Papers by Author: Sang Bong Yi

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Authors: Karl Ulrich Kainer, Joachim Wendt, Kerstin Hantzsche, Jan Bohlen, Sang Bong Yi, Dietmar Letzig
Abstract: Commercial magnesium alloys such as AZ31 exhibit strong crystallographic textures during massive deformation such as rolling. A randomisation of the texture, however, was found in alloys with rare earth (RE) elements in solid solution. This paper describes the development of microstructure and texture during rolling of the Al-free RE-containing wrought magnesium alloy ZEK100 during hot rolling. This alloy develops a strong texture with a pronounced component towards the transverse direction (TD) of the sheets. This TD component forms already after the first rolling pass, persists through all following passes and is further enhanced by subsequent heat treatment. These results are contrasted with results from a study on texture development of binary RE containing Mg-alloys, which show that the presence of RE elements alone is not responsible for the tilt of basal planes towards the TD. There is, however, a threshold concentration at which the texture begins to weaken.
Authors: Sang Bong Yi, Dietmar Letzig, Kerstin Hantzsche, Rodolfo Gonzalez Martinez, Jan Bohlen, Igor Schestakow, Stefan Zaefferer
Abstract: The influences of rare earth elements addition on the crystallographic texture and microstructural evolutions are examined during rolling and annealing of Mg-sheets. In case of Nd or Y additions, dynamic recrystallisation is suppressed such that the deformed microstructure is observed after hot rolling with relatively large strain per pass. Cold rolled binary Mg-Nd alloy sheet shows strong texture with splitting of the basal poles in the rolling direction, however, the texture intensity decreases significantly during the recrystallisation annealing. From the comparison of deep drawing behaviours between commercial ZE10 and AZ31 sheets, it is observed that the addition of the rare earth elements and accompanying texture changes result in the improved formability.
Authors: Sang Bong Yi, Heinz Günter Brokmeier, Jens Homeyer
Abstract: The orientation changes during heating of extruded AZ31 were investigated using synchrotron Xrays by in-situ experiments. The as-extruded sample shows a strong <10.0>-fiber texture which is generally observed after round extrusion. The initial <10.0>-fiber texture, in which the <10.0> direction of the hexagonal crystallites is parallel to extrusion direction, starts to change to <11.0>- fiber component at 300 °C. The orientation change rate shows an exponential relation to the heating temperature and the soaking time. The microstructure observations, prior and post heat treatment, indicate that the texture changes occur mainly during grain growth.
Authors: Heinz Günter Brokmeier, Sang Bong Yi, No Jin Park, Jens Homeyer
Abstract: Due to the high penetration power, the high photon flux and the excellent brilliance high energetic synchrotron radiations are a phantastic tool particular in fast experimentation. Therefore in-situ measurements under applied load or at various temperatures have been carried out to investigate the texture influence on the deformation of magnesium and aluminium. An upgraded loading device with a power up to 20 kN was installed at the hard wiggler beamline BW5. Magnesium samples with different initial textures show the strong influence on this texture on the activation of different glide systems and on the activation of twins. First experiments to study the phase transition in titanium were performed, which demonstrate the strong orientation correlation between the α- and the β-phase.
Authors: Muhammad Shahzad, Dan Eliezer, Wei Min Gan, Sang Bong Yi, Lothar Wagner
Abstract: The wrought magnesium alloys AZ80 and ZK60 were extruded at 175°C ≤ T ≤ 350°C at an extrusion ratio of ER = 12. With decreasing extrusion temperatures a marked refinement in grain size was found for both alloys resulting in higher values of yield stress while UTS values were hardly affected. As opposed to AZ80, a marked yield stress differential between loading in tension and compression was observed in ZK60, this effect being explained by the differences in crystallographic texture.
Authors: Sang Bong Yi, Tomasz Ludian, Lothar Wagner
Abstract: The deformation degrees in near-surface zones and related microstructural changes in CuZn30 were determined after shot peening and ball-burnishing. The induced depths of plastic deformation increase with Almen intensity and rolling pressure in shot peening and ball-burnishing, respectively. As the ball size in burnishing increases, the depth of plastic deformation also increases. The local degrees of cold work within the affected depths were estimated by comparing micro-hardness values, penetration depths of induced slip bands as well as X-ray diffraction peak widths with those measured in blanks cold rolled to fixed degrees of plastic strain. The results clearly indicate that very high local deformation degrees above 80% are typically induced by mechanical surface treatments in areas very close to the surface with this deformation degree gradually decreasing with increasing distance to the surface. Depending on the intensity of ball-burnishing, deformation degrees still of the order of 10% were observed in regions having distances to the surface of as large as 1000 (m.
Authors: Dogan Arpacay, Sang Bong Yi, Miloš Janeček, Adem Bakkaloglu, Lothar Wagner
Abstract: The microstructure evolution during high pressure torsion and its influence on the mechanical properties of AZ80 magnesium alloy is presented in this study. Significant grain refinement was observed after high pressure torsion, while the homogeneity of the grain structure increases with the number of revolutions. Grain size decreases to about 50 nm after 15 revolutions. The microhardness profiles measured at through-thickness and through-width directions show no significant variation at different positions of the sample. Moreover, the negligible effect of the revolution number on the microhardness value was observed.
Authors: Dierk Raabe, A. Al-Sawalmih, P. Romano, C. Sachs, Heinz Günter Brokmeier, Sang Bong Yi, G. Servos, H.G. Hartwig
Abstract: In this study we present experimental investigations on the microscopic structure, constituent phases, and crystallographic textures of the exoskeleton of three types of decapod crustaceans, namely, lobster, crab, and horseshoe crab. The carapace of such animals is a biological multiphase nano-composite consisting of an organic matrix (crystalline chitin and non-crystalline proteins) and biominerals (calcite, phosphate). The synchrotron measurements of the crystalline chitin and of the biominerals which are embedded in the chitin-protein matrix (in case of lobster and crab) reveal strong textures. The horseshoe crab does not seem to contain notable amounts of crystalline minerals. The Debye-Scherrer images of the lobster specimen suggest that the biominerals form clusters of crystals with similar crystallographic orientation. TEM images support this suggestion. The crystallographic texture of the chitin is arranged with its longest cell axis parallel to the normal of the surface of the exoskeleton.
Authors: Chris H.J. Davies, Sang Bong Yi, Jan Bohlen, Karl Ulrich Kainer, Heinz Günter Brokmeier
Abstract: The crystallographic response to deformation – texture evolution, internal elastic strain, and twin evolution – was measured for three load/orientation variants for AZ31 magnesium alloy extrudate tested in-situ in a synchrotron beamline. Specimens were loaded in tension parallel to the extrudate transverse direction, in compression along the same axis, and in compression parallel to the extrudate normal direction. The crystallographic responses are correlated with the mechanical behaviour of the extrudate.
Authors: Heinz Günter Brokmeier, Brigitte Weiss, Sang Bong Yi, Wenhai Ye Yi, Klaus Dieter Liss, Thomas Lippmann
Abstract: A new method to investigate thin wires has been tested, which is based on a special sample holder and on a high energy X-rays. Due to the high penetration power of high energy Xrays quantitative texture data will be obtained without any additional corrections such as constant volume correction and absorption correction. The measurements have been carried out at the high energy beam line BW5 at HASYLAB – DESY (Hamburg). In order to overcome grain statistics problems on the investigated Cu-wire of 122µm thickness a special scanning routine together with the sample preparation allows to average over a wire length between 1mm and up to 240 mm.
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