Chan Gyung Park

Strain-induced precipitation of NbC in Nb and Nb-Ti microalloyed HSLA steels

The precipitation start time (Ps) of strain-induced NbC carbides is delayed in Nb–Ti steel in comparison to the case of Nb steel. The delay of precipitation of strain-induced NbC carbides is attributed to both the insufficient solution of Nb during a reheating treatment, and the heterogeneous nucleation of (Nb,Ti)C carbides.

The effects of tungsten addition on the microstructural stability of 9Cr–Mo Steels

Abstract The effects of tungsten addition on the microstructure and high-temperature tensile strength of 9Cr–Mo steels have been investigated by using three different steels: M10 (9Cr–1Mo), W18 (9Cr–0.5Mo–1.8W), and W27 (9Cr–0.1Mo–2.7W) steels. The tungsten-added 9Cr steels have revealed better high-temperature tensile strength. Microchemical analysis for (Cr,Fe) 2 (C,N) revealed that the tungsten addition increased the Cr/Fe ratio, which resulted in the lattice expansion of (Cr,Fe) 2 (C,N), and then the enhanced pinning effect on the glide of dislocation. In addition, in M10 steel, the M 23 C 6 carbides quickly grew and agglomerated, while the tungsten-added 9Cr steels revealed a fine and uniform distribution of M 23 C 6 carbides. Dislocation recovery during tempering treatments was delayed in tungsten-added 9Cr steels, which was correlated with the stabilized precipitates and the decreased self-diffusivity of iron. It is, thus, believed that the excellent high-temperature tensile strength of tungsten-added 9Cr steels is attributed to the stabilized M 2 X carbo-nitrides and M 23 C 6 carbides and the decreased self-diffusivity of iron.

Improved thermal stability of Ni silicide on Si (100) through reactive deposition of Ni

The effect of reactive deposition of Ni on the thermal stability of Ni silicide has been investigated in this study. In the case of room-temperature-deposited Ni, the agglomeration of Ni silicide, which induced the thermal instability during subsequent annealing, started to appear at 600u200a°C and the sheet resistance was increased abruptly after high-temperature anneals. However, when the Ni was deposited on the heated Si substrate (reactive deposition of Ni), the sheet resistance of Ni silicide film exhibited a constant value of about 7.91 Ω/□ at the whole reaction temperature, especially at 900u200a°C.

Effect of microstructural evolution on magnetic property of Mn-implanted p-type GaN

The microstructural evolution of Mn-implanted p-type GaN has been studied using cross-sectional transmission electron microscopy. As Mn3Ga nanoclusters (3–7 nm) with a hexagonal structure were produced by annealing (⩽800u200a°C), the weak ferromagnetic property emerged. Higher-temperature annealing (⩾900u200a°C) reduced the ferromagnetic signal and produced antiferromagnetic Mn-nitride nanoclusters, such as Mn6N2.58 and Mn3N2. This provides evidence that the ferromagnetic property was deeply related to microstructural changes of nanoclusters.

The effect of Fe on the crystallization behavior of Al–Mm–Ni–Fe amorphous alloys

The crystallization behavior and thermal stability of Al86Mm4Ni10−xFex alloys were investigated as a function of Fe content. Alloys, produced by a single roll melt-spinner at a circumferential speed of 52 m/s, revealed fully amorphous structures. The thermal stability of the present amorphous alloys increased with the increase of Fe content. The activation energy for crystallization of α-Al increased as the Fe content increased. This increase of activation energy resulted in the simultaneous precipitation of α-Al and intermetallic phase observed especially in Al86Mm4Ni5Fe5 and Al86Mm4Ni2Fe8 alloys. The glass transition was observed in DSC thermogram only after proper annealing treatment. The effect of alloy composition on the thermal stability could be explained in terms of the atomic structure of the amorphous alloy.

Metal-ZnO Heterostructure Nanorods with an Abrupt Interface

We report on metal-semiconductor heterostructure nanorods with an abrupt interface. The metal-semiconductor nanorods were fabricated simply by evaporating metal on vertically aligned ZnO nanorods. Before the fabrication of heterostructure nanorods, the ZnO nanorods were prepared by metalorganic vapor phase epitaxy. Since no metal catalyst is employed during the nanorod growth, the nanorods do not exhibit any metal clusters on their tips. When metal is evaporated on the vertically aligned nanorods, metal is deposited mainly on the tips of the nanorods. Transmission electron microscopy revealed that the Au-coated ZnO heterostructure nanorods exhibit an atomically sharp interface between ZnO and Au.

Microstructural accommodation of excess Ru in epitaxial SrRuO3 films

The microstructures of Ru-excess SrRuO3 films, which were grown epitaxially on SrTiO3(001) substrates by ion-beam sputtering, were studied by transmission electron microscopy. The excess Ru can be accommodated by forming extended defects on the {100} planes, faulted dislocation loops, by making RuO2 double layers. However, the most stable crystalline phase of the excess Ru in SrRuO3 film was metallic Ru with a hexagonal structure. The orientation relationship between the Ru precipitates, the SrTiO3 substrate, and the SrRuO3 film can be described as follows: (011)Ru//(002)STO//(002)SRO and [100]Ru//[110]STO//[110]SRO where the subscripts STO and SRO indicate SrTiO3 and SrRuO3 respectively. Owing to the difference between the crystal symmetries of Ru and SrTiO3, the precipitates showed different in-plane alignments along two perpendicular directions on the substrate and accordingly different anisotropic growth morphologies. The precipitates degrade the film surface by making deep trenches and act as sources for defect generation.