Papers by Author: Jae Seol Lee

Abstract: The microstructure of as-cast Mg-xAl-2Sn alloys consists of an α-Mg matrix, a Mg17Al12 eutectic phase and a Mg2Sn phase. In as-cast alloys, the Mg17Al12 eutectic phase was located at along grain boundaries. The eutectic phase of the extruded alloys was elongated in the extrusion direction and crushed into fine particles because of deformation during hot extrusion, and the grain size was refined with an increased Al content. The maximum values of the yield strength and tensile strength were 240 MPa and 300 MPa at 9 wt.% containing Al element, respectively.

Abstract: In the present work, the effect of Nd addition on microstructure change, precipitation behaviours and mechanical properties, of Mg-5Sn-3Ca based alloys were investigated. With increasing of Nd addition, size of the Mg-Sn-Ca ternary phases with fine needle shape (A-type) was decreased and volume fraction of these phase were increased. As Nd was increase from 0.5, 1.0 and 3.0 wt.%, size and volume fraction of Mg2Sn with coarse needle shape phase (B-type) was decreased. However, in 3.0 wt.% Nd containing alloy, size of these Mg2Sn phase was smaller than that of 0.5 and 1.0 wt% Nd addition alloy and volume fraction of these phase was decreased.

Abstract: The effect of Ca addition on microstructure change, precipitation behaviours and mechanical properties, in the present work, of Mg-Sn based alloys were investigated. With increasing of Ca additions, size of the Mg-Sn-Ca ternary phases (A-type) with fine needle shape was decreased and volume fraction of these phase were increased. As Ca was increase from 1 wt.%, 2 wt.%, size and volume fraction of Mg2Sn with coarse needle shape phase (B-type) was increased. However, in 3 wt.% Ca containing alloy, size of these Mg2Sn phase was smaller than that of 2 wt% Ca addition alloy and volume fraction of these phase was decreased.

Abstract: The aims of this study are to investigate the microstructure evolution of AZ31 Mg alloys with normal rolling and cross rolling as the large strain hot rolling affects microstructure, texture and mechanical properties of AZ31 Mg alloys. In the microstructures of as-rolled both samples, twins are clearly apparent, small and recrystallized grains are visible along some grain boundary and twinned regions. The Lankford values of large strain cross rolled sample obviously demonstrate the higher average r-value and lower planar anisotropy value. The press formability of cross rolled Mg alloy might be improved due to control of texture and grain size by severe deformation.

Abstract: The layers of ZrO2-Al2O3 were coated on the surface of SiC fibers (Tyranno SA fiber) by dip-coating process. The citric acid-ethylene glycol solution containing Al and Zr ions was polymerized at 150oC to obtain a polymerized complex precursor. The 2D-woven SiC Tyranno SA fibers were dipped into this precursor few times. Heat treatment for coated fiber was conducted at 800-1200oC for 1 h in air. Thin film of mixed zirconium/aluminum oxides on SiC fibers were successfully elaborated using the sol-gel process. The coated zirconium/aluminum oxides layer of SiC Tyranno SA fiber was about 200-1000 nm. The coated fibers were stable in an oxidizing environment even at high temperatures.

Abstract: The n-type Bi2Te2.7Se0.3 compounds were fabricated to investigate the characterization of spark plasma sintering with various SbI3 dopant contents. The Bi2Te2.7Se0.3 compounds with SbI3 dopant content is exhibited n-type conduction characterization, but the Bi2Te2.7Se0.3 compounds without SbI3 dopant content is exhibited p-type conduction characterization. The maximum Seebeck coeficient represented with 0.05wt.% SbI3 dopant content. The Seebeck coefficient of the sintered sample with increasing sintering temperature is increased from -158 to -182 μV/K. The electrical resistivity and thermal conductivity with 0.05wt.% SbI3 dopant content were 1.0 m and 1.33 W/mK, respectively.

Abstract: The mechanical properties of the nano-size primary α-Ti phase in Ti55Zr18Ni6Cu7Be14 bulk metallic glass (BMG) have been examined through various annealing temperatures and times. The yield strength and ductility were changed upon variation of α-Ti volume percent originate from heating conditions. The strength mechanism and associated heating conditions (heating rate, time and temperature) are discussed in terms of the microstructural observations and mechanical properties.

Abstract: AZ31 Mg / 5083 Al clad sheet was fabricated by the hot rolling method and its mechanical properties were investigated in this study. The tensile strength and yield strength of Mg- Al clad samples were slightly higher than that of AZ31 Mg sample, resulting in high strength 5083 Al alloy. Also, in the case of the AZ31 Mg sample, tensile strength indicated different values to the rolling directions. The thickness of interface layers between magnesium and aluminum materials increased with increasing rolling temperature. The thickness of interface layer was about 1.2 μm and 1.6 μm, respectively. The difference of thickness on the interface layer with variation of rolling temperature was attributed to promote the diffusion between magnesium and aluminum materials. The Vickers hardness of Mg-Al interface layer was around 125 Hv. The interface layer composed of hard inter-metallic phases which may act a increment of Vickers hardness depending upon its thickness.

Abstract: The as-cast microstructure of Mg-5Al-3Ca-2Sm alloy consists of equiaxed α-Mg matrix, (Mg, Al)2Ca eutectic phase and Al-Sm rich intermetallic compounds. This eutectic phase of the extruded alloys was elongated to extrusion direction and size of this phase was finered compare to that of as-cast alloys because of severe deformation during hot extrusion. After hot extrusion, the average grain size of Mg-5Al-3Ca and Mg-5Al-3Ca-2Sm alloys was 4.8 *m and 3.8 *m, respectively. In load-unload hardness test, penetration depth was decreased with added Sm and after extrusion procedure because of grain size refining by addition Sm and large deformation. Hardness value of the alloys containing Sm was higher than that of Mg-5Al-3Ca alloy due to grain refining and formation Al-Sm rich intermetallic compound at gain boundary and α-Mg matrix. Maximum hardness value was obtained at the extruded Mg-5Al-3Ca-2Sm alloy at elevated temperatures.

Abstract: The aims of this research are to investigate the effect of Sm addition in Mg-Al-Ca alloys on microstructure and mechanical properties. Sm addition to Mg-5Al-3Ca based alloys results in the change from dendritic to equiaxed grain morphorlogy and formation of Al-Sm rich itermetallic compounds at grain boundary and α-Mg matrix. And these Al-Sm rich intermetallic compounds were dispersed homogeously and stabilized at high temperature. And maximum yield and ultimate strength value was obtained at Mg-5Al-3Ca-2Sm alloys at elevated temperature because of homogeneous dispersion of stable Al-Sm rich intermetallic compound at high temperature.