Yanling Lu

Nanoscale potassium niobate crystal structure and phase transition

Nanoscale potassium niobate (KNbO3) powders of orthorhombic structure were synthesized using the sol-gel method. The heat-treatment temperature of the gels had a pronounced effect on KNbO3 particle size and morphology. Field emission scanning electron microscopy and transmission electron microscopy were used to determine particle size and morphology. The average KNbO3 grain size was estimated to be less than 100 nm, and transmission electron microscopy images indicated that KNbO3 particles had a brick-like morphology. Synchrotron X-ray diffraction was used to identify the room-temperature structures using Rietveld refinement. The ferroelectric orthorhombic phase was retained even for particles smaller than 50 nm. The orthorhombic to tetragonal and tetragonal to cubic phase transitions of nanocrystalline KNbO3 were investigated using temperature-dependent powder X-ray diffraction. Differential scanning calorimetry was used to examine the temperature dependence of KNbO3 phase transition. The Curie temperature and phase transition were independent of particle size, and Rietveld analyses showed increasing distortions with decreasing particle size.

The Effect of Silicon Additions on the Thermal Stability and Morphology of Carbides in a Ni‐Mo‐Cr Superalloy

The effect of silicon additions on the thermal stability and morphology of carbides has been investigated in a Ni-Mo-Cr superalloy. It is found that granular M6C carbides form at the grain boundaries in the standard heat. The size and morphology of M6C carbides remain unchanged in the aging treatment at 700–800 °C up to 1000 h. While in the non-silicon heat, the intergranular carbides are examined as M2C type carbides by TEM at the initial stage of the aging treatment. With the aging time increasing, the fine granular M2C carbides transform into coarse planar M6C carbides. It can be concluded that silicon additions can lead to the more stable intergranular carbides in the Ni-Mo-Cr superalloy.

TEM Investigation on the Microstructural Evolution of Hastelloy N Induced by Ar+ Ion Irradiation

Hastelloy N alloy has been selected as the primary structure material for molten salt reactor. In this article, Hastelloy N alloy samples were irradiated to different doses at room temperature using 300 keV Ar+ ions. The microstructural evolution was investigated by transmission electron microscopy (TEM) and energy‐dispersive spectroscopy (EDS). Black dot defects emerged in sample irradiated at low dose (0.4 displacement per atom (dpa)), and they grew up with irradiation doses (0.4–2 dpa). A high density of small dislocation loops (nano meters in size) were observed in the sample irradiated to 4 dpa. When the ion dose increased to 12 dpa, complicated structures with defects (including dislocation lines, larger loops and smaller black dots) were observed. Dislocation networks were detected from high‐angle annular dark field (HAADF) images. Larger dislocation loops (size: 30–80 nm) were visible in the sample irradiated to 40 dpa. Irradiation with dose of 120 dpa led to the formation of face‐centered cubic nanocrystallites with preferred orientations. Microsc. Res. Tech. 77:161–169, 2014.

Microstructure and properties of epitaxial layers by laser glazing

Abstract Various processes of laser surface melting are employed to modify the features of epitaxially grown microstructures in the melt. Three kinds of microstructures are demonstrated in this investigation. The one with complex microstructures possesses the best resistivity against hot stress corrosion.

Carbides Evolution in a Ni-16Mo-7Cr Base Superalloy during Long-Term Thermal Exposure

The effect of long-term thermal exposure on the carbide evolution in a Ni-16Mo-7Cr base superalloy was investigated. The results show that M12C carbides are mainly precipitated on the grain boundaries during thermal exposure, and the primary massive M6C carbides can be completely transformed to M12C carbides in situ at temperatures above 750 °C for long-term thermal exposure. The transformation from M6C carbides to M12C carbides is attributed to the release of C atoms from M6C, which results in the morphology changes of massive carbides, and stabilization of the sizes of M12C carbides precipitated on the grain boundaries.

Effect of Mg on the Microstructure and Corrosion Resistance of the Continuously Hot-Dip Galvanizing Zn-Mg Coating

The microstructure of continuously hot-dip galvanizing Zn-Mg coating was investigated in order to obtain the mechanism of the effects of Mg on the corrosion resistance. In this paper, the vertical section of the Zn-0.20 wt % Al-Mg ternary phase diagram near the Al-low corner was calculated. The results indicates that the phase composition of the Zn-0.20 wt % Al-Mg ternary phase diagram near the Al-low corner is the same as Zn-Mg binary phase diagram, suggesting Al in the Zn-Mg (ZM) coatings mainly concentrates on the interfacial layer between the coating and steel substrate. The microstructure of continuously hot-dip galvanizing ZM coatings with 0.20 wt % Al containing 1.0–3.0 wt % Mg was investigated using tunneling electron microscopy (TEM). The morphology of Zn in the coating changes from bulk to strip and finally to mesh-like, and the MgZn2 changes from rod-like to mesh-like with the Mg content increasing. Al in the ZM coatings mainly segregates at the Fe2Al5 inhibition layer and the Mg added to the Zn bath makes this inhibition layer thinner and uneven. Compared to GI coating, the time of the first red rust appears increases by more than two-fold and expansion rate of red rust reduces by more than four-fold in terms of salt spray experiment. The ZM coating containing 2.0 wt % Mg has the best corrosion resistance. The enhanced corrosion resistance of ZM coatings mainly depends on different corrosion products.