Papers by Author: Dang Moon Wee

Abstract: The catalytic activity of oxidation-reduction pre-treated Ni3Al powder for methane steam reforming was examined. The oxidation-reduction pre-treatment consisted of two steps: oxidation in air at various temperatures from 973 to 1373 K, and then followed by reduction in H2 at 873 K. It was found that the oxidation-reduction treatments significantly reduced the onset temperature of activity, i.e., improved the activity of Ni3Al powder at low temperatures. The characterization of Ni3Al surface showed that an outer surface layer of fine NiO particles were formed on the surface of Ni3Al after oxidation. These NiO particles were reduced to metallic Ni by the subsequent reduction treatment, resulting in the high activity for methane steam reforming. These results indicate that the Ni3Al can form highly active surface structure with oxidation-reduction treatment, having excellent heat resistance.

Abstract: Evolution of surface morphology in Ni(γ)/Ni3Al(γ´) two-phase foil of binary Ni-18 at.%Al was examined during the electrochemically selective etching in the electrolyte of distilled water including 1 wt.% (NH4)2SO4 and 1 wt.% citric acid. In the early stage (0.5 h), only the γ matrix was etched and the outmost γ´ particles were protected by a preexisting surface product. As the γ matrix was etched more, the side surfaces of the outmost γ´ particles and the γ´ particles that were located inside were exposed in the electrolyte. They were dissolved, and had a high density of fine dimples. However, the dissolution rate of the γ´ particles was slower than that of the γ matrix and thus the selective etching was retained in this stage. Finally, at 5h, more γ´ particles were exposed and the flat and smooth surfaces of the outmost γ´ particles were completely eliminated by the dissolution on the side surfaces. From these observations plus the saturation of the current density observed in the electrochemical test, we concluded that the change in the surface morphology was finished at this stage. Thus, the surface became more rough and irregular, which resulted from the original two-phase microstructure and the fine dimple structure by transpassivation.

Abstract: Ni3Al shows the unique feature of increasing strength with increasing temperature. However, it is too brittle to use as a structural material due to grain boundary weakness. Ductility could be enhanced by controlling grains using directional solidification. In order to increase the ductility or strength of Ni3Al alloys, a ductile γ (Ni-rich) phase of dendrite fibers or a strong β (NiAl) phase of dendrite fibers were arrayed in the γ´ (Ni3Al) matrix by directional solidification. The dendrite spacing could be controlled by varying the solidification rates, and the volume fraction of the γ or β phase could be changed by using alloy compositions, from 23 to 27 at. % Al-Ni alloy. With increasing solidification rates, the dendrite spacing decreased, which caused the tensile strength to be enhanced and the elongation to decrease, evidently due to the phase boundary augmentation. With increasing Al content, the γ dendritic microstructure changed to β dendrites in the γ´ matrix, which resulted in a decrease in elongation as a result of an increase in the volume fraction of the brittle β dendrites in the γ´ matrix.

Abstract: Directional or single crystal technique was applied to enhance the ductility, and two phases of γ (Ni) phase or β (NiAl) phase in γ‘(Ni3Al) matrix were also considered to increase the strength and ductility. In this study, directionally solidified rods were prepared at the solidification rate of 50µm/s in 23-27 at.% Al-Ni alloys, and tensile strengths of these rods were analyzed at room temperature. Directionally solidified samples showed the γ dendrite fibers formed in the Ni3Al matrix in the hypo eutectic composition of 23 at.% Al, the γ‘ single phase in the eutectic composition of 24.5 at. % Al, and the β dendrite fibers in the γ‘ matrix in the hyper eutectic compositions of 25, 26, 27 at.% Al. The hypoeutectic alloy including γ dendrites with γ‘ matrix exhibited a large elongation of over 70% with ductile transgranular fracture at room temperature. With increasing Al contents, the γ dendritic microstructure changed to the β dendrite in the γ‘ matrix, which resulted in decreasing the elongation by increasing the volume fraction of the brittle β dendrites in the ductile γ’ matrix.

Abstract: Anisotropic deformation behavior of single crystalline Ni3Al-base intermetallic compounds, including Ni3Al single-phase and Ni/Ni3Al two-phase alloys, was systematically studied by the plane strain compression tests. Plastic flow behavior of single phase Ni3Al is strongly dependent on the initial crystal orientation and the flow stress becomes higher with increasing the numbers of the operative slip planes. In the case of the Ni/Ni3Al two phase alloys, the flow behavior is found to be divided into two stages. Such flow behavior is considered to be closely related to the difference in the deformation behavior between Ni solid solution and Ni3Al precipitates.

Abstract: Ni3Al has been considerable research area due to its high temperature behavior increasing strength with increasing temperature. A series of directional solidification studies showed that the eutectic occurred between g’/b and the metastable eutectic of g/b forms under slightly different conditions, however, it is not well established whether the eutectic is composed of g/g‘, g’/b, or g/b . In order to understand solidification behavior of the eutectic structure, directional solidification experiments have been carried out with solidification rate near the Ni3Al composition in this study. The effects of the solidification rate and composition on formation of the equilibrium and metastable eutectics have been discussed. The (g’+g) coupled phase was also shown to form with the eutectic at the solid/liquid interface.