발간논문

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This study was carried out to investigate the precipitation behavior of χ phase and effect of χ-phase which precipitated at the initial stage of aging on mechanical properties of 25%Cr-7%Ni-4%Mo-0.2%N super duplex stainless steel. χ-phase was precipitated mainly at the interface of ferrite/austenite phases and inside of the ferrite phase at the initial stage of aging, and it was transformed into σ-phase with an increase of aging time. The ferrite phase was decomposed into new austenite(γ2)phase and σ-phase by aging treatment. The hardness and tensile strength of the initial stage of aging when χ-phase was precipitated did not changed considerably, while elongation rapidly decreased. Accordingly, it is considered that χ-phase didn`t affect the hardness and strength significantly, but it affected the elongation.

keyword : super duplex stainless steel, impact toughness, χ phase, σ phase

Bake hardening steels have to resist strain aging to prevent the yield strength increment and stretcher strain during press process and to enhance the bake hardenability during baking process after painting. The bake hardening steels need to control the solute carbon and the solute nitrogen to improve the bake hardenability. Ti and/or Nb alloying for nitride and carbide precipitation and low carbon content below 0.003% are used to solve strain aging and formability problem for automotive materials. However, in the present study, the effect of micro-precipitation of copper sulfide on the bake hardenability and fatigue properties of extremely low carbon steel has been investigated. The bake hardenability of Cu-alloyed bake hardening (Cu-BH) steel was slightly higher (5 MPa) than that of Nb-alloyed bake hardening (Nb-BH) steel, but the fatigue limit of Cu-BH steel was far higher (45 MPa) than that of Nb-BH steel. All samples showed the ductile fracture behavior and some samples revealed distinct fatigue stages, such as crack initiation, stable crack growth and unstable crack growth.

keyword : metals, annealing, fatigue, transmission electron microscopy, TEM, BH steel

To improve the low ductility and high strain-rate sensitivity in Sn-Bi based solder alloys, the influences of the minor additions of alloying elements (Ag, Mn, In) were investigated. The strain-stress curves of various Sn-40Bi(-X) alloys, including a pre-suggested Sn-40Bi-0.1Cu composition were measured using a tensile testing machine. As a result, the elongation and ultimate tensile strength (UTS) values were compared. The small addition (0.5 wt.%) of Ag significantly enhanced the ductility and high strain-rate sensitivity of the alloys at strain rates of 10(-4) to 10(-2) s-1 mainly due to the increase and refinement of eutectic lamellar structures. The microstructure change increased the area of grain boundaries, thus ameliorating the grain boundary sliding mode. It was also found that Mn is an effective element in enhancing the ductility, especially at the strain rates of 10(-3) to 10(-2) s-1. The enhancement is likely attributed to the fine and homogeneous microstructure in the alloys containing Mn.

keyword : Sn-Bi solder, alloys, soldering, ductility, tensile test

The cold spray process is an emerging technology that utilizes high velocity metallic particles for surface coating. Metallic powder particles are injected into a converging-diverging de Laval nozzle and accelerated to a high velocity by a supersonic gas flow. The cold spray process normally uses a circular nozzle that has a rather narrow spraying range. To overcome this fault, a slit nozzle was considered in this study. The slit nozzle is anticipated to reduce the coating process time because it has a wider coating width than the circular nozzle. However, the slit nozzle can reduce the coating efficiency because it does not allow as much gas and particle velocity as the circular nozzle. To improve the coating efficiency of a slit nozzle, the shape of the slit nozzle was modified. And the results of gas flow and particle behaviour according to the nozzlers shape were compared by the a numerical analysis. As a results, as Expansion Ratio(ER) of 7.5 was found to be the most optimal condition for enhancing the spraying efficiency when the ER was changed by the variation of nozzle neck and exit size.

keyword : alloy, cold spraying, numerical simulation, slit nozzle, critical velocity

Nanopowder of Fe2O3, Al, Cr and Si was fabricated by high energy ball milling. A dense nanostuctured A2O3 and 6.06Fe0.33Cr0.16Al0.23Si0.29 composite was simultaneously synthesized and consolidated using high frequency induction heated sintering method within 1 minute from mechanically activated powders of Fe2O3, Al, Cr and Si. The grain sizes of Al2O3 and Fe0.33Cr0.16Al0.23Si0.29 in composite are 80 and 18 nm, respectively.

keyword : mechanical alloy, TiCrAlSi, nanomaterials, hardness, rapid sintering

The effect of shearing speed on the shear force and energy of Sn-1.0Ag-0.5Cu solder ball was investigated. Various UBM (under bump metallurgy)`s on Cu pads were used such as ENEPIG (Electroless Nickel, Electroless Palladium, Immersion Gold; Ni/Pd/Au), ENIG (Electroless Nickel, Immersion Gold; Ni/Au), OSP (Organic Solderability Preservative). To fabricate a shear test specimen, a solder ball, 300㎛in diameter, was soldered on a pad of FR4 PCB (printed circuit board) by a reflow soldering machine at 245℃. The solder bump on the PCB was shear tested by changing the shearing speed from 0.01 m/s to 3.0 m/s. As experimental results, the shear force increased with a shearing speed of up to 0.6 m/s for the ENIG and the OSP pads, and up to 1.0 m/s for the ENEPIG pad. The shear energy increased with a shearing speed up to 0.3 m/s for the ENIG and the OSP pads, and up to 0.6 m/s for the ENEPIG pad. With a high shear speed of over 1.0 m/s, the ENEPIG showed a higher shear force and energy than those of the ENIG and OSP. The fracture surfaces of the shear tested specimens were analyzed, and the fracture modes were found to have closer relationship with the shear energy than the shear force.

keyword : electrical/electronic materials, soldering, mechanical properties, scanning electron microscopy, SEM, shear test

This study describes the feasibility of producing high-strength Al alloy sheet with a high solute content using a combined technique of twin-roll strip casting and asymmetric rolling. The Al sheet produced in this study exhibited excellent formability (r=1.0, Ar=0.16) and mechanical properties (σTS~305 MPa, ε~33%), that, cannot be feasibly obtained via the conventional technique based on ingot casting and rolling. The structural origin of the observed properties, especially enhanced formability, was clarified by examining the evolution of textures associated with strip casting and subsequent thermo-mechanical treatments. Our evaluation of the mechanical properties and formability leads us to conclude that the combination of strip casting and asymmetric rolling is a feasible process for enhancing the formability of Al alloy sheets to the level beyond what the conventional techniques can reach.

keyword : Al alloy, asymmetric rolling, strip casting, mechanical properties, textures

The basic properties and electrical characteristics of sputtering films deposited with a commercial cast target and Spark Plasma Sintering (SPS) were compared and analyzed. From the results, the Cu film prepared heating 60℃/min (SPS process) showed a similar specific resistance compared to the commercial cast Cu film. Also, auording to the results of XRD, SIMS, and TEM. There was not much difference in the crystal structure and impurities between the two films. Consequently, the SPS Cu target was found to have quite similar properties with the commercial one and it is expected to be applied in futare research to the metal wiring material for semiconductor/display devices.

keyword : Cu target, sputtering film, spark plasma sintering, evaluation

We have synthesized Al-Co/AlN-Co multilayer films with different layer thicknesses by using a Two-Facing Target Type dc Sputtering (TFTS) system. The deposited films were annealed isothermally at different temperatures and their microstructure, magnetic properties and resistivity were investigated. The magnetization of the as-deposited films was found to be very small, irrespective of layer thickness. It was found that the annealing conditions and layer thickness ratio of Al-Co to AlN-Co (LTR) were able to control the microstructure, as well as the physical properties of the prepared films. The resistivity and magnetization increased and the coercivity decreased with a decreasing LTR. A high resistivity of 2500 μΩ-cm, a magnetization of 360 emu/cm3, and a coercivity of 5Oe were obtained for the films with LTR=0.175.

keyword : thin films, sputtering, magnetic properties, X-ray diffraction, resistivity

Hydrogen and hydrogen energy have been recognized as clean energy sources and high energy carrier. Mg and Mg alloys are attractive hydrogen storage materials because of their lightweight and low cost materials with high hydrogen capacity (about 7.6wt.%). However, the commercial applications of the Mg hydrides are currently hinder by its high absorption/desorption temperature, and very slow reaction kinetics. However, Ti and Ti based hydrogen storage alloys have been thought to be the third generation of alloys with a high hydrogen capacity, which makes it difficult to handle because of high reactivity. One of the most methods to develope kinetics was addition of transition metal. Therefore, Mg-Ti-Cr-Nb alloy was fabricated to add TiCrNb by hydrogen induced mechanical alloying. TiCrNb systems have included transition metals, low operating temperatures and hydrogen storage materials. As-received specimens were characterized using X-ray Diffraction analysis (XRD), Scanning Electron Microscopy (SEM) and Thermo Gravimetric analysis/Differential Scanning Calorimetry (TG/DSC). Mg-TiCr10Nb systems were evaluated for hydrogen kinetics by Sievert`s type Pressure-Composition-Isotherm (PCI) equipment. The operating temperature range was 473, 523, 573 and 623 K.

keyword : hydrogenation, mechanical alloy, Mg-5wt.%TiCr10Nbx(x=1, 3, 5), kinetics, composite