Eun Joo Shin

Quantitative study of multi-phases in TRIP-aided steels by means of neutron diffraction: a review

The retained austenite and solute carbon in C-Si-Mn TRIP steels were quantitatively analyzed by neutron diffractometry, and their effects on the mechanical properties of the steels were studied. The decreasing of austempering temperature increased the volume fraction of retained austenite as well as the solute carbon in an austenite phase. However, the samples containing lower silicon showed a large variation in the amount of retained austenite and solute carbon in austenite with austempering temperature. The amount of retained austenite was proportional to the solute carbon in austenite. The elongation in TRIP steel was affected by both the volume fraction of retained austenite and the solute carbon in austenite.

Size and Strain Analysis of Deformation Microstructure in High-Nitrogen Austenitic Stainless Steels

Deformation microstructure of Fe-18Cr-10Mn alloys with different nitrogen content (0.39 and 0.69N) were investigated using neutron diffraction. The deformation mode changed from strain-induced martensitic transformation (SIMT) to deformation twinning (DT), as nitrogen content was increased. The measured neutron profiles of tensile-strained bulk samples were fitted by modified Thomson-Cox-Hasting profile-shape function and the Lorentzian and Gaussian integral breadths were calculated based on Rietveld refinement. With the calculated integral breadths, the mean-squared (MS) strain could be evaluated using the size and strain analysis of double-Voigt approach. Based on the analysis of peak shift of individual reflections, the stacking fault probability (SFP) was determined. Although the calculated SFP and MS strain varied with the degree of deformation, the ratio of MS strain to SFP remained constant for each alloys regardless of tensile strain, but increased with nitrogen content. As a result, it was found that the SFE increased with increasing nitrogen content. The nitrogen-induced transition of deformation microstructure was accounted for by the change in SFE, as reported in other fcc materials.

Nondestructive SANS Study of Reactor Pressure Vessel Steel after Irradiation

The irradiation induced defects of irradiated reactor pressure vessel(RPV) steel were investigated by a small angle neutron scattering. The degradation of the mechanical properties of RPV steels during an irradiation in a nuclear power plant is closely related to the irradiation induced defects. The size of these defects is known to be a few nanometers, and the small angle neutron scattering technique is regarded as the best non destructive technique to characterize the nano sized inhomogeneities in bulk samples. The RPV steel was irradiated in the HANARO reactor in KAERI. The small angle neutron scattering experiments were performed at SANS instrument in the HANARO reactor. Both unirradiated and irradiated RPV steels were measured and the SANS data of both steels were compared. The nano sized irradiation induced defects were quantitatively analyzed by SANS. The type of defects was also analyzed based on the SANS results, and the effect of the chemical composition of the RPV steel on the irradiation induced defects was discussed.

SANS Study of Precipitate and Recrystallization in Ti-Added Extra Low Carbon Steels

Effects of precipitation on the recrystallization behavior of three Ti-added extra low carbon sheet steels were compared. The recrystallization temperature of the high Ti-containing steel is higher than that of the low Ti-containing steel. Small angle neutron scattering (SANS) measurements were performed both for cold rolled samples and for annealed samples. The size distribution and the volume fraction of the nano-sized precipitates were quantitatively determined from the corrected scattering curves using a direct model fitting with a sphere. SANS results revealed no additional carbo-nitrides precipitation during the recrystallization annealing procedure in the low Ti-containing steel. However, in the high Ti-containing steel, new TiC precipitates smaller than 10 nm formed during the recrystallization annealing process. This dynamic interaction of the precipitation of the fine TiC particles with the recovery of dislocations seems primarily responsible for the retardation of the recrystallization in the high Ti-containing extra low carbon steel.

Quantitative Phase Analysis of the Multi-Phase Alloys Using Neutron Diffraction

Alloys including steels usually consist of more than two phases, and the volume fraction of each phase in the materials is closely related to their physical properties. Therefore, accurate data of the volume fraction of each phase in the materials is very useful to control the mechanical properties of the materials. Based on the Rietveld profile refinement of the neutron diffraction patterns and a new texture analysis, a quantitative phase analysis (QPA) method is proposed and examined in artificially produced multi-phase alloys having strong textures. The preferred orientation factors (POFs) of the diffraction peaks was extracted from the inverse pole figure calculated from the orientation distribution function (ODF) that was computed from the averaged pole figures measured by the rotating sample stage providing a faster and simpler texture measurement. The reliability of this method was examined in binary alloy mixtures of known fractions for the zirconium and aluminum. In addition, we applied the QPA method to the evaluation of the amount of retained austenite in a commercial steel strip.