Ki-Bae Kim

Enhancement of the glass forming ability of Cu−Zr−Al alloys by Ag addition

The thermal stability and glass forming ability of Cu50-xZr43Al7Agx (x=0, 1, 3, 5, and 7) bulk metallic glass alloys have been investigated. The glass forming ability in the Cu−Zr−Al−Ag alloys increased proportionally to the Ag content and show good correlations with thermal parameters such as ΔTx(=Tx-Tg), Trg(=Tg/T1) and γ(=Tx/(Tg+T1)). For the Cu43Zr43Al7Ag7 alloy, fully amorphous rods of 8 mm diameter were successfully fabricated by copper mold casting. Mechanical tests on this composition revealed also remarkable properties with compressive strength around 2000 MPa and large ductility.

Corrosion studies on Fe-based amorphous alloys in simulated PEM fuel cell environment

Abstract In this paper, we introduce a novel idea of using metallic amorphous material as a bipolar plate for polymer electrolyte membrane fuel cell (PEMFC). The major requirement of the metallic bipolar plate material is their high corrosion resistance. Owing to its chemical homogeneity, and absence of defects (like grain boundaries, dislocations, etc.), amorphous materials exhibit a combination of attractive properties for bipolar plates such as high corrosion resistance and high strength (~ about 2 GPa). The corrosion properties of Fe48Cr15Mo14Y2C15B6 and of a newly developed Fe50Cr18Mo8Al2Y2C14B6 bulk metallic amorphous alloys have been investigated under conditions that simulate the fuel cell environment and are compared with those of a stainless steel. Hydrogen gas and pressurized air were bubbled into a 1 M H2SO4+2 ppm F- solution at 75 °C solution, throughout the experiment to simulate the respective anodic and cathodic PEMFC environment. In comparison to the stainless steel, the Fe50Cr18Mo8Al2Y2C14B6 amorphous alloy displays significantly higher corrosion resistance.

Effects of casting speed on microstructure and segregation of electro-magnetically stirred Aluminum alloy in continuous casting process

Abstract Recently, a semi-solid metal processing has been acknowledged as a cost-effective technique to be able to manufacture high quality product for the transportation industry. In this study a hypo-eutectic Al alloy was fabricated by means of an electromagnetic stirrer in continuous casting process and the microstructural change during solidification due to a fluid flow by electromagnetic stirring was examined. Due to the forced fluid flow during solidification a dendritic phase of primary α phase of Al alloy was turned into a globular phase, which can make the Al alloy get a thixotropic behavior in the semi-solid region. In order to establish the quantitative relationship between microstructure and the process parameters, the morphology shape, a silicon distribution and a size of primary α phase were observed according to casting speed in continuous casting machine. The primary α phase was turned into the degenerate dendrites approaching a spherical configuration with increasing casting speed. The fine-grained and equiaxed microstructure appeared at higher casting speed. A segregation behavior of Si element was declined with increasing casting speed and a very uniform distribution of Si element was observed on the billet at a casting speed of 600 mm·min−1. A thickness of the solidifying shell of the billet was shortened with increasing the casting speed.

Globulization mechanism of the primary Al of Al−15Cu alloy during slurry preparation for rheoforming

Semisolid slurries of Al-15Cu alloy were produced for rheoforming by a low temperature pouring technique. To investigate the morphological change of the slurry in terms of the particle mean diameter and the roundness factor, samples were extracted during the continuous cooling and the isothermal holding stage of the slurry by a simple technique of interrupt quenching. Results demonstrated that the fine-grained equiaxed dendritic structure, which formed during low temperature pouring, is changed to a globular structure when held at a semisolid temperature for sufficiently long holding time. With regard to the globulization mechanism of the primary α-phase, local melting is considered to take place at the neck of equiaxed dendrites, leading to the separation of small new particles during continuous cooling. These newly formed particles eventually grow during isothermal holding in the semisolid temperature by obeying theD3=Kt kinetic law, which suggests coarsening by Ostwald ripening.