Papers by Author: Seung Hoon Nahm

Abstract: To apply nano-structured materials in micro/nano system, understanding of the mechanical properties of nano-structured materials is required. In order to perform the mechanical test of nano-structured materials, the mechanical testing system was installed in a scanning electron microscope (SEM). The nano-manipulator was set up in the SEM, and the force sensor formed as a cantilever was mounted on the nano-manipulator. Then, the force sensor can be controlled by using the nano-manipulator. The nano-structured materials were dispersed on the transmission electron microscope (TEM) grid, and both end of the nano-structured materials were welded on the TEM grid and the tip of force sensor by exposing E-beam of the SEM. The tensile tests for carbon nanotubes, ZnO nanorods and ZnS nanowires were carried out in the SEM, respectively. The load response during the mechanical test was obtained by force sensor. The dimension of nano-structured materials was obtained by determining the configuration measured from the TEM. And, strain-stress curve was obtained after mechanical test. The elastic modulus of the nano-structured materials after the tensile tests were calculated and compared. The elastic modulus for multi-walled carbon nanotubes, ZnO nanorod and ZnS nanowire were ~0.98 TPa, ~59 and ~39 GPa, respectively.

Abstract: Al alloy is used extensively in several fields because specific strength is good and workability is superior. It is known that If Sc is added to Al alloy, strength is increased and re-crystallization temperature rises because microstructure becomes fine. The high cycle fatigue properties of four kinds of Al-Mg-Si alloys without and with only scandium of 0.20 % or with both scandium(Sc) of 0.20 % and zirconium(Zr) of 0.12% were investigated. The fatigue strength was determined at R = -1.0 under constant amplitude loading conditions in air. The alloy with scandium of 0.20 % showed a little higher fatigue strength values. The alloy with 0.20 % Sc and 0.12 % Zr showed highest tensile yield strength and highest fatigue strength. The fine grained Al 6061+0.20Sc+0.12Zr alloy exhibited a higher resistance against fatigue crack nucleation in comparison to the coarse grained Al 6061 alloy. The results can be explained mainly with the micro-structural differences among four alloys. This results are due to the presence of coherent Al3 (Sc, Zr) precipitates and a very fine sub-grain structure.

Abstract: The effects of the precipitate and martensite lath on the softening behavior have been investigated for 11Cr-3.5W-3Co steel during creep at 700
. During creep, the precipitate on the PAG (prior austenite grain) boundaries and martensite lath boundaries coarsened. The recovery of dislocation density and an increase of martensite lath width took place. It is shown that the inverse of the size of the precipitates and the inverse of the square root of the martensite lath width have a linear relation with the Vickers hardness, which corresponds to the Hall-Petch relation and particle looping mechanism.

Abstract: We have attempted to observe straining responses of an individual multi-walled carbon nanotube (MWNT) by performing an in-situ tensile testing inside scanning electron microscope (SEM). The both ends of an individual MWNT was attached on the rigid support and the tip of the force sensor using electron beam and was elongated by a nano-manipulator. The nano-manipulator was automatically controlled by personal computer. Linear deformation and fracture behaviors of MWNT were successfully observed and its force-displacement curve was also measured from the bending stiffness and displacement of the force sensor and manipulator. The tensile properties of individual MWNT were evaluated from the tensile test results.

Abstract: We tried to apply the nanoindentation technique to yield strength characterization by modifying a previous research. Although the yield strength determining technique developed by Kramer et al. has been successfully demonstrated for large scale indentations on bulky metals, its applicability is still doubtful to nanoscale indentations on thin films with severe roughness, anisotropy, and interfacial constraint. In order to overcome these problems, we combined the nanoindentation technique with a three-dimensional indent visualization technique in this study. Nanoindentation tests were performed for Au and TiN thin films and their corresponding indents were scanned by using an atomic force microscope. From the three-dimensional pile-up morphology, a circular pile-up boundary was measured and input into the yield strength formulation as an effective yielded zone radius. The yield strengths calculated were directly compared with those from the microtensile test.

Abstract: High strength materials have been used for critical components in order to ensure the reliability of aircraft gas turbine engine. But it usually costs high to maintain their premium quality. Therefore, the optimum quality level of materials should be defined under operating condition by material property estimation technique in order to meet the requirement of reliability and economical efficiency. Most cast rotating parts of gas turbine engine have casting defects caused during the casting process. The casting defects less than certain acceptable limit can be permissible for usage. So, it is very important to understand material defect shape and distribution, and its effect on mechanical properties exactly. In this study, 17-4PH stainless steel specimens with variable internal casting defects were prepared to investigate the fatigue property characteristics. The defect fraction and condition were estimated in microstructure by statistical analysis. The correlation between estimated defect condition and fatigue property was discussed.

Abstract: A nondestructive rod compression has been proposed as a new strength characterizing technique for in-service components. Using electrical discharge machining, milling machining, and stamping, it was possible to machine small rods, typically about 0.5 mm in both diameter and height on the target surface. Static compressions of the rods were carried out using a flat punch and their deformation behaviors were recorded as load-displacement curves. Referring to initial dimension of the rods, engineering stress-strain curves implying the yield strengths were calculated from the deformation curves. Surface yield strengths from the rod compressions were directly compared with reference compression results and their slight discrepancies were discussed from the influences of pre-deformation and damage of the rod and additive compliance of the base.

Abstract: In spite of frequent defect in industrial boilers, life assessment or diagnostic method for them has not been studied. In this research, SB410 carbon steel used in industrial boilers is simulated with artificial aging heat treatment. To do qualitative life assessment, differences in micro-structures and hardness of SB410 by the degradation time are studied. In addition, variation in material properties by aging was observed with the tensile test at room temperature and 179 °C and changes in ductile to brittle transition temperature was observed with the charpy impact test performed at several test temperature.

Abstract: The nondestructive evaluation technique for the material degradation is necessary because of the limitation of conventional destructive methods. In this study, an ultrasonic velocity measurement method was attempted for the estimation of the creep damage of degraded modified 9Cr-1Mo steel. The specimens with seven different kinds of aging periods were prepared by an isothermal heat treatment at 690
. The ultrasonic velocity was measured by an immersion method. The correlation between the measured ultrasonic velocity and tensile properties were studied. The ultrasonic velocity has an declining tendency with the increase of heat treatment time. A correlation between the ultrasonic velocity and aging parameter were established, which allows one to estimate the material degradation of modified 9Cr-1Mo steel.

Abstract: Many researchers have been interested in the nondestructive measurement methods for examining the microstructural changes and components damage in order to assure the safe operation of steel structure. It has been recognized that the techniques based on magnetic measurement offered a great potential because of high susceptibility to the change of several metallurgical factors. In this study, the effect of isothermal heat treatments, which simulate the microstructural changes observed in reactor vessel material at the service temperature, on the magnetic properties was investigated. 2.25Cr-1Mo steel specimens with several different kinds of aging were prepared by an isothermal heat treatment at three different temperature levels. Magnetic property such as coercive force was measured. The coercive force at room temperature monotonously increased with the extent of degradation of the material. The correlation between the measured magnetic property and the mechanical properties was studied. In addition, the applicability of magnetic properties measurements to the evaluation for toughness degradation of reactor vessel was discussed.