Papers by Author: Ki Bae Kim

Abstract: We present a phase field model for coherent phase transition and its application to the self-assembled arrangement of second phase particles in coherent phase transition. The model developed here is free from the interface instability due to the contribution of chemical free energy and correctly describes elastically inhomogeneous system. It is observed that coherent misfit strain and anisotropy of elastic constants induce the interface instability that brings about the particle splitting and the self-assembled alignment of second phase particles. Their arrangement to the elastically soft direction occurs only in the system where the elastic contant of the precipitates is larger than that of the matrix phase.

Abstract: The production of high purity hydrogen gas is an important technical issue for future energy and environmental problem. The pure hydrogen is the ideal fuel in terms of fuel-cell performance. There have been many investigations on the hydrogen permeation of Pd-coated membrane of amorphous alloys. The purpose of this research is to characterize the surface such as roughness and morphology of surface layer of Ni-based amorphous alloy membrane by atomic force microscopy (AFM). The ribbon of Ni-Nb-Ta amorphous alloy was produced by a single-roller melt-spinning technique. The surface characteristic of the amorphous alloy membrane was discussed in view of the hydrogen permeation behavior. From the 3-dimensional AFM images, a lot of hole was observed on the surface of the amorphous alloy membrane produced in air atmosphere. But, a lot of protrusion were observed in the membrane produced in vacuum due to the selective growth on the high oxygen-affinity element of the amorphous alloy. The average roughness of the membrane produced in air atmosphere and vacuum was measured to be 0.359 and 0.263nm, respectively. Therefore, it was considered that it would be suitable to produce the amorphous alloy membrane in vacuum.

Abstract: In this paper, we present results of an investigation on the physical, mechanical, chemical properties and processing ability of the Cr2AlC ternary carbide bulk material synthesized by hot pressing technique. The combination of excellent properties indicated that Cr2AlC ternary carbide alloy could be potential candidate materials as bipolar plates in polymer membrane fuel cell (PEMFC).

Abstract: The hydrogenation characteristics and embrittlement behavior of Ti50Zr(50-x)Cux alloys (x=25,33,40) are reported. The hydrogenation kinetics decreased with increasing Zr-content. Though the Ti50Zr25Cu25 alloy showed the slowest kinetics, it absorbed large amount of hydrogen (~2.4 wt.%) and exhibited the best resistance against hydrogen embrittlement. The excellent characteristics of Ti- Zr-Cu alloys in hydrogen environment indicated that they are promising materials in future for energy applications.

Abstract: Ti50Zr25Cu25 in-situ composite ribbons consisting of metastable β-Ti crystalline phase in an amorphous matrix was studied for its structural stability and mechanical properties after hydrogenation. On annealing, dissolution of the metastable β-Ti phase occurred. On hydrogenation, upto ~60 at.% hydrogen was obtained and hydrogen-induced amorphization occurred. The fracture strength of the hydrogenated composite indicated that it was mechanically stable even for high hydrogen contents.

Abstract: Through more than three decades of development, a semi-solid metal processing has been successfully established as a unique casting technique to produce a structural component for an automobile industry with high integrity and improved mechanical properties. A slurry-on-demand process to make the semi-solid slurry having a fine and globular microstructure has been very important in the semi-solid metal process. In the present study, the orientation distribution functions (ODFs) calculated from the pole figure data were examined and tried to characterize the bulky morphology of primary solid phase of the semi-solid slurry of Al-Cu alloy produced with various magnetic flux density of 100 to 300Gauss in the specially designed electromagnetic (EM) stirrer. Columnar dendritic structure of primary α phase was turned into a rosette and globular structure by EM stirring during solidification. The primary α phase was refined and globularized with increasing a magnetic flux density of EM stirring. Also, due to the EM stirring the tendency to random orientation was appeared. In the case of unstirred Al-Cu alloy the <110>//ND texture was developed strongly and <100>//ND and <111>//ND texture was weakly developed. But with the increase of the EM stirring strength, <100>//ND and <111>//ND texture were more strongly advanced. Due to EM stirring the texture was almost completely randomized.

Abstract: Recently, a rheocasting process has been interested to produce a structural part for an automobile industry and so the slurry-on-demand process to make the semi-solid slurry having a fine and globular microstructure has been very important to produce a high quality and cost effective part in the rheocasting process. An electromagnetic (EM) stirrer employing for a slurry making process was designed and prepared to induce simultaneously both a circumferential and vertical fluid flow of a melt in order to control a rotation angle of EM stirring of a poured melt. In the present study, the semi-solid slurry of Al-15%Cu alloy was produced in the EM stirrer and its microstructure and the orientation distribution function calculated from the pole figure data was examined in accordance with a various rotation angle of EM stirring between 0˚ (circumferential flow) and 90˚ (vertical flow). The size and morphology of primary α phase was affected with a rotation angle of EM stirring and the finest and the most globular primary α phase could be obtained at a rotation angle of EM stirring of 45˚ and 60˚. Also, due to the EM stirring the tendency to random orientation was appeared. Also, the tendency of random orientation was the most at a rotation angle of 45˚ and 60˚. Therefore, it was considered that the rotation angle of EM stirring of 45˚ and 60˚ was the most effective to induce the non-dendritic growth of primary solid phase of EM stirred Al-Cu alloy.

Abstract: In this study, in order to develop the cast product of Al alloy having a globular microstructure by using the elctromagnetic (EM) stirrer, which was specially designed and manufactured to induce a various fluid flow type of melt during solidification, a morphlogy and size of primary solid phase of the solidifying slurry was investigated with respect to EM stirring condition such as an induced magnetic flux density (MFD) and a frequency of input current. The magnetic flux density of EM stirrer was measured by using a gaussmeter and its distribution and magnetic force within Al melt was simulated in ANSYS program. The induced MFD was increased with decreasing a frequency of input current at the same input voltage due to the increased penetrating depth of magnetic field. But, the magnetic force related directly with a stirring strength of melt was increased with the frequency. Both a roundness and size of primary α phase of Al alloy was decreased with increasing a frequency of input current and MFD within the experimental range. Therefore, the primary α phase was refined and globularized at the higher frequency and MFD.

Abstract: The mechanical properties of electromagnetically stirred billet of Al alloy during continuous casting such as a tensile, impact, and fatigue property was examined with respect to a size and roundness of primary α phase to examine the influence of the globularization and refining of primary α phase on the dynamic mechanical property. The billet was continuously cast in a casting speed of 100 to 600 mm/min during the electromagnetic stirring with a magnetic flux density of 700 Gauss and then was heat-treated. All tensile properties were enhanced with decreasing the size of primary α crystal and the ultimate tensile strength (UTS), yield strength, and elongation was obtained 320MPa, 235MPa, and 17.8%, respectively, at a size of primary α phase of 77 um. The fatigue life at fracture was largely improved by 37% from 1.25×105 to 1.7×105 cycles and the fracture toughness was obtained approximately 7.25 joules as a maximum value at a minimum size of primary α phase on the present experiment condition. Therefore, it indicates that all mechanical properties can be improved at the higher casting speed owing to the fine microstructure of a primary α phase and also eutectic phase due to the higher cooling rate.

Abstract: Semi-solid slurry for a rheocasting process requires a fine and globular structure of primary solid phase. In the present study, in order to find an optimum preheating temperature of a multiplex-type slurry cup in the slurry maker that we recently developed for the rheocasting process, the semi-solid slurry was held for a various time of up to 60 min. in a multiplex-type slurry cup preheated at a different temperature. A size and morphology of primary solid phase of the semisolid slurry was measured and analyzed with the difference of the recorded cooling curve during the solidification with respect to a preheating temperature of the slurry cup. Also, a microstructural evolution and globularization mechanism occurred during the isothermal holding were discussed. Finally, as considering the effect of a different casting condition on the morphology of primary solid phase the optimum manufacturing condition of the semi-solid slurry for the rheocasting process was proposed.