Papers by Author: Jeong Min Kim

Abstract: Small amount of Ag was added to high conductivity Cu-5%Cr alloy in order to increase the strength without sacrificing the conductivity. The typical microstructure of thermo-mechanically processed specimen mainly consists of Cu matrix and relatively large Cr phase. Although this typical microstructure was not changed with varied process variables, both the micro-hardness and conductivity were significantly affected by them. The electrical conductivity of alloy was slightly decreased by the Ag addition, but the micro-hardness could be enhanced by it. TEM analyses indicated that very fine Cr precipitates were formed in addition to the relatively large Cr phase in the Cu matrix. It is suggested that the properties of Cu-5Cr-xAg alloys can be optimized by carefully controlling the precipitation of the fine precipitates.

Abstract: Aluminum-Silicon based die-casting alloys have been extensively utilized in various industrial applications, but their relatively low electrical and thermal conductivities make them unsuitable as high conductivity parts. In this research, silicon content was restricted to a comparatively low level for higher conductivity and magnesium was added to enhance the castabilities. Al-1Fe-0.5Si-xMg alloys showed significantly higher electrical conductivity than conventional Al-Si based alloys. As the Mg content was increased, the mold filling ability measured using a fluidity serpentine test mold was a little decreased, however the hot cracking susceptibility was observed to be first increased and then decreased. The relationship between solidification characteristics and castabilities of Al-1Fe-0.5Si-xMg alloys was discussed based on the cooling curve analysis and microstructural characterization results.

Abstract: Mo-Si-B alloys have been received an attention due to the high temperature strength and phase stability. However, the nature of poor oxidation resistance often limits the application of the alloy system. In order to resolve the poor oxidation resistance of the alloy system, in this study, the oxidation behaviors of Si diffusion coated Mo-Si-B alloys have been investigated in order to identify the underlying mechanism for the effect of the constituent of the phase combination of Mo-Si-B alloys. The oxidation tests performed at 1100 °C show that the produced MoSi₂ phase, as a result of the coatings, give an excellent oxidation resistance at prolonged high temperature exposure in air. The oxidation behaviors of uncoated and Si coated Mo-Si-B alloys have been discussed in terms of microstructural observations during oxidation tests.

Abstract: The effects of Sn addition on the cast microstructure, mechanical properties, and corrosion behavior were comprehensively investigated. Cast AM60 alloy mainly consisted of primary Mg matrix and Mg17Al12 phase, and the microstructure virtually remained unchanged by the Sn additions up to 2%. Most of added Sn solute atoms were observed to dissolve into the Mg17Al12 phase and only a few fine MgAlSn precipitates were found in the high Sn alloys. Tensile strength at both room and 175oC was rather deteriorated by increasing Sn content, however the creep strength was significantly enhanced by it. Polarization characteristics of cast AM60 based alloys were not remarkably affected by the Sn content.

Abstract: Cast IN738LC alloy mainly consists of primary gamma matrix, gamma prime precipitates, and carbides. SEM-EDS analysis results suggested that most of the carbides are MC type ones that possess high Ti or Ta contents. MC carbides were partly dissolved into the matrix during the solid solution treatment, and the morphology and size of carbides were influenced by the solid solution treatment temperature. Characteristics of gamma prime precipitates were also significantly affected by the solid solution treatment conditions. Single or duplex size distributions of gamma prime precipitates were obtained depending on the solid solution treatment condition. Higher tensile strength was obtained in the case of finer precipitation size and in the case of single size distribution as compared with that of duplex size distribution.

Abstract: The nature of high specific strength compared with other structural materials has led to wide application of Mg alloys. However, Mg alloys often exhibit relatively low strength and/or low surface stability, which can limit the practical application of the alloy system. In order to achieve high strength, the commercial AZ61 alloys were extruded, so that the application of the alloy system can be extended towards new structural parts for requiring the light nature of the alloy system. In this study, a high temperature extrusion has been carried out for the commercial Mg alloys (AZ61). The productivity and mechanical properties of the alloy were critically affected by the extrusion conditions such as temperature, extrusion ratio and extrusion speed. The texture development and alloy strengths with respect to the extrusion conditions have been discussed in terms of microstructural observations and phase analyses.

Abstract: High strength high conductivity Cu-1%Cr-Mg-P alloy was selected as a base composition and Ag was added to the alloy in order to further increase the strength without sacrificing the conductivity. SEM and TEM analyses indicated that very fine MgP and Ag(Mg) precipitates were formed in addition to relatively large Cr phase in the Cu matrix. Significantly high strength could be obtained through the special cold rolling at an extremely low temperature using liquid nitrogen. The electrical conductivity of alloy was slightly decreased by the Ag addition, but the tensile strength could be further enhanced by it.

Abstract: Mo-Si-B alloys have been received an attention due to the high temperature strength and phase stability. However, the nature of poor oxidation resistance often limits the application of the alloy system. The unstable MoO₃ phase is naturally produced when the alloys were exposed at low and /or high temperature in an air atmosphere. In order to resolve the poor oxidation resistance of the alloy system, several attempts have been made via surface coatings and/or component additions. In this study, the oxidation behaviors of the Ti powder thermal spray coated Mo-Si-B alloys have been investigated in order to identify the underlying mechanism for the effect of precursor Ti coatings on Mo-Si-B alloys. The oxidation tests performed at 1100 °C show that the Ti powder was tightly bonded and reacted with the surface of the substrate, and TiO₂ layer was formed at the outer surface of the coated Ti layer as a result of oxidation exposure. The oxidation behaviors of pure elemental component coated Mo-Si-B alloys have been discussed in terms of microstructural observations during oxidation tests.

Abstract: It was observed that both primary and secondary gamma prime precipitates were more significantly influenced by the solid solution treatment temperature as compared to the following cooling rate. Single and duplex size precipitate distribution with different average size could be obtained through the appropriate solid solution and aging treatment. Micro-hardness test result indicated that single size distribution of gamma prime precipitates provides higher hardness than duplex size distribution.

Abstract: Mg alloys are one attractive material in applications of transportations and mobile electronics due to their high specific strength compared with other structural materials. However, Mg alloys often exhibit relatively low strength and/or low surface stability, which can limit the practical application of the alloy system. In this perspective, thermo-mechanical treatments of the alloy can give enhanced materials strength, so that the application of the alloy system can be extended towards new structural parts for requiring the light nature of the alloy system. In this study, a high temperature extrusion has been carried out for the commercial Mg alloys (AZ31). The extrusion temperature and extrusion ratio critically affected the alloy strength and ductility. The texture development and alloy strengths with respect to the extrusion conditions have been discussed in terms of microstructural observations and phase analyses.