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Seol, Jae Bok,Kwak, Chang Min,Han, Jong Chan,Baek, Kyung Hum,Jeong, Young Kyu Elsevier 2019 APPLIED SURFACE SCIENCE - Vol.479 No.-
<P><B>Abstract</B></P> <P>Laser-assisted atom probe tomography (APT) has commonly been performed to determine the composition of non-conductive oxide materials at the atomic scale. However, its field evaporation mechanism is still insufficiently understood due to the complexity of laser-matter interaction on the surface of oxide tips. To understand the physical mechanism underlying laser-assisted field evaporation in a bulk oxide LaAlO<SUB>3</SUB> (LAO), we conducted an interrupted experiment that combines transmission electron microscopy with APT. This correlated technique visualized the shape evolution of LAO tips during laser-assisted APT. We demonstrated that the evaporation field strength of the tips depends on the APT experimental parameters including applied laser energy and base temperature. An increase in the laser energy led to a reduction in the field evaporation strength of the tips and the formation of apex asymmetries on the tips, thereby considerably influencing the mass resolution of the technique. Moreover, an increase in the base temperature caused a slight decrease in the evaporation strength but did not significantly affect the mass-resolving ability of APT. The results discussed in this work suggest that it is the employed laser energy that primarily affects APT's resolving capability and the field evaporation strength of oxide materials. Our interrupted technique provides fundamental insight into the field evaporation sequences of LAO oxide tips under laser illumination, which can be feasibly applied to the study of various metallic and oxide materials.</P> <P><B>Highlights</B></P> <P> <UL> <LI> We have performed the laser-assisted field evaporation in a bulk oxide LaAlO<SUB>3</SUB> by means of a technique combining transmission electron microscopy with atom probe tomography. </LI> <LI> The results show that the laser energy is the parameter that has the strongest effect on the mass resolution and <I>F</I> <SUB>evap</SUB> in bulk oxides, thereby substantially influencing the compositional stoichiometry of LaAlO<SUB>3</SUB>. </LI> </UL> </P>
A Brief Comment on Atom Probe Tomography Applications
Seol, Jae-Bok,Kim, Young-Tae,Park, Chan-Gyung Korean Society of Microscopy 2016 Applied microscopy Vol.46 No.3
Atom probe tomography is a time-of-flight mass spectrometry-based microanalysis technique based on the field evaporation of surface atoms of a tip-shaped specimen under an extremely high surface electric field. It enables three-dimensional characterization for deeper understanding of chemical nature in conductive materials at nanometer/atomic level, because of its high depth and spatial resolutions and ppm-level sensitivity. Indeed, the technique has been widely used to investigate the elemental partitioning in the complex microstructures, the segregation of solute atoms to the boundaries, interfaces, and dislocations as well as following of the evolution of precipitation staring from the early stage of cluster formation to the final stage of the equilibrium precipitates. The current review article aims at giving a comment to first atom probe users regarding the limitation of the techniques, providing a brief perspective on how we correctly interprets atom probe data for targeted applications.
Jae-Bok Seol,Young-Tae Kim,Bo-Hwa Kim,박찬경 대한금속·재료학회 2016 METALS AND MATERIALS International Vol.22 No.1
The characterization of ZnO nanowires is crucial for developing nanostructured devices together with related compounds and alloys with an atomic-scale regime. This study describes the effects of laser energy on the atom probe tomography analysis of a single ZnO nanowire with a high aspect ratio, diameters of 80−100 nm and lengths of 10 μm. We observed both an asymmetrical evolution in the apex morphology and the compositional nonuniformities of Zn and O ions with respect to the laser energies ranging from 5 to 40 nJ. When the higher laser illumination exposed to the ZnO nanowires, non-uniform field strength becomes noticeable especially at the laser incident side of the samples. Moreover, we measured the charge state ratios of Zn + and Zn 2+ ions as a function of the applied laser energies. Our results proved important for accurate quantitative characterization and better interpretation for the laser-pulsed atom probe tomography of ZnO-based devices.
Seol, Jae Bok,Haley, Daniel,Hoelzer, David T.,Kim, Jeoung Han Elsevier 2018 Acta materialia Vol.153 No.-
<P><B>Abstract</B></P> <P>Doping with interstitials influences the grain boundary (GB) composition of metallic alloys, enabling changes in elemental GB enrichment, grain size, and mechanical properties or even promoting nanoparticle formation. Yet, little efforts on these doping effects have been made in oxide dispersion-strengthened (ODS) steels. Here, by combining advanced microscopy techniques, we studied the impact of interstitial concentration and extrusion temperature on the GB structure-dependent solute segregation, Y−Ti−O nanofeatures, and mechanical properties of ferritic Fe–14Cr (wt%) ODS steels fabricated by ball milling. We found that doping with high carbon and oxygen contents causes the GB to be decorated with the interstitials and promotes nanoparticle formation along the GBs, thereby retarding capillary-driven grain coarsening. This effect performs twofold, through grain size refinement and particle hardening. For samples with low interstitial contents, altering the extrusion temperature does not significantly change the material's mechanical properties and microstructure or the nonstoichiometric chemistry of nanoparticles, which are highly stable at high temperatures. Further, for all the samples, Y–Al oxides in the initial precipitation stages rapidly become coarsened at high temperatures, as Al weakens the thermal stability of nanoparticles, thereby transforming them to core-shell structures with Y−Al-rich cores and Ti−O-rich shells in the later precipitation stages.</P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>
A Brief Review of κ-Carbide in Fe-Mn-Al-C Model Alloys
Seol, Jae Bok Korean Society of Microscopy 2018 Applied microscopy Vol.48 No.4
The multiple length scale analysis of previously designed Fe-Mn-Al-C based low-density model alloys reveals the difference in ordered ${\kappa}-carbide$, $(Fe,Mn)_3AlC_x$, between Fe-25Mn-16Al-5.2C (at%) alloy and Fe-3Mn-10Al-1.2C (at%) alloy. For the former alloy composition consisting of fully austenite grains, ${\kappa}-carbide$ showed majorly cuboidal and minorly pancake morphology and its chemical composition was not changed through aging for 24 h and 168 h at $600^{\circ}C$. Meanwhile, for the isothermally annealed ferritic alloy system for 1 hr at 500 and $600^{\circ}C$, the dramatic change in the chemical composition of needle-shape ${\kappa}-carbide$, $(Fe,Mn)_3(Fe,Al)C_x$, was found. Here we address that the compositional fluctuations in the vicinity of the carbides are significantly controlled by abutting phase, either austenite or ferrite. Namely, the cooperative ordering of carbon and Al is an important factor contributing to carbide formation in the high-Mn and high-Al alloyed austenitic steel, while the carbon and Mn for the low-Mn and high Al alloyed ferritic steel.
Boron doped ultrastrong and ductile high-entropy alloys
Seol, Jae Bok,Bae, Jae Wung,Li, Zhiming,Chan Han, Jong,Kim, Jung Gi,Raabe, Dierk,Kim, Hyoung Seop Elsevier 2018 Acta materialia Vol.151 No.-
<P><B>Abstract</B></P> <P>A new class of materials called high-entropy alloys (HEAs) constitutes multiple principal elements in similar compositional fractions. The equiatomic Fe<SUB>20</SUB>Mn<SUB>20</SUB>Cr<SUB>20</SUB>Co<SUB>20</SUB>Ni<SUB>20</SUB> (at%) HEA shows attractive mechanical properties, particularly under cryogenic conditions. Yet, it lacks sufficient yield and ultimate tensile strengths at room temperature. To strengthen these materials, various strategies have been proposed mainly by tuning the composition of the bulk material while no efforts have been made to decorate and strengthen the grain boundaries. Here, we introduce a new HEA design approach that is based on compositionally conditioning the grain boundaries instead of the bulk. We found that as little as 30 ppm of boron doping in single-phase HEAs, more specific in an equiatomic FeMnCrCoNi and in a non-equiatomic Fe<SUB>40</SUB>Mn<SUB>40</SUB>Cr<SUB>10</SUB>Co<SUB>10</SUB> (at%), improves dramatically their mechanical properties, increasing their yield strength by more than 100% and ultimate tensile strength by ∼40% at comparable or even better ductility. Boron decorates the grain boundaries and acts twofold, through interface strengthening and grain size reduction. These effects enhance grain boundary cohesion and retard capillary driven grain coarsening, thereby qualifying boron-induced grain boundary engineering as an ideal strategy for the development of advanced HEAs.</P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>
복합 밀링 공정에 따른 철계 산화물분산강화 합금의 미세조직 및 기계적 특성 변화 연구
설재복 ( Jae-bok Seol ),김경민 ( Kyong Min Kim ),김정한 ( Jeoung Han Kim ) 대한금속재료학회(구 대한금속학회) 2016 대한금속·재료학회지 Vol.54 No.3
A novel approach is established to produce oxide dispersion-strengthened steels with a fine distribution of oxide nanoparticles. This approach involves cryo-milling followed by ball-milling at room temperature. This complementary milling technique allows such steels to exhibit a remarkable combination of strength and ductility. Based on our experimental findings, the associated microstructural factor contributing to the strength is likely to be Y-Ti-O complex oxide particles. On the other hand, an additional milling process performed at room temperature is not effective with regard to deformationinduced sintering. Therefore, it is concluded that milling at room temperature after cryo-milling does not improve the tensile ductility of oxide dispersion-strengthened steels.