Advanced Materials Research Vols. 26-28

Abstract: Phase transformation and crystal growth behavior of Ge2Sb2Te5 were investigated systematically by means of in situ heating (from room temperature to 500 oC) of amorphous Ge2Sb2Te5 alloy in a high voltage electron microscope with real-time monitoring. Large-scale crystallization occurred to amorphous Ge2Sb2Te5 around 200 oC. Large crystal growth developed on heating from 200 oC to 400 oC, and single crystalline grains grew up to 150 nm. Eventually the onset of partial melting of thin Ge2Sb2Te5 foil was at 500 oC and liquid Ge2Sb2Te5 was observed for the first time by high-resolution transmission electron microscopy. Hexagonal Ge2Sb2Te5 phase remains after a subsequent cooling.

Abstract: The optimized terrace formation of SrTiO3 (111) substrates was investigated in various etching conditions. HCl-HNO3 solution was unstable for etching the SrTiO3 (111) substrates with different surface states. It was found that etching in buffered HF (BOE) solution for 2min provides a stable etching condition for SrTiO3 (111) substrates with various surface states and etching process is an important factor for the formation of step-terrace structures. The surface degradation of SrTiO3 (111) in normal atmosphere was also observed and it is considered that the humidity is a important factor for the surface degradation.

Abstract: The microstructure and strain characteristics of self-assembled InAs/GaAs quantum dots (QDs) were studied by using transmission electron microscopy. Compressive strain was induced to uncapped QDs from GaAs substrate and the misfit strain largely increased after the deposition of GaAs cap layer. Tensile strain outside QD was extended along the vertical growth direction; up to 15 nm above the wetting layer. Vertically nonaligned and aligned stacked QDs were grown by adjusting the thickness of GaAs spacer layers. The QDs with a lens-shaped morphology were formed in the early stage of growth, and their apex was flattened by the out-diffusion of In atoms upon GaAs capping. However, aligned QDs maintained their lens-shaped structure with round apex after capping. It is believed that their apex did not flatten because the chemical potential gradient of In was relatively low due to the adjacent InAs QD layers.

Abstract: Steam generators(SG) of the Korean nuclear power plants were replaced with a new one after a 20 year operation. A metallographic examination of the defected tubes is needed for setting up a guide line for a safe operation of the other SG. A destructive analysis of 50 tubes was addressed, and a relationship between the SCC defect location and defect depth was obtained. Tubes extracted from the retired SG were transferred to a hot laboratory. Detailed nondestructive analysis examinations were taken, then the tubes were destructively examined. Types and sizes of the defects were characterized. A location and depth of the SCC were evaluated in terms of the location and height of the sludge. Most axial cracks were in the sludge pile, whereas the circumferential ones were around the top of the tube sheet(TTS) or below the TTS. Cracks were found with various lengths in a specific zone. Average defect depth of the axial cracks was deeper than that of the circumferential ones.

Abstract: Rubber material properties and useful life evaluation are very important in design procedure to assure the safety and reliability of the rubber components. In this paper, the evaluation of characteristics and useful life prediction of rubber component for elevator cabin were experimentally investigated. The material test and accelerated heat-aging test were carried. Rubber material constants were obtained by curve fittings of simple tension, pure shear and bi-axial tension test data. Heat aging test results changes as the threshold are used for assessment of the useful life and time to threshold value were plotted against reciprocal of absolute temperature to give the Arrhenius plot. By using the rubber material and component test several useful life prediction equations for rubber component were proposed. Predicted useful life of rubber component for elevator cabin agreed fairly with the experimental lives.

Abstract: Bending and impact tests were conducted on magnesium alloys AZ31 and AZ80 in different microstructure conditions. It is found that the strength parameters are increased quite a lot, and the ductility parameters (bending flection) decreased with increase in strain rate. The damaged surface analysis in SEM revealed that, the micro crack nucleation modes, micro crack coalescence and final crack path are varied with the stress state on the specimen, and the crack nucleation modes are varied with the initial strain rate. The effect of strain rate on fracture modes is discussed.

Abstract: The fatigue life in arc welded parts of the chassis component of SAPH45 steel in automobile is affected by various defects and residual stress due to welding. Moreover, it is hard to measure directly the crack initiation length of the welded part in chassis component due to the complicated geometric shape. Therefore, to assure the safety of vehicles, it is necessary to evaluate the crack initiation life by using a non-destructive technique (NDT). In this study, the acoustic emission measurement was adapted to monitor the crack initiation of the welded part during the fatigue test. The crack initiation length was verified by a computerized image processing system under stroboscopic light illumination. Besides, to consider the variation of fatigue data, the crack initiation life was statistically evaluated.

Abstract: In order to evaluate the effects of a variation of a supporting springs' shape on the wear behavior of a nuclear fuel rod, sliding wear tests have been performed in room temperature air and water. The objective of the tests is to quantitatively evaluate the relationship between a worn area and a wear volume, and the formation behavior of a worn area with a variation of the slip amplitudes, applied normal loads and supporting spring shapes. The results indicated that the variation behavior of the volume and the wear scar size was influenced by the contact shape between the springs and the fuel rods. Also, it was found to be possible to evaluate a critical ratio (Tc) for each spring shape and test condition when the T was defined as the ratio of an applied normal load (Ln) to a wear scar size (At). Below this Tc, the wear volume was rapidly increased and the Tc was determined by a variation of the At under the same applied normal load condition. This result enables us to evaluate a wear resistant spring shape by using an analysis of a wear scar after wear tests have been completed. Based on the above results, the relationship between At and a worn area (Aw), a wear mechanism and an evaluation method for a wear resistance were discussed.

Abstract: Structural reliability of electrical panels installed in naval vessels is of critical importance from the structural performance viewpoint. The electrical panels may be exposed to vibration and fatigue loadings from internal and external sources as well as wave loadings that result into fatigue crack and fracture due to the decrease of fatigue strength. In this regard, electrical outfitting equipments on naval vessels are required to comply with capabilities against vibration and shock tests, normally tested according to Military Standard. This study introduces the effect of post welding treatment, such as toe grinding as well as post welding heat treatment (PWHT), are examined through a series of fatigue and impact tests according to military standards. On the other hand, the use of elastic mounts that are capable of isolating and mitigating shock and vibration toward electrical panels are investigated. Both experimental and numerical results are presented based on the analysis according to military standards.

Abstract: The development of the instrumented indentation test (IIT), which gives accurate measurements of the continuous variation in indentation load as a function of depth, has paved the way to assessing tensile properties and residual stress in addition to hardness by analyzing the indentation load-depth curve. In this study, analytic models and procedures are presented for evaluating tensile flow properties and residual stress states using IIT. Tensile properties were obtained by defining representative stress and strain beneath the spherical indenter. The evaluation of residual stress is based on the concepts that the deviatoric stress part of the residual stress affects the indentation load-depth curve, and that analyzing the difference between the residual stressinduced indentation curve and the residual stress-free curve permits evaluation of the quantitative residual stress in a target region.