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He, Yinsheng,Yoo, Keun-Bong,Park, Joong Cheul,Lee, Byung-Ho,Yoon, Jeong-Bong,Kim, Jung-Gu,Shin, Keesam Elsevier 2018 Materials characterization Vol.142 No.-
<P><B>Abstract</B></P> <P>This study investigated the corrosion behavior and underline mechanisms of low alloy steels with a minor addition of 0.4 wt% Cu, 0.2 wt% Ni, 0.1 wt% Sb and 0.05 wt% Co developed for a flue gas desulfurization (FGD) system. Corrosion tests were carried out in an aggressive solution of 16.9 vol% H<SUB>2</SUB>SO<SUB>4</SUB> + 0.35 vol% HCl + Bal. H<SUB>2</SUB>O at 60 °C for up to 48 h. The experimental results revealed enrichment of the additional elements (Cu, Sb and Ni) in the corrosion layer, whose concentration increased with test time. As the concentrations of Cu increased, the crystallization and growth of the Cu particles (from several nm to 320 nm)) within the corrosion layer was accelerated. At the initial stage of the test, a continuous amorphous layer rich with the additional elements formed and covered the surface of the steel, which then gradually developed defects, pores and cracks, as the crystallization and growth of Cu particles with the elapse of time. The results indicated that the corrosion resistance of the low alloy steel depends on the existence form of Cu in the corrosion layer, i.e., as solute atoms or as nanoparticles strengthened the corrosion layer and increased corrosion resistance. As the coarsening of Cu particles weakened the corrosion layer and deteriorated the corrosion resistance.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Corrosion mechanism of low alloy steels for flue gas desulfurization was studied. </LI> <LI> An amorphous corrosion layer enriched with Fe, O, S formed on the blank specimen. </LI> <LI> Added Cu concentrated, crystallized and grown to particles in corrosion layer. </LI> <LI> Corrosion resistance of the steels depended on the form of Cu in corrosion layer. </LI> <LI> Sb addition slows the crystallization and growth of Cu particles in corrosion layer. </LI> </UL> </P>
He, Yinsheng,Dong, Jiling,Choi, Woosung,Jung, Jinesung,Shin, Keesam John Wiley Sons, Ltd 2012 Surface and interface analysis Vol.44 No.11
<P>An improved non‐destructive replication (NDR) metallography method was developed and applied successfully for the precipitates study in 1Cr‐1Mo‐0.25V turbine rotor steel. The preparation of the replicas for electron microscopy was carried out with a combination of electropolishing, etching, and acetate film setting for extraction medium. The exposure of the precipitates was improved by electropolishing followed by chemical etching. The electropolishing after application of acetate film, instead of stripping, resulted in a quality improvement of the extraction replica. Three types of precipitates (M<SUB>23</SUB>C<SUB>6</SUB>, Mo<SUB>2</SUB>C, and VC) were identified by TEM analysis of thin foil. The types, sizes, and chemical compositions of the precipitates from the bulk SEM observation and TEM analyses of thin foil specimen and the NDR extraction replica matched very well, indicating NDR extracted all types of the precipitates with high efficiency. Copyright © 2012 John Wiley & Sons, Ltd.</P>
Defect-induced Visible-light Emission in GaN Nanocrystals Synthesized through a Solution-based Route
Yinsheng He,Zhixin Wan,신기삼,Jiling Dong,Youngmoom Yu 한국물리학회 2012 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.61 No.9
In this study, nanocrystalline GaN was synthesized through a metathesis reaction at room temperature. The microstructure of the GaN examined by X-ray diffraction (XRD) and transmission electron microscopy (TEM) showed the single nature with würtzite structure. High resolution TEM (HRTEM) images showed that the defects were formed in the as-prepared GaN nanocrystals. Room temperature photoluminescence (PL) spectra showed the near band-edge emission at 356 nm (3.48 eV) as well as visible light emission at 420 ~ 460 nm (2.70 ~ 2.95 eV). The increase in the intensity of visible light peak was observed by annealing the as-prepared specimen at 850 ℃ for 3 hrs, which was due to the increase in the numeric density of defects caused by de-passivation of impurity during annealing. Therefore, it can be concluded that the visible light emission in the GaN nanocrystals was induced by the defects (possibly N vacancies) in the structure.
He, Yinsheng,Yoo, Keun-Bong,Ma, Houyu,Shin, Keesam Elsevier 2018 Materials letters Vol.215 No.-
<P><B>Abstract</B></P> <P>A gradient-structured layer with initially nanoscale grain size increased gradually to original coarse microscale (∼50 μm) was fabricated on the surface of austenitic (γ) stainless steel SS304 <I>via</I> ultrasonic nanocrystallization surface modification (UNSM) peening. Modified cross-sectional and depth-specific plan-view (DSPV) sample preparation methods were explored with the analysis scales from macro to atom by synchrotron radiation XRD, EBSD, and TEM to determine the depth (strain) dependent deformation microstructures and grain refinement mechanism. The depth-dependent deformation microstructures were ascribed to strain-induced martensitic-transformed (SIMT) α′- and ε-martensite, deformation nanotwins (NT), and dislocation grids. The grain nanocrystallization mechanism was suggested as the formation of α′ and ε grain boundaries that divided the original coarse grains (CG) to the nanoscale. The strains also appeared to play a key role in the crystallographic orientation relationship (OR) of the γ/α′: Kurdjumov-Sachs (K-S) and Pitsch in low strain region and Nishiyama-Wassermann (N-W) in the high strain region.</P> <P><B>Highlights</B></P> <P> <UL> <LI> A gradient-structured layer was fabricated on the surface of SS304 <I>via</I> UNSM peening. </LI> <LI> The depth-dependent microstructure was studied by the modified TEM sampling methods. </LI> <LI> The orientation relationships of the transformed martensite are depended on strain. </LI> </UL> </P>
Microstructural Evolution of Grade 91 Steel upon Heating at 760~1000℃
He, Yinsheng,Chang, Jungchel,Lee, Je-Hyun,Shin, Keesam Materials Research Society of Korea 2015 한국재료학회지 Vol.25 No.11
The microstructural evolution of Grade 91 tempered martensite ferritic steels heat treated at $760{\sim}1000^{\circ}C$ for two hours was investigated using scanning electron microscopy(SEM), energy disperse spectroscopy(EDS), electron backscattered diffraction (EBSD), and transmission electron microscopy(TEM); a microhardness tester was also employed, with a focus on the grain and precipitate evolution process as well as on the main hardening element. It was found that an evolution of tempered martensite to ferrite($760{\sim}850^{\circ}C$), and to fresh martensite($900{\sim}1000^{\circ}C$), occurred with the increase of temperature. Simultaneously, the parabolic evolution characteristics of the low angle grain boundary(LAGB) increased with the increase of the heating temperature(highest fraction of LAGB at $925^{\circ}C$), indicating grain recovery upon intercritical heating. The main precipitate, $M_{23}C_6$, was found to be coarsened slightly at $760{\sim}850^{\circ}C$; it then dissolved at $850{\sim}1000^{\circ}C$. Besides this, $M_3C$ cementite was formed at $900{\sim}1000^{\circ}C$. Finally, the experimental results show that the hardness of the steel depended largely on the matrix structure, rather than on the precipitates, with the fresh martensite showing the highest hardness value.
On-Site Corrosion Behavior of T91 Steel after Long-Term Service in Power Plant
He, Yinsheng,Chang, Jungchel,Lee, Je-Hyun,Shin, Keesam Materials Research Society of Korea 2015 한국재료학회지 Vol.25 No.11
In this work, on-site corrosion behavior of heat resistant tubes of T91, used as components of a superheater in a power plant for up to 25,762 h, has been investigated using scanning electron microscopy(SEM), energy dispersive X-ray spectroscopy (EDS), and electron backscattered diffraction(EBSD), with the objectives of studying the composition, phase distribution, and evolution during service. A multi-layer structure of oxide scale was found on both the steamside and the fireside of the tube surface; the phase distribution was in the order of hematite/magnetite/spinel from the outer to the inner matrix on the steamside, and in the order of slag/magnetite/spinel from the outer to the inner matrix on the fireside. The magnetite layer was found to be rich in pores and cracks. The absence of a hematite layer on the fireside was considered to be due to the low oxygen partial pressure in the corrosion environment. The thicknesses of the hematite and of the slag-deposit layer were found to exhibit no significant change with the increase of the service time.
Microstructural Characterization of SS304 upon Various Shot Peening Treatments
He, Yinsheng,Li, Kejian,Cho, In Shik,Lee, Chang Soon,Park, In Gyu,Song, Jung-il,Yang, Cheol-Woong,Lee, Je-Hyun,Shin, Keesam Korean Society of Microscopy 2015 Applied microscopy Vol.45 No.3
Plastic deformation was introduced to the austenitic (${\gamma}$) stainless steel of SS304 by air blast shot peening, ultrasonic shot peening, and ultrasonic nanocrystalline surface modification. Various deformation structures were formed. The hardness, the deformation structure and the underlying grain refinement mechanism were investigated. In the deformed region, planar dislocation arrays and deformation twin (DT), the DT-DT intersection and ${\varepsilon}$-martensite structures, and ${\alpha}^{\prime}$-martensite were formed in the respective regions of low, medium, and high strain. The grain refinement mechanism is found to be closely related to the 1) sub-division of coarse grains by DT, shear bands and their intersection, and 2) formation of nano-sized ${\alpha}^{\prime}$-martensite due to the high plastic deformation.
He, Yinsheng,Yang, Cheol-Woong,Lee, Je-Hyun,Shin, Keesam Korean Society of Microscopy 2015 Applied microscopy Vol.20 No.1
In this work, various electron microscopy and analysis techniques were used to investigate the microstructural evolution of a 9% Cr tempered martensite ferritic (TMF) steel T91 upon ultrasonic nanocrystalline surface modification (UNSM) treatment. The micro-dimpled surface was analyzed by scanning electron microscopy. The characteristics of plastic deformation and gradient microstructure of the UNSM treated specimens were clearly revealed by crystal orientation mapping of electron backscatter diffraction (EBSD), with flexible use of the inverse pole figure, image quality, and grain boundary misorientation images. Transmission electron microscope (TEM) observation of the specimens at different depths showed the formation of dislocations, dense dislocation walls, subgrains, and grains in the lower, middle, upper, and top layers of the treated specimens. Refinement of the $M_{23}C_6$ precipitates was also observed, the size and the number density of which were found to decrease as depth from the top surface decreased. The complex microstructure and microstructural evolution of the TMF steel samples upon the UNSM treatment were well-characterized by combined use of EBSD and TEM techniques.