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One of the barriers limiting wide applications of magnesium alloys to various industries is their poor corrosion resistance. The corrosion properties of AZ91 magnesium alloy, which is the most popular magnesium casting alloy, are affected by microstructural and environmental factors. The corrosion properties of AZ91 magnesium alloy are affected by the corrosion properties of α-Mg and β phases, the volume fraction and distribution of β phase and area ratio of α-Mg/β phases. The corrosion properties of AZ91 magnesium alloy under various environments also change according to the passivity of films and types of corrosion products formed on its surface. The corrosion resistance of the magnesium alloys can be improved by microstructural control through the addition of alloying elements and optimization of the production process.

keyword : metals, casting, corrosion, magnesium alloys

Volume shrinkage behavior accompanying the cure of resin formulations might be a critical factor when assembly processes using polymer materials are considered. In this study, cure shrinkage behavior with respect to resin formulation type and heating method was measured on sandwich structure samples by a thermomechanical analyzer (TMA). Quartz, used as a cover material for the sandwich structure, indicated the coefficient of thermal expansion close to 0 ppm/℃. When a dynamic heating mode was conducted, a squeezeout region and a cross-linking region for each resin formulation could be separated clearly with overlapping differential scanning calorimeter results on the TMA results. In addition, a cure shrinkage dominant region and a thermal expansion dominant region in the cross-linking region were distinguished. Consequently, the degree of cure at the initiation of the thermal expansion dominant region was successfully measured. Measurement of all resin formulations indicated the thermal expansion behavior exceeded cure shrinkage before full cure.

keyword : cure shrinkage, adhesives, thermomechanical processing, thermodynamic properties, thermal analysis

Recently, because of safety and environmental concerns, there has been a tendency to introduce solid self-lubricating composites for bearing materials. In this paper, we developed Fe-Cr-C-Mn-Cu cast composite alloys as a self-lubricating composite and investigated the effect of carbon on the formation of protective tribofilms during sliding. The wear resistance of these materials was mainly affected by carbon concentrations due to the fact that in particular wear passed from delamination to tribo-oxidation, reducing the wear rate. The improved wear resistance likely resulted from protective tribofilms that formed on the surface during sliding.

keyword : metals, casting, wear, scanning electron microscopy, SEM, smearing

This study investigated changes in phase fraction caused by the addition of Mo, as well as the subsequent behaviour of N and its effect on the mechanical properties of welded 24Cr-N duplex stainless steel weld metals. Filler metal was produced by fixing the contents of Cr, Ni, N, and Mn while adjusting the Mo content to 1.4, 2.5, 3.5 wt%. The delta ferrite fraction increased as the Mo content increased. In contrast, the γ fraction decreased and changed from a round to an acicular shape. Secondary austenite (γ`) was observed in all specimens in a refined form, but it decreased as the Mo content increased to the extent that it was nearly impossible to find any secondary austenite at 3.5 wt% Mo. Both tensile and yield strengths increased with the addition of Mo. In contrast, the highest value of ductility was observed at 1.41 wt% Mo. At all temperatures, impact energy absorption showed the lowest value at 3.5 wt% Mo, at which the amount of -ferrite was greatest. There was no significant temperature dependence of the impact energy absorption values for any of the specimens. As the fraction of γ phase decreased, the amount of N stacked in the γ phase increased. Consequently, the stacking fault energy decreased, while the hardness of γ increased.

keyword : metal, welding, mechanical properties, scanning electron microscopy, SEM, impact test

In this study, a molybdenum disilicide (MoSi2) coating process was investigated by hydrogen reduction and Si-pack cementation. At first, the metallic Mo coating was carried out by hydrogen reduction of MoO3 powder at 750℃ for various holding times (1, 2, 3 h) in hydrogen atmosphere. A 4.3 ㎛thick metallic molybdenum thin film was formed at 3 h. MoSi2 was obtained by Si-pack cementation on molybdenum thin film through hydrogen reduction processing. It was carried out using Si : Al2O3 : NH4Cl = 5 : 92 : 3 (wt%) packs at 900℃ for various holding times (30, 60, 90 min) in Ar atmosphere. When the holding time was 90 min, a MoSi2 layer was coated successfully and a 15.4 ㎛thickness was observed.

keyword : thin films, coating, diffusion, scanning electron microscopy, SEM, hydrogen reduction

Alloy 617 is a Ni-base superalloy and a candidate material for the intermediate heat exchanger (IHX) of a very high temperature gas reactor (VHTR) which is one of the next generation nuclear reactors under development. The high operating temperature of VHTR enables various applications such as mass production of hydrogen with high energy efficiency. Alloy 617 has good creep resistance and phase stability at high temperatures in an air environment. However, it was reported that the mechanical properties decreased at a high temperature in an impure helium environment. In this study, high-temperature corrosion tests were carried out at 850℃-950℃ in a helium environment containing the impurity gases H2, CO, and CH4, in order to examine the corrosion behavior of Alloy 617. Until 250 h, Alloy 617 specimens showed a parabolic oxidation behavior at all temperatures. The activation energy for oxidation in helium environment was 154 kJ/mol. The SEM and EDS results elucidated a Cr-rich surface oxide layer, Al-rich internal oxides and depletion of grain boundary carbides. The thickness and depths of degraded layers also showed a parabolic relationship with time. A normal grain growth was observed in the Cr-rich surface oxide layer. When corrosion tests were conducted in a pure helium environment, the oxidation was suppressed drastically. It was elucidated that minor impurity gases in the helium would have detrimental effects on the high- temperature corrosion behavior of Alloy 617 for the VHTR application.

keyword : metals, high temperature corrosion, microstructure, scanning electron microcopy, SEM, helium

Hot tearing was the most significant casting defect when the castability evaluation of the Al- Zn-Mg-Cu alloy system was conducted. It was related to the solidification range of the alloy. Therefore, the hot tear susceptibility of the AA7075 alloy, whose solidification range is the widest, was evaluated. The hot tear susceptibility was evaluated by using a mold for a hot tearing test designed to create the condition for the occurrence of hot tear in 8 steps. According to the tearing location and shape, a hot tear susceptibility index (HTS) score was measured. The solidification range of each alloy and hot tear susceptibility was compared and thereafter the microstructure of a near tear defect was observed. As a result, the HTS of the AA7075 alloy was found to be 67. Also, the HTS in relation to a change in Zn, Mg, Cu composition showed a difference of about 6-11% compared to the AA7075 alloy.

keyword : aluminum alloy, hot tearing, casting, solidification

A feasible way to fabricate in-situ Al/TiC composites was investigated. An elemental mixture of Al-TiO2-C pellet was directly added into an Al melt at 800-920℃ to form TiC by self-combustion reaction. The addition of CuO initiates the self-combustion reaction to form TiC in 1-2 ㎛at the melt temperature above 850℃. Besides the CuO addition, a diluent element of excess Al plays a significant role in the TiC formation by forming a precursor phase, Al3Ti. Processing parameters such as CuO content, the amount of excess Al and the melt temperature, have affected the combustion reaction and formation of TiC, and their influences on the microstructures of in-situ Al/TiC composites are examined.

keyword : composites, self-propagating synthesis, casting, X-ray diffraction, TiC

Decalcomania is a new method for SOFCs (solid oxide fuel cells) unit cell fabrication. A tight and dense 5 ㎛Yttria-stabilized zirconia (8YSZ) electrolyte layer on anode substrate was fabricated by the decalcomania method. After 8YSZ as the electrolyte starting material was calcined at 1200℃, the particle size was controlled by the attrition mill. The median particle size (D50) of each 8YSZ was 39.6 ㎛, 9.30 ㎛, 6.35 ㎛, and 3.16 ㎛, respectively. The anode substrate was coated with decalcomania papers which were made by using 8YSZ with different median particle sizes. In order to investigate the effect of median particle sizes and sintering conditions on the electrolyte density, each sample was sintered for 2, 5 and 10 h, respectively. 8YSZ with a median particle size of 3.16 ㎛which was sintered at 1400℃ for 10 had the highest density. With this 8YSZ, a SOFCs unit cell was manufactured with a 5 ㎛layer by the decalcomania method. Then the unit cell was run at 800℃. The Open Circuit Voltage (OCV) and Maximum power density (MPD) was 1.12 V and 650 mW/cm2, respectively.

keyword : SOFC, YSZ, Yttria-stabilized Zirconia, decalcomania, electrolyte densification, electrical properties

Calcium phosphate ceramic composites, consisting of hydroxyapatite(HA) and tricalcium phosphate (TCP), were fabricated by solid state sintering in order to investigate the effect of their initial compositions on microstructural evolutions and biocompatibility. All the sintered calcium phosphate ceramics exhibited almost full densification, while the grain growth of the composites increased with an increasing TCP content in the green body. The TCP phase transformed into a Ca-deficient HA phase during sintering via the diffusion of calcium ions from the HA phase into the TCP phase. The phases formed in the composites significantly affected the biocompatibility of the composites. The HA-matrix ceramic composites with TCP had a better cellular response than the pure HA ceramics, presumably due to the newly formed Ca-deficient HA.

keyword : ceramics, sintering, diffusion, SEM, cellular response