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The effects of the ordering reaction on high temperature mechanical behavior is investigated by tensile tests at 2×10-2/s-3.3×10-5/s up to 745˚C. The tensile deformed region is examined by differential scanning calorimeter (DSC), TEM, and high resolution neutron diffraction (HRPD). The results showed that a plateau of tensile strength appeared at 150-500˚C whereas the elongation minimum occurred at about 600˚C. This suggests that the occurrence of a plateau does not cause the elongation minimum. The temperature of the elongation minimum decreases with the strain rate, HRPD results show a lattice contraction in the tensile deformed specimen at the temperature of the plateau occurring region. The plateau of tensile strength, the lattice contraction, and the occurrence of serration appeared in the same temperature region.

keyword : Alloy 600, ordering reaction, strain induced ordering, lattice contraction, serration

Abstract: In this study, the effect of aging treatment on the microstructure and tensile properties of AZ61-xPd (x = 0, 1 and 2 wt %) alloys were investigated. The microstructure of as-cast AZ61-xPd alloys mainly consisted of α-Mg, Mg17Al12 and Al4Pd phases. After solution treatment, most of the Mg17Al12 phases were dissolved into the Mg matrix. Thereafter, Mg17Al12 phases were finely formed and distributed near thermally stable Al4Pd phases and inside the grains through aging treatment at 220℃ during 88 hours. With the aging at 220℃, the peak aged AZ61-xPd alloys showed higher hardness than as-cast and solution treated AZ61-xPd alloys. In particular, the AZ61-1Pd alloy was optimized due to refined Mg17Al12 and Al4Pd phases. Further, the peak aging time was reduced with increasing Pd addition (>1 wt %). Tensile strength was increased by Pd addition at 25℃, 150℃, both as-cast and peak aged AZ61-xPd alloys. After aging treatment, room and high temperature tensile strength were increased more than the as-cast specimens. The AZ61-1Pd alloy especially showed the largest strength increase range. Elongation was decreased with addition Pd at 25℃ and 150℃.

keyword : Magnesium alloy, aging, microstructure hardness, tensile test

This paper evaluates the reliability of creep rupture life under service conditions of Alloy 617, which is considered as one of the candidate materials for use in a very high temperature reactor (VHTR) system. A Z-parameter, which represents the deviation of creep rupture data from the master curve, was used for the reliability analysis of the creep rupture data of Alloy 617. A Service-condition Creep Rupture Interference (SCRI) model, which can consider both the scattering of the creep rupture data and the fluctuations of temperature and stress under any service conditions, was also used for evaluating the reliability of creep rupture life. The statistical analysis showed that the scattering of creep rupture data based on Z-parameter was supported by normal distribution. The values of reliability decreased rapidly with increasing amplitudes of temperature and stress fluctuations. The results established that the reliability decreased with an increasing service time.

keyword : Creep, Z parameter, Larson-Miller parameter, creep life, Monte-Carlo simulation

This study investigated the effect of water absorption on the tensile properties of carbon-glass/ epoxy hybrid composites at room temperature and -30℃. To investigate the effect of the position of glass fabric in the hybrid composite on the tensile properties, the stacking pattern of the fiber fabrics for reinforcing was created in three different ways: (a) glass fabrics sandwiched between carbon fabrics, (b) carbon fabrics sandwiched between glass fabrics and (c) alternative layers of carbon and glass fabrics. They were manufactured by a vacuum-assisted resin transfer molding (VARTM) process. The results showed that there was surprisingly little difference in tensile strength at the two different temperatures with dry and wet conditions. However, the water absorption into the hybrid system affected the tensile properties of the hybrid composites at RT and -30℃. When the glass fabrics were at the outermost layers, the hybrid composite had the lowest tensile properties. This is attributed to the fact that the composite had a relatively high water absorption rate.

keyword : Hybrid composites, water absorption, mechanical properties

Abstract: The aim of this study was to analyze the effect of the cooling rate after heat treatment on the microstructure and mechanical properties of 2507 duplex stainless steels. Heat treatment was carried out at 1050℃ for 1 hr, followed by controlled cooling. The cooling rates were 175.6 × 10-3℃/s, 47.8 × 10-3℃/s, 33.3 × 10-3℃/s, 16.7 × 10-3℃/s, 11.7 × 10-3℃/s, 5.8 × 10-3℃/s and 2.8 × 10-3℃/s, which resulted in variations of the microstructure, such as the fractional change of the ferrite phase and sigma phase formation. Fatigue, hardness, impact and tensile tests were performed on the specimens with different cooling rates. The precipitation of the σ phase caused a hardness increase and a sharp decrease of toughness and tensile elongation. The fatigue limit of the sample with a cooling rate of 5.8 × 10-3℃/s was 26MPa higher than that of the sample with a cooling rate of 175.6 × 10-3℃/s. Our observations of the fracture surface confirmed that the higher fatigue resistance of the specimen with a cooling rate of 5.8 × 10-3℃/s was caused by the delay of the fatigue crack growth, in addition to higher yield strength.

keyword : Duplex stainless steel, fatigue test, heat treatment, cooling rate, sigma phase, annealing

The Kroll process for magnesium reduction of titanium tetrachloride is used for mass production of titanium sponge. The present study was conducted in a lab scale reactor to develop a better understanding of the mechanism of titanium sponge formation in the Kroll reactor with respect to reaction degrees and the feeding rate of TiCl4. The MgCl2 produced during the initial stage of the reaction was not sunk into the molten magnesium, but covered the surface of the molten magnesium. As a result, subsequently fed TiCl4 reacted with Mg exposed on the edge of molten MgCl2 in the crucible. Therefore, titanium sponge grew toward the center of the crucible from the edge. The temperature of the molten magnesium increased remarkably with the increasing feeding rate of TiCl4. Consequently, fed TiCl4 reacted at the upper side of the crucible with evaporated Mg, and produced titanium on the upper surface of the crucible wall, which increased considerably with the feeding rate of TiCl4.

keyword : Kroll process, TiCl4, titanium sponge, magnesium, reduction

The reduction behavior of low level oxides such as (T.Fe), (MnO) and (P2O5) in molten EAF slag was investigated using commercial reductants. In an air atmosphere, the slag volume increased and the reduction rate of the slag was very low due to the oxidation loss of reductants by oxygen in the air. The reduction rate of the slag was also low when a commercial reductant was used alone in an Ar gas atmosphere. The reason is probably because the material transfer through the interface between the slag and reductant is difficult due to the formation of high melting point oxide. When reductants were mixed with burnt lime in order to form low melting point reaction products, the reduction rate of the slag increased up to the range of 45-70%. By using the mixtures of reductants and burnt lime so as to form a low melting point slag at the reaction end, the reduction rate of the slag was improved up to 60-85%.

keyword : Slag, oxides, reduction, reduction rate, reductant

The uniform and highly smooth nanofibers of biocompatible poly (ε-caprolactone) (PCL) composited with different contents of multiwalled carbon nanotubes (MWCNTs) were successfully prepared by electrospinning. Experimental parameters were MWCNTs addition to a PCL solution and applied voltages. The topographical features of the composite nanofibers were characterized by scanning electron microscopy and its electrical properties were measured by a four-point probe method. The surface resistance gradually decreased with an increasing content of MWCNTs in PCL fibers because of the excellent electrical conductivity of MWCNTs. The nanofiber diameter could be regulated by varying the solution viscosity and voltages. Our results establish that this kind of electrospinning PCL/MWCNTs nanofibers with the control of fiber diameter and electrical conductivity may be a promising candidate for the application of scaffolds in tissue engineering.

keyword : Nanostructured materials, electrical properties, scanning electron microscopy, spray, electrospinning

Mg-x wt% Fe2O3-y wt% Ni samples were prepared by reactive mechanical grinding in a planetary ball mill, and their hydrogen-storage properties were investigated and compared. Activations of Mg-5Fe2O3-5Ni was completed after one hydriding (under 12 bar H2) - dehydriding (in vacuum) cycle at 593 K. At n = 2, Mg-5Fe2O3-5Ni absorbed 3.43 wt% H for 5 min, 3.57 wt% H for 10 min, 3.76 wt% H for 20 min, and 3.98 wt% H for 60 min. Activated Mg-10Fe2O3 had the highest hydriding rate, absorbing 2.99 wt% H for 2.5 min, 4.86 wt% H for 10 min, and 5.54 wt% H for 60 min at 593 K under 12 bar H2. Activated Mg-10Fe2O3-5Ni had the highest dehydriding rate, desorbing 1.31 wt% H for 10 min, 2.91 wt% H for 30 min, and 3.83 wt% H for 60 min at 593 K under 1.0 bar H2.

keyword : Hydrogen absorbing materials, mechanical alloying/milling, microstructure, X-ray diffraction, Fe2O3 addition

The effect of thermal annealing on the in-situ growth characteristics of intermetallics (IMCs) and the mechanical strength of Cu pillar/Sn-3.5Ag microbumps are systematically investigated. The Cu6Sn5 phase formed at the Cu/solder interface right after bonding and grew with increased annealing time, while the Cu3Sn phase formed at the Cu/Cu6Sn5 interface and grew with increased annealing time. IMC growth followed a linear relationship with the square root of the annealing time due to a diffusion-controlled mechanism. The shear strength measured by the die shear test monotonically increased with annealing time. It then changed the slope with further annealing, which correlated with the change in fracture modes from ductile to brittle at a critical transition time. This is ascribed not only to the increasing thickness of brittle IMCs but also to the decreasing thickness of the solder, as there exists a critical annealing time for a fracture mode transition in our thin solder-capped Cu pillar microbump structures.

keyword : Intermetallics, metals, soldering, diffusion, microbumps