Papers by Author: Ik Min Park

Abstract: Carbon Nano Tube (CNT) reinforced AZ91 metal matrix composites (MMC) were fabricated by the squeeze infiltrated method. Properties of magnesium alloys have been improved by impurity reduction, surface treatment and alloy design, and thus the usage for the magnesium alloys has been extended recently. However there still remain barriers for the adaption of magnesium alloys for engineering materials. In this study, we report light-weight, high strength heat resistant magnesium matrix composites. Microstructural study and tensile test were performed for the squeeze infiltrated magnesium matrix composites. The wear properties were characterized and the possibility for the application to automotive power train and engine parts was investigated. It was found that the squeeze infiltration technique is a proper method to fabricate magnesium matrix composites reducing casting defects such as pores and matrix/reinforcement interface separation etc. Improved tensile and mechanical properties were obtained with CNT reinforcing magnesium alloys

Abstract: The fracture behavior of Mg-5Al-1Zn and Mg-5Al-1Zn-3Sn alloy was investigated by direct observation of microfracture process using an in-situ loading stage installed inside a scanning electron microscope chamber. Crack was initiated at the interface of Mg/second-phase particles or second-phase particles. Fracture of the alloys was predominantly dimple or/and quasi-cleavage failure. The improvement of what could be explained by mechanisms of blocking of crack or shear band propagation, formation of multiple shear bands, crack blunting and shear band branching.

Abstract: In the present work, the effect of Sn addition on the corrosion behavior of Mg–5Al–1Zn alloys was investigated. Microstructure, potentiodynamic polarization and immersion tests were carried out in 3.5% NaCl solution of pH 7.2 to estimate the corrosion behavior of AZ51 alloys with and without Sn addition. Mg17Al12 and Mg2Sn phases were mainly precipitated in inter-dendrite structures. With increasing the Sn content, the volume fraction of the Mg2Sn phase was increased and coarsening tendency was observed. The corrosion resistance was increased by Sn addition. Especially, the AZ51-5wt.%Sn alloy characterized the superior corrosion resistance among the four alloys. The Sn is known for a high hydrogen overvoltage and the secondary phases effectively formed the network structure, resulting in a drastically decreasing corrosion rate of AZ51 alloy.

Abstract: Porous HAp/chitosan-alginate composite scaffolds were successfully synthesized by insitu co-precipitation method. During the preparation of HAp/chitosan-alginate composite scaffolds, the interaction between chitosan-alginate molecules would be reduced with increasing HAp content, with the resulting that the chitosan-alginate molecules were homogeneously dispersed in the composite scaffolds. The chitosan-alginate content was found to be almost consistent as initially added during the preparation. These results imply that chitosan-alginate was almost perfectly incorporated into the composites. It was found that the pore structure of the composite scaffolds with low HAp content was similar to chitosan-alginate scaffolds, and the morphology of uniform microstructure was unaffected by the presence of HAp. However, the pore diameter decreased with increasing the HAp content up to HAp content of 30 wt%, eventually the pore structure was collapsed and the composites scaffolds appeared to be agglomerated at higher HAp content.

Abstract: Plasma radical nitriding was performed to harden the surface of SCM 440 steel for 1-10 hours at temperature range of 450-550°C. No compound layer was formed during this process except the experiment carried out at 500
for 10 hours. A diffusion depth increased with increasing treatment temperature and time ( up to about 250). The surface hardness of radical nitrided layer was two times higher than that of the untreated surface. The main phase produced in the diffusion zone was identified to be γ΄-Fe4(N,C). The residual stress of the diffusion layer also increased with increasing treatment temperature and time due to the increase of precipitates.

Abstract: P-type thermoelectric material Si0.8Ge0.2 was fabricated by mechanical alloying(MA) and Hot-Press Process(HP) The effect of Boron(0.25~2wt%) addition on the thermoelectric properties of p-type Si0.8Ge0.2 alloy was reported. Experiments showed that the electrical conductivity decreased with Boron content at temperature ranging from 500K to 1250K. The carrier concentration measured by the Hall effect measurement also decreased as a function of doping level. With increasing temperature, the Seebeck coefficient and the power factor increased with boron content. Based on measured results, the Figure of Merit (Z) value of 0.5 wt% Si0.8Ge0.2 alloy increased with the small addition of Boron, and reached maximum rapidly; the Z value was 0.9×10-3/K, the highest value among the prepared alloys.

Abstract: The direct extrusion process using the powder as raw materials was applied to prepare the thermoelectric materials. The mechanically alloyed powders of Ag added (Bi0.25Sb0.75)2Te3 were extruded by pulse discharge sintering method in the temperature range of 345°C ~ 425°C. High quality products were obtained by hot-extrusion method and their texture and thermoelectric properties were measured. The intensity of (110) plane increased with extrusion temperature up to 385°C and altered in the range of above 405°C
which coincided with the variation of power factor. The measured Power factor ranged from 3.5 ~ 4.0 × 10-3 W/K2·m. The figure of merit (Z) of the material extruded at 385°C was 3.1 × 10-3 /K, the highest value among the prepared materials.

Abstract: Porous HAp scaffolds have been prepared by using the slurry including HAp and magnesia based on the replication of polymer sponge substrate. The influence of HAp and MgO content in slurry on the pore morphology and size, and density, porosity, and mechanical strength of porous HAp scaffolds was investigated. The obtained scaffolds with average pore sizes ranging 150 to 300 μm had open, relatively uniform, and interconnected porous structure regardless of HAp and MgO content. As the MgO content increased, the pore network frame of scaffolds became to be relatively stronger, even though the pore size was not much changed. The compressive strength of the scaffolds increased rapidly with the increase of MgO content at a fixed HAp content because of increasing the pore wall thickness and density of the scaffolds. As a result, the porosity, density, and compressive strength of the porous HAp scaffolds scaffolds prepared by the sponge method were significantly affected by the addition of MgO.

Abstract: The effect of Al content on the microstructural characteristics, tensile and creep properties of Mg-xAl-Zn alloy was investigated. The molten Mg-(3~11)Al-Zn alloys of 700°C were poured into the permanent mould designed to prepare the specimens used in this study. Tensile and creep tests were carried out for each Mg-Al based alloys. Results show that creep resistance decreased while tensile strength improved with increasing Al contents. Microstructural study revealed that morphology and distribution of non equilibrium Mg17Al12 phase is main factor determining the tensile and creep properties of as cast Mg-xAl-Zn alloys. The final purpose of this study is to construct the database of already used Mg-Al based alloy and to develop the heat resistant magnesium alloys to apply transportation systems such as automobile.

Abstract: The effects of tin (Sn) and strontium (Sr) additions on the microstructure and creep properties of Mg-5Al-2Si (AS52) alloys were investigated. Results showed that the addition of Sn did not affect the morphology of secondary phase Mg2Si particles but induced the fine and uniform precipitation of thermally stable Mg2Sn at the grain boundary in the AS52 alloys. However, the morphology modification of the secondary phase Mg2Si particles from Chinese script shape to refined polygonal shape was greatly affected by the addition of Sr. Tensile strength and creep resistance were improved and tensile elongation was also increased in the modified alloy with addition of both Sr and Sn. Sn is the effective element for precipitating thermally stable Mg2Sn finely and uniformly at the grain boundary and Sr is the effective element for refining microstructure. Therefore both Sn and Sr are effective for modifying the microstructure and improving the creep properties of AS52 alloys.