Byoung-Gi Moon

Microstructural change and precipitation hardening in melt–spun Mg–X–Ca alloys

Abstract Mg–Al–Si–Ca and Mg–Zn–Ca base alloys were rapidly solidified bymelt spinning at the cooling rate of about a million K/s. The melt-spun ribbons were aged in the range 100–400%C for 1 h. The effect of additional elements on microstructural change and precipitation hardening after heat treatment was investigated using TEM, XRD and a Vickers microhardness tester. Age hardening occurred after aging at 200%C in the Mg–Al–Si–Caalloys mainly due to the formation of Al2Ca and Mg2Ca phases, whereas in the Mg–Zn–Ca alloys mostly due to the distribution of Mg2Ca. TEM results revealed that spherical Al2Ca precipitate has the coherent interface with the matrix. Considering the total amount of additional elements, Mg–Zn–Ca alloys showed higher hardness and smaller size of precipitates than Mg–Al–Si–Ca alloys. With the increase of Ca content, the hardness values of the aged ribbons were increased. Among the alloys, Mg–6Zn–5Ca alloy showed the maximum value of age hardening peak(Hv:180) after aging at 200%C for 1h.

Effect of crystallization condition on the Microwave properties of Fe-based amorphous alloy flakes and polymer composites

The electromagnetic (EM) wave absorption properties with a variation of crystallization temperature have been investigated in a sheet-type absorber made of the amorphous Fe73Si16B7Nb3Cu1Finemet powder. With the variation of the annealing temperature, the magnetic and dielectric properties of the crystallized Fe-based absorber with a nano-structure were changed. The complex permittivity increased with increasing the annealing temperature, whereas the complex permeability was maximized after annealing at 530°C for 1 hour. The absolute value of the reflection parameter, |S11|, increased with increasing annealing temperature of the nanocrystalline alloy powder. On the contrary, the transmission one, |S21|, showed the highest value after annealing at 530°C for 1 hour, which is regarded as the optimum temperature for the improvement of EM wave absorption properties.

Purification of Highly Contaminated Magnesium Melt

The steering wheel core is chosen as the first target for the development of a purification technology for highly contaminated magnesium melt, because it contains abundant foreign matter such as polyurethane, copper electrodes, and steel inserts, which have high potential to form non-metallic inclusions and to deteriorate the corrosion resistance of recycled alloys. Various melt treatment technologies have been investigated for refining AM50 magnesium alloy contaminated with polyurethane. The NMI content in magnesium alloy scrap contaminated with polyurethane was effectively reduced by a sequential refining process consisting of filtration, fluxing, and gas bubbling treatments. The filtration step reduced most large inclusions such as carbon residues from the decomposed polyurethane. The subsequent fluxing and gas bubbling treatments effectively removed the small inclusions such as carbonates and oxides.

Effect Of Crystallization Annealing And Surface Oxidation On Electromagnetic Wave Absorption Behavior Of FeSiBNbCu Nanocrystalline Alloy

The oxidation effect of Fe73Si16B7Nb3Cu1 nanocrystalline powder on the electromagnetic (EM) wave absorption behavior has been investigated. The oxidation did not improve the absorption characteristics of the nanocrystalline soft magnetic material. The pulverization and milling lowered the optimum crystallization temperature of the nanocrystalline material because of the accumulated internal energy due to impact during fragmentation and powder thinning.