Hyung-Ha Jin

Fabrication of a TEM sample of ion-irradiated material using focused ion beam microprocessing and low-energy Ar ion milling

Cross-section-view TEM samples of ion-irradiated material are successfully fabricated using a focused ion beam (FIB) system and low-energy Ar ion milling. Ga ion-induced damages in FIB processing are reduced remarkably by the means of low-energy Ar ion milling. There are optimized ion milling conditions for the reduction and removal of the secondary artifacts such as defects and ripples. Incident angles and accelerated voltages are especially more important factors on the preservation of a clean surface far from secondary defects and surface roughing due to Ga and Ar ion bombardment.

Atom probe tomography analysis of radiation-induced solute clustering in austenite stainless steels

ABSTRACT Various types of defects are produced by the irradiation of energetic particles onto a structural material. The large number of mobile vacancies and self-interstitial atoms during irradiation induce defect fluxes and the diffusion of solute atoms in the matrix. The preferential interaction between the solute atoms and radiation-induced defects leads to the enrichment/depletion or clustering of the solutes at defect sinks. In the current work, atom probe tomography (APT) was used for the analysis of radiation-induced solute clustering in ion-irradiated austenite stainless steel 316. Quantitative analysis of the localised clustering of chemical elements was implemented and a parameter selection procedure was proposed. The number density and size distribution of Si clusters in APT specimens irradiated at various temperatures were examined. At high temperature, the number density of the clusters decreased and their size increased. The localized Si atoms in variously shaped defects were clearly identified. The APT method was demonstrated to be suitable for identifying defect structures and for the quantitative analysis of clustering in irradiated specimens.

Microstructures and high-temperature tensile properties of mechanically alloyed and spark plasma-sintered 304SS-CNT composites:

We fabricated and investigated a 304 stainless steel and carbon nanotube (304SS-CNT) composite with an aim to study its microstructures and high-temperature tensile properties. 304SS powders were mixed with carbon nanotubes using ball milling and consolidated using the spark plasma sintering technique. Tensile specimens made from the consolidated samples of 304SS-CNT were tested in a temperature range from 299 K (26℃) to 773 K (500℃). An induction coil was used for high-temperature tensile tests. The yield strength and the work hardening of the 304SS-CNT sample were found to be higher than those of a sample fabricated from 304SS without carbon nanotubes for all tested temperatures. Microstructure analysis carried out using optical microscopy, scanning electron microscopy, and transmission electron microscopy showed that the 304SS-CNT sample has a microstructure significantly different from the 304SS sample, e.g. reduced grain size and many small cuboidal particles. Composition analysis using energy-dispersive spectroscopy revealed that the cuboidal particles are chromium carbides, and the chromium content is reduced in the 304SS-CNT matrix. Retained carbon nanotubes could not be observed; it is thought that the carbon nanotubes may decompose, induce the reduced grain size and chromium carbides.

Microstructural Characterization of Hydrogen Irradiated Austenitic Stainless Steel

Irradiation assisted stress corrosion cracking by neutron irradiation has been recognized as a significant concern for extended operation of commercial nuclear reactors [1, 2]. Recently, the retention of helium and hydrogen has been raised for a promising factor affecting the IASCC susceptibility of nuclear internals [3, 4]. In this work, we are attempting microstructural characterization of hydrogen irradiated austenitic stainless steels to get more information on effect of hydrogen on microstructural changes in austenitic stainless steel. A commercial austenitic stainless steel (SS316 type) was used for this work. An ion irradiation experiment was carried out with a multi-purpose ion implanter in the Korea Institute of Geoscience & Mineral Resources (KIGAM). In the ion irradiation, hydrogen ions (H2) were used with various energy ranges from 50keV to 490keV for the development of uniform radiation damage and implanted ion concentration in the experimental sample. Since the ion-irradiated layer was calculated to be about 1 μm in depth by “Stopping Range of Ions and Matter (SRIM)” [5]. TEM lamellae were prepared by FIB milling and low energy argon ion milling. A transmission electron microscope (TEM) equipped with an energy dispersive spectrometer (EDS) system was used for the analyzing of radiation induced defects and radiation induced segregation (RIS) behavior at grain boundaries.

Investigation of Cr-atom clustering in Fe‒Cr model alloys under irradiation

Irradiation-induced microstructure in Fe‒Cr model alloys, 0.5 MeV-He ion-irradiated at room temperature, was investigated by atom probe tomography (APT). The APT results showed the formation of Cr-atom clustering depending on the ion-penetration depth. Although the Cr-atom clustering was observed in the irradiation damaged zone, this effect was not dominant in the less-damaged zone. In addition, we performed computer simulations using the Metropolis–Monte Carlo (MMC) method for investigating the tendency to form Cr-atom clustering in binary Fe‒Cr alloys. The simulation results revealed the formation of Cr-atom clustering. The degree of Cr-atom clustering for the APT analysis and the MMC simulation was verified by plotting the Cr‒Cr radiation distribution function. It was found that the number of Cr atoms, located in the first and second nearest-neighboring sites, increased significantly. Both results support the formation of Cr-clustering, which is believed to be a source of radiation hardening. The application of two techniques, APT and the MMC simulation, provided complementary information on the radiation-induced microstructure.

Microstructural Analysis of Radiation Defect in Ion-Irradiated Austenitic Stainless Steel using TEM and 3D-APT

Austenitic stainless steel (ASS) used for internal components in a nuclear reactor is exposed to severe neutron irradiation at high doses (up to 100 dpa in a pressurized water reactor). Microstructural changes by neutron irradiation are known to lead to material degradation such as radiation hardening and irradiation assisted stress corrosion cracking (IASCC) during operation [1-3]. The formation of radiation defects and the development of solute segregation at the grain boundary have been considered as typical radiation induced phenomena to affect the material degradation in an ASS. Since radiation hardening is strongly correlated with the evolution of radiation defects, extensive studies have still been progressed to understand the characteristics of a radiation defect in irradiated ASS using various analytical techniques such as transmission electron microscopy (TEM) and 3D-atom probe tomography (3D-APT). In this work, we focused on the change in characteristics of radiation defects with irradiation temperature. We used ion irradiation to produce the simulated material, and performed a TEM examination and 3D-APT analysis to reveal the characteristics of the radiation defect in the ion-irradiated ASS.