발간논문

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Extensive studies of the metastable structures in beta Ti alloys have investigated the ambiguity of the unstable phase. Their microstructural characteristics have been observed to have a modulated structure, and spot satellites of diffraction in TEM, however the formation mechanisms have not been successfully interpreted because of the resolution limit of the nano lattice structure. High resolution TEM was performed to observe the metastable nano structure and for analysis of the relationship between the nano lattice and spot satellites. A metastable structure was formed just before α precipitation by overlapping a new lattice onto the β matrix, inducing a modulated structure as a kind of Moire fringe. The new lattice structure of the 3a phase had 3 times the distance of the β lattice parameter and had lattice relationships with the β/ω/3a phases of β(112)/ ω(1100)/3a[⅓(112)] and β(222)/ ω(1101)/3a[⅓(222)]. It was unreasonable to interpret the 3a phase as a definite precipitation of the ω phase, because only the limited zone axes of <113> and <110> were present in the TEM diffraction results of the metastable structures. Other zone axes, such as <100> or <111> etc., were not able to break the distinct metastable features of the spot satellites. Rather than precipitation, the 3a phase was activated by the slip of <113>/(112) and <110>/(112) in the TEM diffraction. It exhibited 3 times the distance of the β lattice parameter, which could not be achieved by typical precipitation. (Received August 30, 2016; Accepted January 3, 2017)

keyword : metastable beta Ti, omega phase, spot satellite, 3a lattice parameter, slip, lattice relationship

The effects of alloying elements, especially Cr and Si, and transformation temperature on the microstructures and mechanical properties of hyper-eutectoid steels were investigated. Increases in Si and Cr changed the morphology of cementite in upper bainite, while the morphology of pearlite seemed to be unchanged by the amount of alloying elements, except for variation in interlamellar spacing. Increasing Si and Cr content also caused increases in tensile strength and a reduction of area of the steels containing pearlitic microstructure, while decreasing tensile strength and reduction of area in steels transformed into upper bainitic microstructure. The tensile strength of steels consisting of pearlite, upper bainite and a mixture of pearlite and upper bainite, can be expressed with the equation The values calculated using the above equation coincided well with the measured tensile strength of hyper-eutectoid steels containing Cr and Si. (Received December 1, 2016; Accepted February 7, 2017)

keyword : isothermal transformation, hyper-eutectoid steel, pearlite, upper bainite

A feasibility study on microstructure design has been carried out to improve creep rupture life in alumina-forming austenitic (AFA) Fe-14Cr-20Ni-2.3Al stainless steel with and without 2 wt% W addition. After creep tests under 780 °C/80 MPa conditions, it was observed that internal cracks initiated primarily from grain boundaries in the W-free steel while voids nucleating at primary coarse NbC carbides elongated into fractures in the W-added steel. This result indicates that the W addition shifts the fracture mode from intergranular to transgranular. Based on this observation, the fine intragranular NbC carbides, which need to precipitate as much as possible during the initial stage of creep, can be a promising metallurgical factor for enhancing the creep resistance of both the AFA steels. Cold work at 5% strain prior to creep accelerated the heterogeneous precipitation of nano-sized intragranular NbC during the early stage of creep, which was responsible for the strong hardening effect by pinning the dislocations. The creep life was found to be 3 times longer as compared to the unpre-strained AFA steels. Applying 5% cold work induces the generation of dislocations in the matrix, which can lead to the nucleation of nano-scale NbC without recrystallization. The intragranular NbC carbide showed excellent resistance to coarsening during creep: their size was as small as around 30 nm after creep rupture. (Received Nobember 10, 2016; Accepted February 7, 2017)

keyword : alloys, cold working, creep, transmission eletron microscopy (TEM), AFA stainless steel

The hot deformation behavior of as-cast and homogenized Fe-22Cr-9.5Mn-3.3Ni stainless steel was investigated over a temperature range between 600~1,250 ℃. The deformability of the as-cast Fe-22Cr-9.5Mn-3.3Ni steel decreased with increasing temperature up to 850 ℃, and then increased at temperatures between 900 and 1200 ℃ caused by dynamic recrystallization at high temperatures above 900 ℃. At temperatures between 800 and 900 ℃, the discontinuous precipitation of Cr₂N and an decrease in ductility were observed in the as-cast steel. The ductility minimum at 850 ℃ in the as-cast Fe-22Cr-9.5Mn-3.3Ni could be associated with cracking at the interface of the lamellar chromium nitrides. The presence of fatal cracking at 1,250 ℃ and the loss of hot ductility can be attributed to the partial melting at austenite grain boundaries at high temperatures. (Received November 10, 2016; Accepted January 31, 2017)

keyword : high temperature strength, elongation, casting, austenite stainless steel, dynamic recrystallization

A new Ni based superalloy is considered as bucket material because of high fatigue resistance with fine grain. However a bucket is usually exposed to high temperature for a long time, so creep rupture tests were conducted to understand the microstructure evolution during creep deformation under various stresses at temperatures of 566, 593, 621, 649 and 670 ℃. The hardness applied to the crept specimen of these alloys increased until the value of LMP reached 25, and decreased while the value of LMP exceeded 25. The microstructure analysis with TEM, SEM and OM were conducted on ruptured specimen to observe the changes of γ″ morphology, carbides, and fracture mode. As a result of investigating microstructural changes and creep properties, it can be concluded that the effect of grain size on creep properties is dominant regardless of the size of γ″. (Received December 5, 2016; Accepted January 14, 2017)

keyword : alloys, forging, creep, microstructure, TEM

Pt-functionalized RGO samples were prepared and subsequently irradiated with 2-MeV electron beams (e-beams). The effect of e-beam dose on the structural, electrical, and gas sensing properties of the samples was investigated. E-beam irradiation was found to induce significant microstructural changes in the Pt-functionalized RGO. Measurements of NO₂ sensing at room temperature showed that the dose of 100 kGy led to better NO₂ sensing properties compared to those of unirradiated sensors. On the other hand, the dose of 500 kGy appeared to be excessive, leading to extensive damage of the RGO structure and a significant reduction in the response to NO₂ gas. E-beam irradiation can be beneficial for improving the gas sensing of Pt-functionalized RGO, but its dose needs to be optimized to obtain the best sensing properties. (Received November 1, 2016; Accepted December 19, 2016)

keyword : reduced graphene oxide, gas sensor, E-Beam irradiation, Pt nanoparticles, NO₂.

All-solid-state Li-S batteries were fabricated with lithium silicide anodes (Li_4.4Si), sulfide-based glass electrolyte (Li₂S-P₂S₅), and a sulfur composite cathode. The sulfur based composite cathode was prepared using two methods; (Case 1) mortar-mixing and (Case 2) planetary ball milling at a rotation speed of 370 rev/min for 15 min. SEM analysis results indicated that the `Case 2` method improved the distribution of sulfur, carbon and solid electrolyte particles, with a reduction in particle size. This effect was reflected in impedance spectra, and eventually in charge-discharge capacity: the electrode/electrolyte interface resistance was reduced so that the average charge-discharge capacities increased with the `Case 2` cathode. Thus, the present work indicates the method of preparing the sulfur composite cathode profoundly affects the performance of solid state Li-S batteries. (Received December 19, 2016; Accepted February 24, 2017)

keyword : all-solid-state batteries, lithium-sulfur batteries, sulfur composite cathode

Silver nanowire (AgNWs) networks offer a promising approach for producing cost effective transparent electrode materials. However, high temperature annealing is necessary to achieve low sheet resistance, which limits the use of AgNWs for low temperature processing applications such as flexible electronics, and perovskites solar cell fabrication. This study shows that the low temperature processing of efficient AgNW electrodes can be achieved with the choice of an appropriate dispersion solvent. Ethanol shows promising behaviors over IPA by forming uniform percolation networks even without thermal annealing. The key factors affecting percolation uniformity are the solubility of organic contaminants and the viscosity of the solvent which determines the droplet sizes. This study contributes to future studies of transparent conductors and flexible devices. (Received December 13, 2016; Accepted February 11, 2017)

keyword : transparent conductors, silver nanowires, spray coating, dispersion solvent, annealing

Casting defects such as gas traps and oxide particles must be removed in the die-casting process in order to obtain good cast products. Experimental research is costly and time-consuming, so computer simulations are typically performed in advance and then compared to experimental results. In this paper, a magnesium alloy die-casting process for a thin-walled mobile phone housing with 0.4mm thickness was investigated. The study examined the effects of important parameters such as overflow geometry and injection speed on surface defect and solid fractions. The overflow shape and second plunger speed were determined by simulations and experiments using an actual manufacturing process. In addition, the simulations predicted in this study showed in good agreement with the experimental results. (Received December 13, 2016; Accepted February 22, 2017)

keyword : diecasting, simulation, magnesium, overflow, porosity

Heat transfer plays a critical role in the fabrication of high efficiency electronic devices. Double-layer clad metals consisting of stainless steel (STS) and aluminum (Al) alloy were fabricated by the roll bonding process, and their thermal conductivity was calculated using measurements of density, specific heat, and thermal diffusivity obtained by light flash analysis. The microstructure at the interface between the STS and Al plates was characterized by scanning electron microscopy. The results revealed a big difference in thermal conductivity along the thickness direction compared to the value obtained from the series model. The difference was due to the interface characteristics of the double-layer samples. In particular, the measured thermal conductivity was 6~10% smaller than the arithmetic average (mean value) of the parallel and series models. From this study, it can be concluded that the thermal conductivity of the double-layer clad metals approaches the mean value obtained with parallel and series models when the interface is narrow and pure. (Received December 1, 2016; Accepted February 15, 2017)

keyword : thermal conductivity, double-layer, series model, interface effect, clad metal, heat transfer