Young-Jig Kim

High-temperature oxidation behavior of pure Ni3Al

The high-temperature oxidation behaviour of pure Ni3Al alloys in air was studied above 1000°C. In isothermal oxidation tests between 1000 and 1200°C, Ni3Al showed parabolic oxidation behavior and displayed excellent oxidation resistance. In cyclic oxidation tests between 1000 and 1300°C, Ni3Al exhibited excellent oxidation resistance between 1000 and 1200°C, but drastic spalling of oxide scales was observed at 1300°C. When Ni3Al was oxidized at 1000°C, Al2O3 was present as θ-Al2O3 in a whisker form. But, at 1100°C the gradual transformation of initially formed metastable θ-Al2O3 to stable α-Al2O3 was observed after oxidation for about 20 hr. After oxidation at 1200°C for long times, the formation of a thick columnar-grain layer of α-Al2O3 was observed beneath a thin and fine-grain outer layer of α-Al3O3. The oxidation mechanism of pure Ni3Al is described.

Dehydrogenation properties of nano-/amorphous Mg2NiHx by hydrogen induced mechanical alloying

Abstract In recent years, nanocrystalline materials have been of interest for hydrogen storage applications. Especially, the great improvement of the hydrogenation properties of Mg alloys that can be achieved by nanocrystallization. In this study, nanocrystalline Mg2NiHx is fabricated from Mg and Ni chips by hydrogen induced mechanical alloying (MA) for 96 h under a high-pressure hydrogen atmosphere. The balls to chips mass ratios (BCR) are 30:1 and 66:1. The particles obtained are characterized by XRD and TEM, and absorbed hydrogen contents (AHC) were measured by TGA. For dehydrogenation kinetics, activation energies are calculated by isothermal thermogravimetry analysis (ITGA) and pressure–composition-isotherm (PCI) analysis. The results of XRD and TEM revealed that the Mg2NiHx peaks are broadened in the case of high BCR and the particles are composed of the nanocrystalline phases less than 10 nm with the amorphous phase. The results of ITGA and PCI analysis show that the dehydrogenation kinetics are greatly improved by nanocrystallization. The results show that BCR conditions mainly affect the size and fraction of the nanocrystalline phases, the resultant AHC and the dehydrogenation kinetics.

Synthesis and hydrogenation behavior of Mg-Ti-Ni-H systems by hydrogen-induced mechanical alloying

Mg and Mg alloys are attractive hydrogen storage materials because of their lightweight and high absorption capacity. Their range of applications could be further extended if their hydrogenation properties and degradation behavior could be improved. The main emphasis of this study was to find an economic manufacturing method for Mg–Ti–Ni–H systems, and to investigate their hydrogenation properties. (Mg10−xTix)–10, 20 mass% Ni systems were prepared by hydrogen-induced mechanical alloying (HIMA) using Mg and Ni chips and sponge Ti. The particles synthesized were characterized by X-ray diffraction, and their morphologies were observed by means of scanning electron microscopy (SEM) with energy dispersive spectrometry (EDS). The absorbed hydrogen capacity (AHC) was measured by using thermogravimetry analysis (TGA) after HIMA. In addition, the crystal structures were analyzed in terms of their bright-/dark field images and the selected area diffraction pattern (SADP) of transmission electron microscopy (TEM). In order to examine hydrogenation behavior, a Sieverts type automatic pressure–composition–isotherm (PCI) apparatus was used and the experiments were performed at 423, 473, 523, 573, 623 and 673 K. The results of TGA reveal that the absorbed hydrogen contents are around 2.5 mass% for (Mg9Ti1)–10 mass% Ni. With increased Ni content, the absorbed hydrogen content decreases to 1.7 mass%, whereas the dehydriding starting temperatures are lowered by some 70–100 K. The results of PCI on (Mg9Ti1)–20 mass% Ni show that its hydrogen capacity is around 5.3 mass% and its reversible capacity and plateau pressure are also excellent at 523 and 573 K. In addition, the reaction enthalpy, ΔHD.plateau, is −30.6±5.7 kJ/mol H2.

Microstructural evolution and semisolid forming of SiC particulate reinforced AZ91HP magnesium composites

Abstract The aim of this paper is to report the experimental results concerning the microstructural evolution and formability of semisolid forming SiC particulate reinforced AZ91HP Mg composites. The composite materials were produced in the form of cylindrical billets by a rotating cylinder method. The microstructural evolution was characterised by conventional parameters (liquid fraction, average size of globules, and number of globules per unit area) as functions of temperature and isothermal holding time in the semisolid state. In addition, special attention was paid to the shapes of the globules and the liquid droplets entrained within the globules and to the variation of hardness with regard to microstructural evolution. Finally, the formability of the composites was evaluated through thixocasting trials, and then correlated with the effective liquid fraction of the composites.

Serum 1,5-anhydroglucitol is associated with diabetic retinopathy in Type 2 diabetes

Diabet. Med. 29, 1184‐1190 (2012)

Investigation of interface reaction between TiAl alloys and mold materials

This paper describes the investment casting of TiAl alloys. The effects of mold mateiral and mold preheating temperature for the investment casting of TiAl on metal-mold interfacial reaction were investigated by means of optical micrography, hardness profiles and an electron probe microanalyzer. The mold materials examined were colloidal silica bonded ZrO2, ZrSiO4, Al2O3 and CaO stabilized ZrO2. When compared with conventional titanium alloy, the high aluminum concentration of TiAl alloys helps to lower their reactivity in the molten state. The Al2O3 mold is a promising mold material for the investment casting of TiAl in terms of the thermal stability, formability and cost. Special attention need to be paid to thermal stability and mold preheating when developing the investment casting of TiAl alloys.

The effect of Si3N4 additions on the oxidation resistance of TiAl alloys

The oxidation kinetics of TiAl alloys with and without 3 and 5 wt.%additions of Si3N4 particles were studied at 1173 and1273 K in 1 atm of air. The Si3N4 dispersions wereunstable in the matrix phase, so that some of them reacted with titaniumduring sintering to form Ti5Si3 and dissolvednitrogen. The oxide scale formed on TiAl–Si3N4alloys consisted of an outer TiO2, an intermediate(Al2O3+TiO2), and an inner(TiO2+Al2O3) mixed layers. The enhancedalumina-forming tendency, the presence of discrete SiO2 particlesbelow the outer TiO2 layer, and the improved scale adhesion bySi3N4 dispersions were attributable mainly to theincreased oxidation resistance compared to the Si3N4-freeTiAl alloys. Marker experiments showed that, for TiAl–Si3N4 alloys, the primary mode of scale growth was the outward diffusion oftitanium ions for the outer scale and the inward transport of oxygen ionsfor the inner scale.

Rotating cylinder manufacturing method and investment casting of SiCp/AZ91HP magnesium composites

Abstract This paper describes the rotating cylinder method for manufacturing, and investment casting for forming, composite slurry. Microstructural features, such as SiC particle distribution and grain refinement of the as cast composites, were investigated. Also the effect of SiC particle fraction and size, and process parameters on the microstructure and the mechanical properties are discussed. Attempts were made to evaluate the thermal stability of oxides against molten AZ91HP magnesium alloy. The oxides examined included CaO, CaZrO3, and silica bonded Al2O3 and zircon flour. Finally, the tensile properties, hardness, and wear resistance of the as cast composites were evaluated and the results are compared with those of the as cast alloy.

Investment casting of AZ91HP magnesium alloy

This paper describes AZ91HP magnesium alloy investment casting. The aim of this study is to optimize the process for magnesium investment casting. Special attention was given to evaluating the thermal stability of oxides against molten AZ91HP magnesium alloy. The oxides examined included CaO, CaZr03, and silica bonded A1203 and ZrSi04. Also, the microstructural features of the as-cast alloy were investigated, and the effect of the processing parameters on the microstructure and mechanical properties were evaluated describing the grain size, hardness and ultimate tensile strength of the as-cast alloy.

Rotation-cylinder method for fabrication of SiC particle reinforced magnesium composites

A new molten metal mixing process, RCM (Rotation-Cylinder Method) for incorporating reinforcement particles into molten magnesium, has been developed for manufacturing SiC particle reinforced magnesium composites in an ambient atmosphere. The RCM can be characterized by the U-shaped laminar melt surface with the Rankine vortex, which significantly reduces particle agglomeration and entrapped slags, such that composites of improved microstructure and properties can be manufactured successfully in conjunction with the subsequent fabrication processes. The RCM can incorporate SiC particles (5 urn size and larger) into molten magnesium with particle volume fractions of up to 20 vol.%. In this paper, the development background, the characteristic vortical motions, the optimum manufacturing conditions and the in-process wetting procedures of the RCM are described.