Papers by Author: Bo Young Hur

Abstract: Mg72.5Zn26Y1.5 quasicrystal alloys were investigated under different solidification conditions. The specimens of Mg-Zn-Y alloys with cooling rates from 13.2K/s to 69.8K/s were gathered by a designed multi-channel temperature acquiring system and then the microstructures and phase evolution of the alloys were analyzed. The results show that the precipitation temperature of icosahedral quasicrystal phase (I-phase) increased with cooling rate increased from 13.2 K/s to 69.8K/s. The microstructure was mainly made up of α-Mg, I-phase and Mg7Zn3 phase. Meanwhile, the quasi-crystalline morphology was significantly different in the experiments. It changed from five (six) petals to the big pentagon with the decreased cooling rate.

Abstract: Mg72.5Zn26Y1.5 (at.%) quasicrystal alloys were investigated under different solidification conditions. The cooling curves were gathered by the multi-channel temperature acquiring system and the corresponding microstructures were analyzed. The morphology, microhardness and volume fraction of quasicrystals were detailedly studied. The effects of cooling rate on the above three aspects were also studied and the relation schema among them were exhibited. The results show that the quasicrystal size tend to smaller, its microhardness and volume fraction gradually increased, and the quasicrystal morphology changed from bulk polygon to petal-like and finally changed to plat X-shape with the elevated cooling rate from 2.3K/s to 181.2K/s. However, the quasicrystal nucleation will be restrained and amorphous matter will be created if the cooling rate exceed the critical point which make against the formation of quasicrystals with high performance.

Abstract: Al foams whose matrix contains dispersed AlN particles (Al/AlN composite foams) were prepared by a melt foaming method, and the effect of foaming temperature on the pore morphology of the prepared foams was investigated. First, Al/AlN composites were prepared by non-compressive infiltration of Al powder compacts with molten Al alloy in nitrogen atmosphere. Next, the prepared composites were melted by induction heating and foamed at various temperatures using TiH2 powders as blowing agents. The porosity of prepared Al/AlN composite foams slightly decreases with increasing foaming temperature, and the pore morphology of the foam becomes homogeneous simultaneously. When the foaming temperature is 1123 K, homogeneous pores are formed in all over the ingot. This pore homogeneity is probably achieved by the stabilization of the foaming behavior due to the formation of Al3Ti particles in the melt and dispersion of AlN particles.

Abstract: The purpose of this research is to investigate effect of rare-earth elements (Y and Dy) on industrial pure Mg through the ignition point test and oxide film analysis. The results show that the 0.5%Y can make the ignition point of pure magnesium about 30°C higher. However, the ignition point of pure magnesium can be multiplied about 50°C by the mixed additions of 0.5wt%Y and 5wt%Dy. The SEM analysis indicates the oxide film of Mg-0.5Y-5Dy is more compact and tenacious than that of Mg-0.5Y. The XRD analysis indicates that the oxide film of Mg-0.5Y-5Dy consists of MgO, Y2O3 and Dy2O3. The most of all the three oxides are Dy2O3, which can prevent magnesium from further oxidizing.

Abstract: In-situ spherical icosahedral quasicrystal phase (I-phase) was fabricated in Mg-Zn-Y-based alloys by metal mold casting route. The effects of Mn and/or Cu on spheroidized process, morphology and microhardness of primary I-phase were studied in this paper. The results showed that the solidified morphology of I-phase depended on the stability of spherical I-phase during the subsequent growth and critical radius Rr of the spherical I-phase. Only when actual radius smaller than critical radius Rr can spherical IQC be formed. The effects of the fourth component and degree of undercooling on Rr were studied for the first time. Amazing spherical minisize IQC had been produced by improving cooling rate using a water-cooled copper mold. Multicomponent spherical IQC is hopeful of being strengthening phase to novel metal composites.

Abstract: Aluminum alloy foams, new materials belonging to a special class of porous materials, have been prepared using melt foaming method. Silicon was chosen alloying element due to its low density, high strength, effective casting and reduced shrinkage. Melt foaming method is cost-effective method to fabricate metal foam. Usually, TiH2 is applied to blowing agent, but its cost is high. CaCO3 is one of candidates to substitute TiH2 in the economic view-point. For the comparison of formability, Mg alloy foams were prepared by TiH2 and CaCO3. However, the decomposition temperature of CaCO3 is higher than that of TiH2. This paper will be discussed on the possible usage of CaCO3 in the Al melt.

Abstract: Fabrication of Aluminum foam with near net shape has been investigated by powder metallurgy method and conventional pouring method. PM method is good for fabrication of near net shape foam, but it needs high cost compared with pouring method. More cost-effective methods are needed to make near net shape al foam to be applied various field. Therefore, novel method for fabrication Al foam was researched in this paper. In order to prepare near shape Al foams with homogeneous pore structures, the so-called upward foaming method was designed and applied. By using this method, two kinds of molds were designed, one is stainless mold used for melting and foaming Al and another is the plaster mold with near net shape. The fabrication procedures of near net shape Al foam are as following: (1) a quantity of Al ingot was melted in the stainless mold; (2) Ca particles was added in the Al melt to increase its viscosity; (3) TiH2 was introduced in the thickened Al melt to make melt being foamed; (4) the plaster mold was put on the stainless steel one to make enlarging Al melt foam fill with the plaster one; (4) the plaster mold was removed and was cooled. In this study, in order to get near net Al foam with relative good pore structures, the plaster molds were designed with three different upside styles and their effects on the pore structures (pore size, porosity and cell wall thickness) of Al foams were investigated. The results showed that the Al foam had the relative good pore structures when the plaster mold with a void was applied.

Abstract: Al-Mg alloy foams were synthesized via conventional melt foaming method. Ca and TiH2 were introduced into molten Al-Mg alloys with different magnesium contents. The macrostructures of resultant alloy foams were analyzed and correlated with compressive properties estimated by compression test. It is shown that the pore structures observed in alloy foams degraded with increasing Mg contents. This tendency was shown to be consistent with compressive characteristics of corresponding alloy foams. In detail, plateau strength was high for Al-1wt%Mg alloy foams, exhibiting a gradual decrease in plateau strength with increasing magnesium content.

Abstract: We have been investigated application of the lotus type porous metal that is manufactured by the unidirectional solidification of the melt in pressurized gas atmosphere such as hydrogen or nitrogen for an air bearing development. In this study, wear properties of porous copper having cylindrical pores have been investigated. Generally, wear test is influenced by a shape, a wear test method, atmosphere and a way of lubrication. Wear test used pin on disk type's wear test.

Abstract: Al foams with high porosity were fabricated via melt foaming method. In this process, Ca and TiH2 are used as thickening and foaming agent. The macrostructure of aluminum foams exhibited homogeneous distribution of fine pore sizes at shorter holding time. From comparison of the actual pore sizes with ones calculated with modified Ostwald ripening, the measured pore sizes were well fitted to the calculated ones. These results suggest that hydrogen diffusion through pore walls from smaller pores to a larger one is the main process of pore evolution at least in the later period of holding. The detailed are discussed in the context.