Byong-Whi Lee

Anatase-to-rutile transition of titania thin films prepared by MOCVD

Phase transitions of TiO{sub 2} thin films prepared by MOCVD were investigated using X-ray diffraction, scanning electron microscope, transmission electron microscope, and secondary ion mass spectroscope. It was found that the in-situ deposited thin films showed the onset of anatase-to-rutile transition at a much lower temperature of 400 C. In order to reveal the mechanism responsible for the low temperature transition, the transition of titanium iso-propoxide derived and titanium tetra-chloride derived powders were investigated. The onset temperatures of transition were 450 C and 700 C, respectively. It is therefore suggested that source effect is the main mechanism of low temperature transition. It is also suggested that the heterogeneous nucleation on the surface of substrate is responsible for the transition shown by in-situ deposited and post-annealed titania thin films being much faster than that shown by titanium powders.

Plastic zone size in fatigue cracking

Fatigue tests were performed on the specimens of Type 304 stainless steel and Inconel 718. To investigate the effects of specimen thickness on crack tip deformation and fatigue crack growth rate (FCGR), specimens of different thickness were used. To validate fatigue crack propagation in terms of plastic zone size, elastic plastic fracture mechanics (EPFM) are studied in this investigation. Results show that FCGR is a function of specimen thickness, which worsens as the specimen thickness increases. It is considered that plastic zone size is an important fatigue crack propagation parameter in conjunction with applied stress level, specimen thickness and crack closure.

Effect of heat treatment on fatigue crack growth rate of Inconel 690 and Inconel 600

Abstract The effects of heat treatment on fatigue crack growth rates (FCGRs) of Inconel 690 and Inconel 600 have been investigated in terms of carbide morphology and grain size. Cycling tests in air at room temperature have shown that FCGR in low stress intensity factor range (Δ K) region can be effectively reduced by increasing the grain boundary carbide precipitate size and grain size. Decrease in FCGR is attributed to the crack tip blunting at the precipitates of grain boundary chromium carbides. Compared with test data of as-received Inconel 600, difference in FCGRs between Inconel 690 and Inconel 600 has been also discussed in detail.

Effect of specimen thickness on fatigue crack growth rate

Fatigue tests were performed on the compact tension (CT) specimens of Type 304 stainless steel and Inconel 718. To investigate the effects of specimen thickness on crack tip deformation and fatigue crack growth rate (FCGR), specimens of different thickness were used. In the analysis, the elastic plastic fracture mechanics (EPFM) parameter known as the cyclic J-integral, ΔJ was adopted to observe the local plasticity at the crack tip and compared with the linear elastic fracture mechanics (LEFM) parameter known as the stress intensity factor range, ΔK. The results show that FCGR is a function of specimen thickness, the effect of which is accelerated as specimen thickness increases. Therefore, it is thought that not only applied stress level but also specimen thickness should be taken into account in the measurement of FCGR, which is not considered in ASTM E 647 (ASTM E 647, 1995. Standard test method for measurement of fatigue crack growth rates.).

Nondestructive evaluation of isothermally annealed 12% CrMoV steel by magnetic BN measurement

Abstract The effects of microstructural evolution during isothermal annealing on structure-sensitive magnetic properties and Barkhausen noise (BN) characteristics have been investigated for 12% CrMoV steel. A rapid decrease in coercive force, remanence, hysteresis loss and hardness took place in accordance with the release of internal stresses from supersaturated martensite after annealing for only 20 minutes at 923 K. BN energy is correspondingly related with three stages of microstructural evolution such as the recovery of strain energy, the increase of precipitates size as a result of Ostwald ripening and the annihilation of dislocation density during isothermal annealing of the specimen. The linear relation between hardness and BN parameters in the magnetization region of irreversible domain wall displacement of 2.4 kA/m suggests that hardness and microstructural evolution could well be evaluated nondestructively by using BN measurements.

Low temperature synthesis of PbTiO3 thin films by MOCVD without carrier gas

Abstract A new MOCVD (metal–organic chemical vapor deposition) technique employing the precursor delivery without carrier gas was developed in the deposition of PbTiO 3 thin films, which enables low temperature (450°C) synthesis and high grow rates (∼6.0 μ m/h). Substrates used were bare Si, TiO 2 buffered Si, and platinized Si. Tetraethyl lead, titanium tetra-isopropoxide, and oxygen were used as precursors. Perovskite PbTiO 3 thin films were successfully grown on all kinds of substrates investigated at the substrate temperature of 450°C. However, the C–V characteristics of perovskite thin films prepared at low temperature did not always show normal ferroelectric behavior. Without carrier gas, impinging of non-adsorbing species with substrates would be suppressed as much as possible. The difference in growth behaviors with or without carrier gas was compared with each other.

Fuzzy power control algorithm for a pressurized water reactor

AbstractA fuzzy power control algorithm is presented for automatic reactor power control in a pressurized water reactor (PWR). Automatic power shape control is complicated by the use of control rod...

Correlation between the domain structure and ferroelectricity in PbTiO3 thin films

Two types of PbTiO3 thin films were prepared on Pt/SiO2/Si substrates by metal–organic chemical vapor deposition (MOCVD). One type was in situ deposited at 450°C, while the other type was post-annealed at 650°C which is above the Curie temperature (~490°C) after deposition at 450°C. Both types of films possessed the perovskite structure, but only the film post-annealed at 650°C displayed ferroelectricity evidenced by the capacitance–voltage (C–V) characteristics, the displacement–electric field (D–E) hysteresis, and the domain wall structure.

Effects of crack tip plasticity on fatigue crack propagation

Abstract A simple model for fatigue crack propagation has been proposed based on the modified Dugdale model of crack tip plasticity and energy balance approach to stable crack propagation. To verify the proposed model, fatigue tests were performed on the specimens of Type 304 stainless steel and Incone1718. To measure the effect of crack tip bluntness on the fatigue crack propagation, specimens of different thickness were used. Results show that the theoretical prediction of fatigue crack propagation agreed well with the experimental test results. It is suggested that the prediction of fatigue crack propagation should take account of the different plasticity related to the variation of specimen thickness, stress state and materials tearing modulus in the crack tip region.

Micromagnetic nondestructive evaluation of isochronally tempered 12% CrMoV steel

The martensitic stainless steels, based on the nominal 12% Cr alloy composition, have been used at temperatures up to about 800 K, because of their non-oxidizing and superior high temperature mechanical properties. Literature established the microstructural evolution of 12% Cr steel during heat treatment and related it to the mechanisms of material degradation due to aging in high temperature environment. The purpose of the present investigation is to evaluate mechanical properties, like hardness and microstructural evolution due to thermal aging, by measuring structure-sensitive magnetic properties and BN.