Papers by Author: Hahn Choo

Abstract: After an overload was imposed during a constant amplitude fatigue experiment, a retardation period was observed. The deformation in the vicinity of a crack tip was studied using neutron and x-ray microbeam-diffraction techniques, which provide millimeter and submicrometer spatial resolutions, respectively. From the neutron-diffraction measurements, compressive lattice strains and higher dislocation densities at the macroscale were observed in front of the crack tip, which indicates a plasticity induced crack-closure phenomenon. Furthermore, Laue patterns obtained from the microbeam diffraction at different locations near the crack tip show alternating regions with high and low dislocation densities at the mesoscale.

Abstract: The creep-fatigue crack-growth behavior of HAYNES® 188, a cobalt-based superalloy, was studied at the temperatures of 649, 816, and 927 oC under isothermal conditions. Various hold times at the maximum load were introduced to study the effects of hold time and temperature on the crack-growth behavior. The experiments were conducted under constant stress-intensity-factorrange control modes. Crack lengths were measured by a direct current potential method. The introduction of hold times led to an increase in the cyclic crack-growth rate. As the temperature increases, the time-dependent crack-growth behavior was dominant.

Abstract: Residual strain profiles in friction-stir processed (FSP) AZ31B magnesium-alloy plates were measured using neutron diffraction. Two different specimens were prepared to investigate the influences of the tool shoulder and the tool pin on the residual-strain profiles: (Case 1) a plate processed with both the stirring pin and tool shoulder, i.e., a regular FSP plate subjected to both the plastic deformation and frictional heating, and (Case 2) a plate processed only with the tool shoulder, i.e., subjected mainly to the frictional heating. The results show that the strain profiles of both cases are qualitatively quite similar. The longitudinal strain is mainly tensile with its maximum near the bead of the FSP plate. On the other hand, the transverse and normal strains are mildly compressive in both Cases 1 and 2.

Abstract: The compressive behavior was investigated on an electrodeposited nanocrystalline Ni-20%Fe alloy with a grain size of about 22 nm at room temperature (RT), 298 K, and the liquid nitrogen temperature (LN2T), 77 K. The sensitivity of the yield strength and plastic strain to the test temperature at different grain sizes was discussed. Moreover, through the Transmission Electron Microscopy (TEM) examination and microhardness measurement, the microstructures before and after the compression test at RT and LN2T were studied.

Abstract: Neutron incoherent scattering measurements were conducted on Zircaloy-4 round bars. The specimens were charged in a tube furnace at 430 °C, using a 12.5 vol. % hydrogen in an argon mixture for 30, 60, and 90 minutes at 13.8 kPa pressure. The volume-average neutron diffraction measurements showed the presence of the face-centered-cubic delta zirconium hydride (δ-ZrH2) phase in the hydrogenated specimens. The assessment of the background in the diffraction profiles due to the incoherent scattering from the hydrogen atoms was carried out by performing inelastic scans around zero energy transfer and at a fixed two-theta value for which there was only flat background and no coherent scattering. To estimate the relative amount of hydrogen in the Zircaloy-4 samples, the increase in incoherent scattering intensities with hydrogen content was calibrated using samples for which the hydrogen content was known. Measurement of the background scattering from locations within the round bar was also performed to map the distribution of hydrogen content.

Abstract: ULTIMET® alloy, a cobalt-based superalloy with good corrosion and wear resistant properties, exhibits a deformation-induced phase transformation from the face-centered-cubic (FCC) phase to the hexagonal-close-packed (HCP) phase. The HCP phase formation during monotonic tensile loading was investigated using in-situ neutron diffraction. The HCP phase is first observed at a stress level of 810 MPa, which is well beyond macroscopic yielding. Strain analysis is performed on the FCC phase diffraction data in order to relate the lattice-strain development with the evolution of the new HCP phase. A method of calculating the effective macroscopic stress associated with the measured lattice strains is presented here. The effective stress can then be compared to the applied macroscopic stress in order to draw conclusions about the load-partitioning behavior of the material as a new phase develops.

Abstract: The martensitic phase transformation in an ultra fine grained (UFG) TRIP (transformation induced plasticity) steel with combination of high strength and high elongation was investigated during room temperature tensile test using in situ neutron diffraction. Two types of specimens, namely coarse grained (grain size of about 50 μm) and ultra-fine-grained (grain size of about 350 nm) specimens were examined. The lattice strain evolution of the austenite and martensite phases was observed and the load partitioning between the phases was identified.

Abstract: Localized heating was imposed on a 6061-T6 aluminum alloy plate to cause thermal strains, and simultaneously interplanar spacing changes were measured using in-situ time-resolved neutron diffraction techniques. Two different methods were used: 1) direct real-time measurements of the transient behavior and 2) a series of measurements based on the quasi-steady state (QSS) principle. A comparison of the two results shows that the QSS method can represent the transient behavior under the current experimental conditions.