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The resistance of hydrogen induced cracking (HIC) in two different microstructures of API X65MS steel plates was investigated with plastic deformation. A banded type of polygonal ferrite/acicular ferrite (PF/AF) microstructure and a single phase acicular ferrite (AF) were produced by controlling the rolling and cooling conditions. As plastic deformation increased, dislocation density was increased, particularly in the banded AF within the two phase PF/AF microstructure. In contrast, the dislocation density was increased uniformly in the single phase AF microstructure. It was found that HIC resistance deteriorated with increasing plastic deformation. Between the two different types of microstructures, the uniform AF showed better HIC resistance and higher crack propagation resistance than PF/AF, which can be attributed to the more uniform distribution of dislocation density in the AF microstructure.
In this study, we have demonstrated that the Marc v.11 simulation program, based on heat transfer, was able to estimate the thermal distribution of a graphite mold and sintered compact, for sintering a Ruthenium (Ru) target material. The thermal distribution simulation analysis was conducted as a function of setting temperatures to obtain basic thermal behaviors, and to determine whether physical properties such as the density, grain size and compactness of the sintered Ru target material were influenced by temperature distribution in the graphite mold. It was found that a very small difference in temperature between the center and edge of the sintered compact could be observed at the simulation temperature of 1200 ℃. The highest relative density of 99.1% was achieved when the Ru target material was sintered at 1200 ℃ by spark plasma sintering(SPS). Also, it was confirmed that the grain size of the sintered Ru target was considerably increased with increasing sintering temperature, in spite of the fast heating rate and short dwell time. From these results, a very meaningful thermal characteristic simulation technique was confirmed that can predict the optimized conditions needed to obtain high quality sintered materials prior to SPS process.
A typical brightener-propargyl alcohol-was added to enhance the cutting performance during Ni-diamond composite electrodeposition. Electrochemical analysis was performed and mechanical properties such as hardness and wear resistance were examined. In addition, the surface morphology of composite coating layers was observed using an optical microscope, and using image analysis software, the dispersivity of the diamond particles was analyzed to calculate the number of single diamond particles. A galvanostatic test was employed to identify the electrodeposition mechanism as a function of the concentration of propargyl alcohol. When 0.1 mg/ℓ of propargyl alcohol was added, the dispersivity and adhesion performances of the Ni-diamond coating layer were optimized for cutting tools. (Received September 22, 2015)
In the present work, the effect of Ca additions on microstructure and corrosion characteristics of high pressure die-cast Mg-4Al-2Sn alloy has been investigated. Mg-4Al-2Sn-xCa (x= 0, 0.3 and 0.7wt.%) alloy was prepared by using a high pressure die-casting method. Results indicated that the microstructure of Mg-4Al-2Sn alloy consisted of α-Mg, Mg17Al12 and Mg2Sn phase. With increase of Ca additions, CaMgSn phase was newly formed and grain size was sharply decreased. From the test results, the corrosion resistance of die-cast Mg-4Al-2Sn alloy was significantly improved by Ca addition. It is considered that stabilization of Mg(OH)2 layer and refinements of microstructure with increase of Ca additions. (Received October 14, 2015)
The influence of the size distribution and particle interaction on packing behaviors is investigated for water/gas-atomized STS 316L powders with polydisperse and bimodal size distributions. In most cases, the experimentally estimated packing fraction matched well with the results predicted by the Desmond model, which implies that the packing fraction increases with the polydispersity and skewness. When the powder size decreases below ~10 μm, however, the calculation starts to positively deviate from the experimental data, presumably because particle interaction plays a significant role in the powder packing behavior. The Desmond model is modified to consider the effects of particle interaction as well as of size distribution on the packing behavior, and the modified model shows better accordance with the experimental data, particularly for fine powders, than does the original Desmond model. †(Received August 25, 2015)
In order to mitigate the disadvantage of the Mg hydrides, several studies have been conducted that have used MgHx intermixed with carbon. Graphene is a kind of carbon allotrope that is easily subject to a desorption reaction at low temperatures because such a reaction is exothermic. In this work, an MgHx-graphene mixture has been prepared by reactive mechanical grinding. The synthesized powder was characterized by X-ray diffraction analysis (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and simultaneous thermogravimetric (TG) and differential scanning calorimetric (DSC) analyses. The hydrogenation behaviors were evaluated using a Sievert’s type automatic pressure-composition-temperature (PCT) apparatus without activation treatment. From the characteristics of the absorption kinetics and the curves observed, the role of graphene as a catalyst in hydrogen absorption was determined. According to the results of the PCI curve, the available hydrogen storage amounts for MgHx-5 wt% graphene composites had maximum values of 3.69, 5.09, and 5.72 wt% at 423, 523, and 623 K, respectively. Those values for MgHx-10 wt% graphene were 5.08, 5.45, and 5.83 wt% at 423, 523, and 673K, respectively.
ZnO thin films were spin-coated onto a Zn seed layer that was deposited on a quartz substrate by thermal evaporation. This was found to enhance the structural and optical properties of the resulting ZnO thin film, with a particular improvement in its optical transmittance. Furthermore, by varying the temperature at which the Zn seed layer was thermally annealed, a temperature of 100℃ was identified as being optimal to ensure a maximum increase in the crystallinity and optical properties of a ZnO thin film. On the basis of these results, it is proposed that the use of a thermally annealed Zn seed layer is an effective means of achieving high-quality ZnO thin films.
Rare-earth (RE) mono-nitrides can be used as magnetocaloric materials for the magnetic refrigerant for liquefying hydrogen. In this study, holmium nitride (HoN) particles were prepared by sol-gel method with microwave irradiation. HoN particles were successfully synthesized by 2-step heat treatment in N2 atmosphere form gel-type intermediate by microwave irradiation. X-ray diffraction patterns of the synthesized HoN particles exhibited the six sharp peaks in good agreement with each plane of crystallographic structure of HoN. Especially, the synthesized HoN particles reveal a high magnetic entropy change (△Sm) of 36.6 J/kgK in applied field of 5T at 14.2 K which is comparable with those of rare earth mono-nitrides by carbothermic reduction method or plasma arc discharge technique.
Hydrogen pre-charged Zr-1.0Nb alloy was prepared to study the effect of hydrogen uptake on its mechanical properties. The apparent activation energy of the Zr-1.0Nb alloy for hydriding was about 27 kcal/mol. The tensile strength of the Zr-1.0Nb alloy with a hydrogen content of up to 700 ppm was similar to that of an as-fabricated specimen, in that its elongation was drastically decreased after 400 ppm and became nearly zero above about 1200 ppm. The critical hydrogen content range needed to produce the ductile-brittle transition was above 400 ppm.