Kee-Do Woo

Surface plasmon resonances, optical properties, and electrical conductivity thermal hystersis of silver nanofibers produced by the electrospinning technique.

In the present study, silver metal nanofibers have been successfully prepared by using the electrospinning technique. Silver nanofibers have been produced by electrospinning a sol-gel consisting of poly(vinyl alcohol) and silver nitrate. The dried nanofiber mats have been calcined at 850 degrees C in an argon atmosphere. The produced nanofibers do have distinct plasmon resonance compared with the reported silver nanoparticles. Contrary to the introduced shapes of silver nanoparticles, the nanofibers have a blue-shifted plasmon resonance at 330 nm. Moreover, the optical properties study indicated that the synthesized nanofibers have two band gap energies of 0.75 and 2.34 eV. An investigation of the electrical conductivity behavior of the obtained nanofibers shows thermal hystersis. These privileged physical features greatly widen the applications of the prepared nanofibers in various fields.

Effect of high energy ball milling on displacement reaction and sintering of Al−Mg/SiO2 composite powders

High-energy ball milling and low temperature sintering were successfully employed to fabricate a metal matrix composite of Al reinforced with Al2O3 particulate. Nano- and/or submicro-sized SiO2 particles embedded in an Al−Mg matrix particle can be obtained by high-energy ball milling. No new phases were found in the high-energy ball milled Al-0.4 wt.% Mg-14 wt.%SiO2 powder. Milling of the Al−Mg−SiO2 powder increased the sintering rate and decreased the sintering temperature. The hardness ofthe sintered Al−Mg−SiO2 composite using the ball-milled powder was about twice that of a sintered composite using a mixed powder due to the fine and homogeneous distribution of Al2O3 particles formed by the displacement reaction between Al and SiO2 during sintering.

Effects of grain size on high temperature deformation behavior of Al-4Mg-0.4Sc alloy

Abstract A high temperature tensile test has been performed at 450 °C with various strain rates and grain sizes for an Al–4Mg–0.4Sc alloy. The experimental results show that the total elongation is strongly sensitive to the grain size and is approximately a linear function of an inverse average grain size. The Portevin–Lechatellier (P-L) effect with significant serrations on the stress–strain curves during tensile deformation at high temperature has been shown to be usual for the Al–4Mg–0.4Sc alloy. The average amplitude of the stress oscillation decreases with increasing strain rate at high temperature.

Mechanical Properties and Bio-Compatibility of Ti-Nb-Zr-HA Biomaterial Fabricated by Rapid Sintering Using HEMM Powders

Ti-6Al-4V ELI (Extra Low Interstitial) alloy has been widely used as an alternative to bone due to its excellent biocompatibility. However, it still has many problems, including a high elastic modulus and toxicity. Therefore, nontoxic biomaterials with a low elastic modulus should be developed. However, the fabrication of a uniform coating is challenging. Moreover, the coating layer on Ti and Ti alloy substrates can be peeled off after implantation. To overcome these problems, it is necessary to produce bulk Ti and Ti alloy with hydroxyapatite (HA) composites. In this study, Ti, Nb, and Zr powders, which are biocompatible elements, were milled in a mixing machine (24h) and by planetary mechanical ball milling (1h, 4h, and 6h), respectively. Ti-35%Nb-7%Zr and Ti-35%Nb-7%Zr-10%HA composites were fabricated by spark plasma sintering (SPS) at under 70MPa using mixed and milled powders. The effects of HA addition and milling time on the biocompatibility and physical and mechanical properties of the Ti-35%Nb-7%Zr-(10%HA) alloys have been investigated. , CaO, , and phases were formed by chemical reaction during sintering. Vickers hardness of the sintered composites increases with increased milling time and by the addition of HA. The biocompatibilty of the HA added Ti-Nb-Zr alloys was improved, but the sintering ability was decreased.

Mechanical properties and deformation behavior of Al–4Mg–0.4Sc–1.5Mm alloy at room temperature

Tensile test at room temperature has been performed on the Al–4Mg–0.4Sc–1.5Mm (Misch metal) alloy sheet with different reduction rate at crosshead speed of 0.036 mm min−1. The experimental results show that the fracture strain is strongly sensitive to cold rolling reduction rate, and the fracture strain increases with an increase of cold rolling reduction. The Portevin–LeChatellier (P–L) effect with a significantly serrate oscillation of stress–strain curves has been shown to be very regular for Al–4Mg–0.4Sc–1.5Mm alloy during tensile deformation. The onset strain decreases with an increase of cold rolling reduction rate. The average interval and the amplitude of stress–strain oscillation can be described by an exponential function with the strain.

Calorimetric investigation of precipitation kinetics in Al-Mg-Si-X(Cr,Be) alloys

This study has been carried out by differential scanning calorimetry (DSC) to study the kinetics of precipitation and the dissolution of metastable and stable phases in Al-Mg-Si-(Cr,Be) alloys which were heat treated by T6, two-step aging and RRA (retrogression and reaging) treatment. The heat flow variations by phase transformation in the as-quenched specimen were calculated from DSC thermograms obtained from heating rates of 5, 10, 15 and 20°C/min. Four exothermic peaks may be attributed to the precipitation of G.P.I zone, G.P.II zone(β″), β′ and β (Mg2Si) phases, and three endothermic peaks may be attributed to the dissolution of G.P.I zone, β″ and the β′ phases, respectively. The kinetic equation (dY/dt)=f(Y)koexp(-Q*/RT) can be used to study the precipitation kinetics of Ai-Mg-Si-(Cr, Be) alloys, where Q*, ko, and f(Y)are the activation energy, frequency factors and the function of Y, respectively. The kinetic parameters measured from DSC curves can be used to interpret the transformation kinetics.The formation rate of β″ phase in the Al-Mg-Si alloy increased by the small addition of Be. This is because Be increases the nucleating rate of the β″ phase due to the decrease of the matrix/β″ interface energy. By the addition of Be or Cr and Be in Al-Mg-Si alloy, G.P. zone was easily decomposed during retrogression treatment at 225°C for 3 min. Therefore, maximum hardness can be obtained by RRA (150°C/20 min→225°C/3 min→ 180°C/3O min) in Al-0.8%Mg-1.0%Si-0.05% Be and Al-0.8% Mg-l.0% Si-0.l% Cr-0.05% Be alloys owing to the high density of β″ and β′ precipitates.

Texture and mechanical properties of Al-0.5Mg-1.0Si-0.5Cu alloy sheets manufactured via a cross rolling method

The relationship between the texture and mechanical properties of 6xxx aluminum alloy sheets processed via cross rolling was investigated. The microstructures of the conventional rolled and cross rolled sheets after annealing were analyzed using optical micrographs (OM). The texture distribution across the thickness in the Al-Mg-Si-Cu alloy, conventional rolled sheets, and cross rolled sheets both before and after annealing was investigated via X-ray texture measurements. The texture was analyzed in three layers from the surface to the center of the sheet. The β-fiber texture of the conventional rolled sheet was typical of the texture obtained using aluminumoll ring. After annealing, the typical β-fiber orientations were changed to recrystallization textures: cube{001}〈100〉 and normal direction (ND)-rotated cubes. However, the texture of the cross rolled sheet was composed of an asymmetrical, rolling direction (RD)-rotated cubes. After annealing, the asymmetrical orientations in the cross rolled sheet were changed to a randomized texture. The average R-value of the annealed cross rolled sheets was higher than that of the conventional rolled sheets. The limit dome height (LDH) test results demonstrated that cross rolling is effective in improving the formability of the Al-Mg-Si-Cu alloy sheets.

Tensile Behavior of Al-4 %Mg-0.4 %Sc-0.5 %Misch Metal Alloy at Room Temperature

The effects of strain rate and pre-deformation in Al−4 wt.%Mg−0.4 wt.%Sc−0.5 wt.%Mm (misch metal) alloy on tensile behavior and P-L effect have been investigated. Pre-deformation of Al−4 wt.%Mg−0.4 wt.%Sc−0.5 wt.%Mm alloy clearly enhances the yield strength and ultimate strength, though it decreases the fracture strain. The yield strength of pre-deformed Al−4 wt.%Mg−0.4 wt.%Sc−0.5 wt.%Mm alloy is higher than that of commercially used Al−Mg based alloys. The strength of Al−4 wt.%Mg−0.5 wt.%Sc−0.5 wt.%Mm alloy was changed slightly at a strain rate between 2×10−5s−1 and 2×10−3s−1, but changed significantly when predeformation was introduced. Tensile test results of as-cast Al-4 wt.%Mg-0.4 wt.%Sc-0.5 wt.%Mm alloy show a significant oscillation of serration during deformation at room temperature, and the critical strain (εc), which is the strain at the start of serration, decreases with increasing strain rate. Pre-deformation of Al−4wt%Mg−0.4wt%Sc−0.5wt%Mm also affects the serration oscillation: it decreases the critical strain at lower strain rate and increases it at higher strain rate (>2×10−4s−1).

Effects of solidification structure on fatigue crack growth in rheocast and thixocast Al–Si–Mg alloys

Abstract Fatigue crack growth test was performed for rheocast and thixocast Al–Si–Mg aluminum alloys. At small stress intensity factor range (Δ K ), fatigue crack growth (FCG) rate of sample with coarse acicular Si particles decreased slightly compared with specimen with small acicular Si particles. However, at large Δ K , fatigue crack growth rate of specimen with coarse acicular Si particles drastically increased. This is because large acicular Si particles induce high strain hardening at small Δ K , but such particles are easily cracked with the increase in Δ K . Morphology of the Si particles strongly affects striation formation.

Wear characteristics of Al−Si−Mg−(Cu)/Al2O3 composites fabricated by thermit reaction

Al2O3 particles could be formed by a thermit reaction in an Al-12Si-4Mg-1.5Cu/Al2O3 composite due to thein-situ reaction between Al-12Si-4Mg-(1.5Cu) molten metal and SiO2 particles in preform, which took place at 1173 K for 24 hours, resulting in the decomposition of SiO2 particles and the formation of Al2O3 particles simultaneously. The mechanically mixed layers (MMLs) consisting of α-Fe and Fe oxides existed on the subsurface layers beneath the worn surface in composites or mother alloys, which improved the wear resistance. The characteristics of wear resistance and hardening of an Al-12Si-4Mg-1.5Cu/Al2O3 composite are superior to those of the Al-12Si-4Mg/Al2O3 composite and Al-12Si-4Mg-1.5Cu alloy.