diffraction experiments

We report a high-pressure behavior of carbon nanohorns (CNHs) to 40 GPa at ambient temperature by in situ Raman spectroscopy and synchrotron radiation x-ray diffraction (XRD) in a diamond anvil cell. In Raman measurement, multiple structural transitions are observed. In particular, an additional band at ∼1540 cm−1 indicative of sp3 bonding is shown above 35 GPa, but it reverses upon releasing pressure, implying the formation of a metastable carbon phase having both sp2 and sp3 bonds. Raman frequencies of all bands (G, 2D, D + G, and 2D′) are dependent upon pressure with respective pressure coefficients, among which the value for the G band is as small as ∼2.65 cm−1 GPa−1 above 10 GPa, showing a superior high-pressure structural stability. Analysis based on mode Gruneisen parameter demonstrates the similarity of high-pressure behavior between CNHs and single-walled carbon nanotubes. Furthermore, the bulk modulus and Gruneisen parameter for the G band of CNHs are calculated to be ∼33.3 GPa and 0.1, respectively. In addition, XRD data demonstrate that the structure of post-graphite phase derives from surface nanohorns. Based on topological defects within conical graphene lattice, a reasonable transformation route from nanohorns to the post-graphite phase is proposed.We report a high-pressure behavior of carbon nanohorns (CNHs) to 40 GPa at ambient temperature by in situ Raman spectroscopy and synchrotron radiation x-ray diffraction (XRD) in a diamond anvil cell. In Raman measurement, multiple structural transitions are observed. In particular, an additional band at ∼1540 cm−1 indicative of sp3 bonding is shown above 35 GPa, but it reverses upon releasing pressure, implying the formation of a metastable carbon phase having both sp2 and sp3 bonds. Raman frequencies of all bands (G, 2D, D + G, and 2D′) are dependent upon pressure with respective pressure coefficients, among which the value for the G band is as small as ∼2.65 cm−1 GPa−1 above 10 GPa, showing a superior high-pressure structural stability. Analysis based on mode Gruneisen parameter demonstrates the similarity of high-pressure behavior between CNHs and single-walled carbon nanotubes. Furthermore, the bulk modulus and Gruneisen parameter for the G band of CNHs are calculated to be ∼33.3 GPa and 0.1, respecti...

Failure Analysis and Fatigue Investigation on Titanium Tubes in a Condenser

This paper presents an analysis of condenser titanium tube leakage in nuclear power plants. Chemical compositions, mechanical properties, metallographic structures, and microscopic morphologies were analyzed. The results show that the titanium tube leakage was mainly caused by fatigue failure on the basis of the fatigue fracture features. Fatigue tests had been carried out in both air and steam environments, and the fatigue resistance of titanium tubes decreased distinctly in a steam environment. Based on the investigations, proper recommendations have been proposed to enhance the prevention of fatigue fracture of titanium tubes in condensers.