발간논문

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The objective of this study is to investigate the ballistic properties of Zr-based amorphous alloy surface composites fabricated by high-energy electron-beam irradiation. The mixture of Zr-based amorphous powders and LiF+MgF2 flux powders was deposited on a pure Ti substrate, and then an electron beam irradiated this powder mixture to fabricate a one-layer surface composite. A four-layer surface composite, in which the composite layer thickness was larger than 3 mm, was also fabricated by irradiating the deposited powder mixture by an electron beam three times on the one-layer surface composite. The microstructural analysis results indicated that a small amount of fine crystalline particles were homogeneously distributed in the amorphous matrix of the surface composite layer. According to the ballistic impact test results, the surface composite layers effectively blocked a fast traveling projectile, while many cracks were formed at the composite layers, and thus the surface composite plates were not perforated. The surface composite layer containing ductile β dendritic phases showed a better ballistic performance than the one without dendrites because dendritic phases hindered the propagation of shear bands or cracks.

keyword : Amorphous materials, surface modification, fracture, impact test, electron-beam irradiation

We present a new joining method for Pd-Cu membrane foils used as permeation tubes to collect CO2. Since foils have poor mechanical strength, joining should be done at low temperatures to reduce residual stresses and without joining pressure. This contradicts the well known conditions for good contact between base materials that determines joint qualities. We selected Sn-Ag-Cu alloys that are highly reactive with Pd and Cu as a filler metal. As the filler melts at joining temperatures as low as 220~280℃, Pd and Cu are dissolved into the melt and react with the filler elements, which raises the melting temperature of the filler based on eutectic structures among the elements. Then, isothermal solidification progresses for the rest of the joining time. Intermetallic compounds (IMC) in the joints, one of the main factors for brittle joints, are inevitably formed. However, by optimizing both joining time and temperature, we balanced the wettability with IMC. Sealing test results confirmed that the joints are mechanically reliable during operation.

keyword : Porous materials, joining, mechanical properties, X-ray diffraction

Thin films have been used in a large variety of technological applications such as solar cells, optical memories, photolithographic masks, protective coatings, and electronic contacts. If thin films experience frequent temperature changes, thermal stresses are generated due to the difference in the coefficient of thermal expansion between the film and substrate. Thermal stresses may lead to damage or deformation in thin film used in electronic devices and micro-machined structures. Thus, knowledge of the thermo-mechanical properties of thin films, such as the coefficient of thermal expansion, is an important issue in determining the stability and reliability of the thin film devices. In this study, thermal cycling of Cu and Ag thin films with various microstructures was employed to assess the coefficient of thermal expansion of the films. The result revealed that the coefficient of thermal expansion (CTE) of the Cu and Ag thin films increased with an increasing grain size. However, the effect of film thickness on the CTE did not show a remarkable difference.

keyword : Thin films, vapor deposition, mechanical properties, thermal analysis, coefficient if thermal expansion

In this study, high temperature oxidation behavior of Alloy 617 was investigated to evaluate the effect of grain size for Alloy 617. The grain size of grain-refined Alloy 617 (GR617) was reduced to 5 μm from 71 μm for as-received Alloy 617 (AR617) by recrystallization after cold rolling. After high temperature aging, the oxide layers of AR617 and GR617 consisted of Cr2O3 external oxide scale and Al2O3 internal oxide. The external oxide scale resulted in a Cr-depleted zone and a carbide free zone below the scale. The depth of the carbide free zone was deeply formed in GR617. On the other hand, the depth of the internal oxide layer in GR617 was shorter than that in AR617. After a 3-point bending test, crack propagation of GR617 was more restricted than that of AR617 because of the different microstructure of the internal oxide.

keyword : alloys, aging, grain refinement, carbide free zone, crack propagation

This study elucidates the formation mechanism of SnO plate observed during the precipitation reaction of a SnCl2 aqueous solution. Sn21Cl16(OH)14O6 and Sn6O4(OH)4 precipitates was formed at pH=3~5 and at pH=11, respectively. When the pH was in the range of 11.5~12.5, the Sn6O4(OH)4 precipitates dissolved into HSnO2 - [Sn6O4(OH)4+4OH-=6HSnO2 -+2H+] and dissolved HSnO2 - ions reprecipitated to SnO plate [HSnO2 -+H+=SnO+H2O]. The Sn6O4(OH)4 precipitates completely transformed into SnO plate through a repeated process of dissolution-precipitation in the range of pH=11.5~12.5.

keyword : oxide, chemical synthesis, precipitates, X-ray diffraction, reaction mechanism

This pitting corrosion study of welded joints of austenitic stainless steels (AISI 304L and 316L) has addressed the differentiating solidification mode using three newly introduced filler wires with a flux-cored arc welding process (FCAW). The delta ferrite (δ-ferrite) content in the welded metals increased with an increasing equivalent weight ratio of chromium/nickel (Cr(eq)/Ni(eq)). Ductility dip cracking (DDC) was observed in the welded metal containing ferrite with none of AISI 304L and 0.1% of AISI 316L. The potentiodynamic anodic polarization results revealed that the Cr(eq)/Ni(eq) ratio in a 3.5% NaCl solution didn`t much affect the pitting potential (E(pit)). The AISI 316L welded metals with a-ferrite content of over 10% had a superior E(pit) value. Though the AISI 316L welded metal with 0.1% ferrite had larger molybdenum contents than AISI 304L specimens, it showed a similar Epit value because the concentration of chloride ions and the corrosion product induced severe damage near the DDC.

keyword : metals, welding, corrosion, optical microscopy, ductility dip cracking

Silver coated copper powders were prepared by a chemical reduction method with controlling the deposition process variables such as the feeding rate of the silver ionic solution and concentration of the reductants at room temperature. The characteristics of the products were evaluated by scanning electron microscope (SEM), X-ray diffractometer (XRD), atomic absorption spectrophotometer (AA) and a 4 probe resistivity measurement system. The optimum condition of the preparation of Ag coated Cu powders was at 0.05 M of potassium sodium tartrate and 2 ml/min of the feeding rate of the silver ionic solution. Our method successfully produced dense, uniform, and well-dispersed Ag coated Cu powder of 2~2.5 μm witha silver layer of 100~200 nm. Additionally, we found that thespecific resistivity of the 30 wt.% Ag coated Cu powder was similar to that of pure silver, so that the composite powder could be used as an alternative electromagnetic shielding material for silver.

keyword : Powder processing, Ag coated Cu powder, chemical reduction, specific resistivity, potassium sodium tartrate

We fabricated flanged outer races for wheel bearing units using 0.52%C clean steels, and then characterized and studied the transformation behavior. The outer races produced by hot forging and high frequency induction hardening in this study were analyzed through microstructural characterization using OM, SEM, TEM, and X-ray diffractometer and their microhardness depth profiles of the raceway contacted by balls were measured using MVH tester. The surface hardened layers with a uniform hardness profile in the raceway consisting of very fine martensite with sub-micron sized retained austenite could be formed for very short time during high frequency induction hardening after hot forging. The very fine martensite may be transformed on rapid cooling, from the inhomogeneous austenite nucleated on rapid heating in small particles of pearlitic cementite fragmentated by hot forging. On the other hand the sub-micron sized retained austenite may be chemically stabilized due to their extremely small size, from the small austenite nucleated at the grain boundaries.

keyword : 0.52%C steels, hot forging, high frequency induction hardening, transformation

ZnO thin films were deposited on Si(100) substrates at low temperatures (44℃~210℃) by atomic layer deposition using DEZn (diethyl zinc) and water as precursors. The film thickness was measured by ellipsometry calibrated with cross-sectional TEM. The phase formation, microstructure evolution, UV-absorbance, and chemical composition changes were examined by XRD, SEM, AFM, TEM, UV-VIS-NIR, and AES, respectively. A uniform amorphous ZnO layer was formed even at 44℃ while stable crystallized ZnO films were deposited above 90℃. All the samples showed uniform surface roughness below 3 nm. Fully crystallized ZnO layers with a band-gap of 3.37 eV without carbon impurities can be formed at substrate temperatures of less than 90℃.

keyword : ALD, thin films, crystallinity, deposition, X-ray diffraction

Metal interconnections of multilayer Al/Ni and TiW/seed-Ni/Ni were formed on glass, and the adhesion strength and nanoindentation response of the composite layers were evaluated. The Al/Ni multilayer was formed by an anodic bonding of glass to Al and subsequent electroless plating of Ni, while the TiW/Ni multilayer was fabricated by sputter deposition of TiW and seed-Ni onto glass and electroless plating of Ni. Because of the diffusion of aluminum into glass during the anodic bonding, anodically bonded glass/Al joint exhibited greater interfacial strength than the sputtered glass/TiW one. The Al/Ni on glass also showed excellent resistance against delamination by bending deformation compared to the TiW/seed-Ni/Ni on glass. From the nanoindentation experiment of each metal layer on glass, it was found that the aluminum layer had extremely low hardness and elastic modulus similar to the glass substrate and played a beneficial role in the delamination resistance by lessening stress intensification at the joint. The indentation data of the multilayers also supported superior joint reliability of the Al/Ni to glass compared to that of the TiW/seed-Ni/Ni to glass.

keyword : Thin films, sputtering, interfaces, indentation, anodic bonding