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This study is concerned with the influence of strain rate on the plastic deformation behavior and microstructure variables of AZ31 magnesium alloy sheet. The tensile properties were measured at room temperature at a strain rate of 0.001 and 100 s^-1 using a universal tensile testing machine and servo-hydraulic high-speed tensile test machine respectively. The microstructure was observed at strains of 5, 10, 15%, produced by tensile elongation. As the strain rate increased, the flow stress and the yield strength increased. The EBSD analysis showed that a texture of the (0001) basal plane was formed parallel to the rolled plate and texture was formed at random in the (10□0) non-basal plane. Double twinning was not observed and extension and contraction twinning were observed at all strain rate levels. The fraction of twin boundaries were increased as strain rate increased. TEM analysis determined that the dislocation density of the specimen strained by 1% at a strain rate of 100 s^-1 was higher than that of a strain rate of 0.001 s^-1. In addition, the < c+a >dislocation was found at all strain rate levels. These results show that the deference in elongation of the AZ31 Mg alloy is dependent on strain rate. (Received October 4, 2019; Accepted March 18, 2019)

keyword : AZ31 magnesium alloy, high-speed tensile test, EBSD analysis, texture, TEM analysis, pyramidal slip

In this study, we characterize the viscoelastic behavior of polymers using nanoindentation. We applied the indentation representative stress approach and elastic solution to the Maxwell model and determined that there was a linear relationship between the inverses of the initial unloading stiffness and indentation unloading rate. From nanoindentation tests with various unloading rates on poly(dimethylsiloxane), the linear relationship between the indentation unloading rate and the initial unloading stiffness was confirmed. We have suggested two parameters such as, elastic coefficient and viscous coefficient, represent the viscosity and elasticity of the polymer material based on their relation. In order to check the dependency of the elastic and viscous coefficients on mechanical properties, we performed nanoindentation on poly(dimethylsiloxane) with different crosslinking densities by mixing different proportions of curing agent. The viscosity and elasticity depend on the crosslinking density of polymer, and it was confirmed that the elastic coefficient and viscous coefficient obtained from nanoindentation varied with the same trend depending on crosslinking density. (Received November 14, 2018; Accepted March 27, 2019)

keyword : nanoindentation, viscoelasticity, PDMS, crosslinked polymer, Maxwell model

This study investigated the microstructure and compressive properties (at room & high temperatures) of Fe-Cr-Ni based AISI 316L alloy, manufactured by selective laser melting process. The effect of stress relieving heat treatment on the microstructure and mechanical properties also examined. Regardless of the stress relieving heat treatment, the SLMed AISI 316L alloy exhibited typical molten pools and fine columnar structures which grew along the laser heat source. A cellular type dendrite structure was also observed inside the molten pool. After heat treatment, the low angle boundary fraction decreased and the high angle boundary fraction increased in the SLMed AISI 316L alloy. In the 25 ℃, 500 ℃, 700 ℃, 900 ℃ compressive results, the heat treated 316L alloy showed lower yield strengths than the as fabricated alloy at all temperature conditions. However, the difference in yield strength between the as fabricated and heat treated alloys gradually decreased with increasing temperature. Surface and cross-sectional fractographies showed that the heat treated alloy accommodated more plastic deformation. This was considered the cause of the more pronounced work hardening of the heat treated alloy, as identified in the stress versus strain curves. The correlations between microstructure, temperature dependent mechanical properties and stress relieving heat treatment were also discussed based on these findings. (Received February 12, 2019; Accepted April 12, 2019)

keyword : selective laser melting, AISI 316L, heat treatment, microstructure, compressive test, high temperature

In this study, Binderless-WC hard materials were fabricated using the pulsed current activated sintering (PCAS) process for a Friction Stir Welding tool and difficult-to-cut tool application. Tungsten carbide (WC) hard materials are used in various industries and possess a superior hardness compared to other hard materials. They have particularly high melting points, high strength, and abrasion resistance. 100 mm diameter and 5 mm thick binderless-WC hard materials were fabricated using a 30,000 A pulsed current activated sintering machine and 0.3 to 0.5 μm size WC powders. Variation of properties in binderless WC hard materials by sintering temperature were increased slightly for 0.11 to 0.37 um with a grain size and densified completely for 70.0 to 99.5% for a relative density. Consequently, these materials were almost completely dense with a relative density of up to 99.5% after simultaneous application of 60 MPa pressure and an electric current for 2 min at 1600 oC, almost without any significant change in the crystallite size. The average WC crystallite size that was produced through PCAS was 0.37 μm at 1,600 °C. In terms of mechanical properties, considering only the densified sintered body that the hardness and fracture toughness of binderless-WC hard materials were about 2,661.5 kg/mm² and 3.90 MPa·m^1/2, respectively. (Received December 14, 2018; Accepted March 27, 2019)

keyword : pulsed current activated sintering process, binderless-WC, hardness, fracture toughness, rapid sintering

Aluminum nitride (AlN) has several attractive properties including high thermal conductivity, excellent electrical insulation, a thermal expansion coefficient close to that of Si and low density. Accordingly, it is considered a promising packing material and substrate for high power integrated circuits. However, its low fracture toughness at room temperature limits its wide industrial application. To improve its mechanical properties, the approach generally utilized has been to fabricate and add nanostructured materials as a second phase, to make composites. In this study highly dense nanostructured AlN and AlNBN composites were sintered for two minutes at 1400 ℃. The effect of BN on the mechanical properties (fracture toughness and hardness) and the microstructure of the AlN-BN composites was investigated. The hardness and fracture toughness of AlN, AlN-1 vol% BN, AlN-3 vol% BN, AlN-10 vol% BN were 943, 1455, 1147, 1110 kg/mm² and 4, 5.7, 5.5, 5.4 MPa·m^1/2, respectively. The addition of BN to AlN simultaneously improved the hardness and fracture toughness of the AlN-BN composite by deterring crack propagation. This study demonstrates that BN can be an effective reinforcing material to improve the fracture toughness and hardness of AlN composites. (Received January 7, 2019; Accepted January 31, 2019)

keyword : nanomaterials, sintering, composite, mechanical properties

We demonstrated an effective method for fabricating a highly efficient flexible transparent electrode in an oxide/metal/oxide configuration based on an ultrathin Cu film. It exhibited low losses in optical transmittance and electrical conductivity while minimizing current loss due to pinhole-related leakages. The Cu film was developed on a chemically heterogeneous ZnO film into a completely continuous and pinhole-free layer with a reduced thickness of approximately 6 nm. This was accomplished using a simple, but highly effective, room-temperature reactive sputtering technique with precisely controlled oxidation of Cu. The pinhole-free morphology of the ultrathin Cu_(O) film is attributed to the dramatically improved wetting ability of Cu_(O) in the presence of a trace amount of oxygen (ca. 2-3 at%). The synthesis of a completely continuous, ultrathin Cu_(O) film in an oxide/metal/oxide configuration, consisting of the Cu_(O) film is sandwiched between top TiO₂ and bottom ZnO films on a polymer substrate, for making a flexible transparent electrode with excellent transparency and no notable current leakage. The superior optoelectrical performance of the TiO₂/ Cu_(O)/ZnO electrode clearly exceeded that of the same geometric configuration using a pure Cu film of the same thickness. The optimized electrode exhibited an average transmittance of 80.8% in the spectral range of 400- 800 nm and a sheet resistance of 13 Ω sq^-1. The proposed TiO₂/Cu_(O)/ZnO electrode was proven as a promising alternative to ITO, and demonstrated excellent mechanical flexibility on flexible polymer substrates. (Received March 18, 2019; Accepted March 28, 2019)

keyword : transparent electrodes, flexible substrates, oxide/metal/oxide, copper, pinholes, current leakage

ZnO/Ag/SnO₂ (ZAS) tri-layer films were prepared on glass substrates via RF and DC magnetron sputtering, and then the influence of the thickness of the ZnO and SnO₂ layers on the optical and electrical properties of the ZAS films was investigated. As deposited ZnO 50 nm/Ag 10 nm/SnO₂ 50 nm films showed a higher figure of merit, 1.08 × 10^-2 Ω^-1, than the other films due to a high visible transmittance of 80.8% and a low resistivity of 1.21 × 10^-4 Ωcm. From the observed results, it can be concluded that the ZnO 50 nm/Ag 10 nm/SnO₂ 50 nm tri-layer films can be used as a substitute for conventional transparent conducting oxide films in various opto-electrical applications. (Received March 20, 2019; Accepted April 4, 2019)

keyword : ZnO/Ag/SnO₂, thin film, x-ray diffraction, electrical properties, optical properties

Tetrahedrite Cu₁₂Sb₄S₁₃ has low lattice thermal conductivity because of the lone-pair electrons of Sb, which cause the Cu atoms to vibrate at a low frequency and high amplitude. When the Cu atoms of Cu₁₂Sb₄S₁₃ are partially substituted with a transition metal, changes in the Cu vibrations can intensify phonon scattering, thereby enhancing the thermoelectric properties. The synthesis of tetrahedrite compounds by conventional melting methods requires sophisticated reactions because the S element vaporizes at low temperature and its homogenization requires a long time. However, a homogeneous and solid-state synthesis can be carried out in a short time using mechanical alloying, because the volatilization of the constituent elements is inhibited and the subsequent heat treatment is not necessary. In this study, Zn-doped Cu_12-xZn_xSb₄S₁₃ tetrahedrites were prepared by mechanical alloying (MA) and hot pressing (HP), and their electronic transport and thermoelectric properties were examined. A single tetrahedrite phase was successfully obtained by the MA-HP process without subsequent heat treatment. The lattice constant increased with the Zn content, confirming that Zn was substituted for Cu. Zn doping led to p-type conduction characteristics; however, it did not effectively increase the power factor. However, the thermal conductivity showed a marked decrease upon Zn doping, due to decrease in carrier contribution. A dimensionless figure of merit of 0.76 was obtained at 723 K for Cu_11.6Zn_0.4Sb₄S₁₃. (Received February 27, 2019; Accepted March 21, 2019)

keyword : thermoelectric, tetrahedrite, mechanical alloying, hot pressing, doping