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The high Mn (≥25 wt%) TWIP steels with low stacking fault energy are currently one of the most attractive materials in the automotive industry due to their unique combination of high strength and good ductility. However, the conventional manufacturing processes have many problems with alloying high Mn. Therefore, many efforts are being made to reduce the Mn content, but the steels could not have an appropriate stacking fault energy (SFE) for twinning with decreasing Mn content. In this study, TWIP steel with lean Mn less than 20 wt% (Fe-18Mn-1.5Al-0.6C) was made. Tensile properties of the lean 18 Mn TWIP steel were measured at a strain rate range of 10.4s.1≤ ε. ≤10.1s.1 to investigate the correlation between twinning behavior and stress at various strain rates. The lean 18 Mn TWIP steel possessed an austenite single phase before and after tensile tests at room temperature and exhibited a total elongation of about 60% and ultimate tensile strength of about 1000 MPa by strain-induced twinning. The stress decreased with an increasing strain rate, showing a negative strain rate sensitivity. That was because the volume fraction of twinned grains decreased with an increasing strain rate at the same strain. Stress was directly related to the volume fraction of twinned grains at quasi-static strain rates.

keyword : metal, deformation, strain rate, twinning, tensile test

This work assesses the hot ductility behavior of high manganese steel under hot working conditions. Reduction of area (RA) behavior of 16 Mn-0.5 C steel was investigated to consider it for production in a slab and subsequent hot rolling process. All specimens were machined from the as-cast slab. A hot ductility test was accomplished by a cast simulator through a temperature range of 650 ℃-1200 ℃ with 50 ℃ intervals. The obtained RA values ranged between 30% and 60%, with a maximum at 850℃, and this is far from the general RA behavior of low carbon steel. This result could be attributed to the fact that the specimen is fully austenitic through all test temperatures, and that there is no ferrite at the low temperature end and reduced dynamic recrystallization at the high temperature end. Microstructural evaluation of the fractured specimen revealed that the deformation was concentrated at the grain boundary, and there was little matrix deformation. These results suggest that the grain boundary sliding is the basic cause of the fracture and low values of hot ductility in high manganese steel.

keyword : alloys, hot working, hot ductility, tensile test, high Mn steel, continuous casting

The effects of nitrogen content, cold working, and stress relief treatment on the mechanical properties of high-nitrogen 18Mn-18Cr austenitic steels with different nitrogen content are investigated with respect to the specimen direction and location. The yield and tensile strengths usually increased with increasing nitrogen content and cold working, and they additionally increased after stress relief treatment. Based on these results, we propose a semi-empirical equation to predict the yield and tensile strengths of high-nitrogen 18Mn-18Cr austenitic steels. On the other hand, we discuss the ductile-brittle transition behavior depending on cold working and stress relief treatment in terms of the temperature sensitivty of yield stress.

keyword : alloys, cold working, mechanical properties, tensile test

Fire resistance is an important factor in sustaining the structural stability of steel framed buildings on fire. However, evaluation of the fire resistance of steel columns has been conducted using rolled steels for general structures, SS 400. Recently, rolled steels for welded structures, such as SM 400 and SM 490, have been used frequently because they have better performance of welding than the SS 400. However, there has been doubt about how much fire resistance SM 400 and SM 490 have. To evaluate by calculation the fire resistance of an H-section column made of SS 400 its mechanical and thermal properties were derived and suggested respectively in the form of regressive equations and the analysis was done based on heat transfer and thermal stress analysis. In this study, the results of the evaluation of H-section columns made of SS 400 with loaded fire tests turned out to be conservative. As a result, a new guideline is required to get the exact fire resistance of another structural steel.

keyword : metals, deformation, mechanical property, tensile test, thermal property

Ferritic stainless steel Crofer 22 APU specimens with various surface roughnesses were prepared by grinding with different grit SiC grinding papers. The samples were then thermally cycled 20 times between 298 K and 1073 K (a total oxygen exposure time of 500 h at 1073 K). Observations of the microstructure, measurements of the area specific resistance (ASR), and analysis of the atomic percentages of elements by energy dispersive X-ray (EDX) spectroscopy were performed. As the grit number of the grinding paper, with which the sample was ground, increased from grit 80 through grit 800, the particle size decreased. The particle sizes of the sample ground with grit 800 and the polished sample were similar. A trench of a Crofer 22 APU sample ground with grit 80 after 120 thermal cycles between 298 K and 1073 K (a total oxygen exposure time of 3000 h at 1073 K), observed by scanning electron microscopy (SEM) after focused ion beam (FIB) milling, exhibited an alloy substrate phase, Cr2O3 layer, and (Cr, Mn)3O4 spinel phase from the inside to the surface of the sample. The Cr2O3 layer was undulated with a thickness of about 3 μm. At the same measuring temperature, the ASR increased as the surface of the sample became rougher. Plots of ln (ASR/ T) versus 1/ T for samples ground with grit 120, grit 240, and grit 800 after 20 thermal cycles exhibited good linearity.

keyword : fuel cells, oxidation, microstructure, X-ray diffraction, thermal cycling

Abstract: Two Al-Mg alloys were subjected to extrusion for fabricating ferrules and the mechanical behavior of the extruded ferrules was investigated. To estimate the proper extrusion conditions, the power dissipation efficiency and plastic instability of the Al-Mg alloys were assessed by the construction of deformation processing maps. The Al-1.5Mg alloy ferrule had a higher work hardening capacity than the Al-0.9Mg alloy ferrule, which resulted in a higher load bearing capacity in the former during the extension test of wire rope slings.

keyword : ferrule, alloys, extrusion, mechanical properties, tensile test

The replacement for indium tin oxide (ITO) in electronic displays should have comparable optical transmittance and electrical conductivity while being easy to source and manufacture. However, novel materials such as singlewalled carbon nanotubes (SWCNTs) and reduced graphene oxides (RGOs) are incapable of addressing these challenges. We demonstrate a simple method to fabricate good transparent conductive films (TCFs) by combining and leveraging the superior optical transparency of RGOs and the excellent electrical conductivity of SWCNTs. This method affords thin multilayers of SWCNTs and RGOs with excellent optical and electrical properties because these properties are correlated with spraying time and the amount of SWCNTs or RGOs. In general, transmittance is advantageous to RGO as conductance is to CNTs. With a view to finding good TCFs with reduced sheet resistance, but with little sacrifice of transmittance, it is natural to explore the combination of CNT and RGO. The sandwiched multilayer of SWCNTs and RGOs exhibited a low sheet resistance of 214.2 Ω/sq, which was comparable to that of SWCNTs, and a transmittance of 60% at a wavelength of 550 nm. To further reduce the sheet resistance and improve the transparency of the multilayer TCFs, Au doping was carried out. The doping, in combination with controlled spraying of the amount of SWCNTs and RGOs, led to multilayers with resistance/transmittance combinations of 141.3 Ω/sq and 70% and 371.5 Ω/sq and 83%. These properties meet the requisite criteria for an ITO replacement.

keyword : carbon and graphite, coating, electrical properties, optical properties, electrical condustivity resistivity, scanning electron microscopy

Al/Pt and Ti/Pt bilayers to a flat glass substrate were employed as a counter electrode to improve the energy conversion efficiency of a dye-sensitized solar cell device with a structure of glass/FTO/blocking layer/TiO2/N719(dye)/electrolyte/(50 nmPt-50 nmAl) or (50 nmPt-50 nmTi)/glass. For comparison, a 100 nmthick Pt counter electrode on a flat glass substrate was also prepared using the same method. The sheet resistance was examined by a four point probe. The photovoltaic properties, such as the short circuit current density, open circuit voltage, fill factor, energy conversion efficiency, and impedance, were characterized using a solar simulator and potentiostat. The phases of the bilayered films were identified by X-ray diffraction. The measured energy conversion efficiencies of the dye-sensitized solar cell devices with Al/Pt and Ti/Pt bilayer counter electrodes were 5.36% and 5.03%, respectively. The interface resistance at the interface between the counter electrode and electrolyte decreased when the Al/Pt and Ti/Pt bilayer thin films were applied. The new phases of AlPt3 and Pt3Ti led to a decrease in resistivity and an increase in catalytic activity. This suggests that Al/Pt and Ti/Pt bilayer thin films might improve the efficiency of dye-sensitized solar cells.

keyword : solar cells, sputtering, optical properties, solar simulator, bilayer

The purpose of this research is to make new alloys and evaluate their corrosion behavior and biocompatibility by adding different amounts of Mo to Ni-Cr alloys (Ni-13Cr-4Mo, Ni-13Cr-6Mo, Ni-13Cr-8Mo, and Ni-13Cr-10Mo). In this research, we used incubated 6th passage human gingival fibroblast cells for the cell viability and cell adhesion experiments. The form and amount of adhesion of human gingival fibroblast cells to alloys were observed using field emission scanning electron microscopy (FE-SEM). In order to evaluate quantitative analysis of cell adhesion and corrosion behaviors, we used the AC impedance method (at 37 ± 1 ℃ in 0.9% NaCl solution) and compared the obtained results with the results of the FE-SEM. The microstructure of the Ni-13Cr-6Mo alloy had a more uniform surface than the other alloys. The Ni-13Cr-6Mo alloy demonstrated the lowest cytotoxicity in the cell viability assay, and the highest cell adhesion and corrosion resistance in an AC impedance test. We concluded that Ni-Cr-xMo alloys have better cell viability and cell adhesion and higher electrochemical impedance than Ni-Cr alloys.

keyword : alloys, corrosion, biocompatibility, FE-SEM, AC impedance

This study investigates the tensile properties of a glass-basalt/epoxy interply hybrid composite. The objective of this work is to develop an environmentally friendly hybrid composite material that has good strength and stiffness. These composites were fabricated through a vacuum-assisted resin transfer molding process. The glass/epoxy and basalt/epoxy composite were also prepared to compare the properties. The tensile strength of the hybrid composite improved as the basalt fiber content increased. However, we did not find a variation in the elastic modulus defined as the initial slope of the stress-strain curve for the hybrid composite under tensile loading. When the basalt fiber content of the total reinforcement became 40%, the fracture strain of the hybrid composite was higher than that of the glass/epoxy composite. From the test result, we knew that there is almost no difference between the tensile properties of the hybrid composites with different stacking sequences.

keyword : fibers, fracture, tensile test, composites