• 대한금속재료학회지
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• 제49권12호
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• pp.989-994
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• 2011

Samples with compositions of 80 wt% Mg-14 wt% Ni-6 wt% $Fe_2O_3$ (named $Mg-Ni-Fe_2O_3$), and 78 wt% Mg-14 wt% Ni-6 wt% $Fe_2O_3-2$ wt% CNT (named $Mg-Ni-Fe_2O_3-CNT$ ) were prepared by reactive mechanical grinding. Hydriding and dehydriding properties and effects of CNT addition on the hydriding and dehydriding rates of $Mg-Ni-Fe_2O_3$ were then investigated. Activation of the $Mg-14Ni-6Fe_2O_3$ sample was completed after three hydriding (under 12 bar $H_2$)-dehydriding (under 1.0 bar $H_2$) cycles at 573 K. The addition of CNT to the $Mg-14Ni-6Fe_2O_3$ sample made the activation process unnecessary, with a small decrease in the hydrogen-storage capacity.

• 소성∙가공
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• 제28권1호
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• pp.15-20
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• 2019

The superplastic forming/diffusion bonding process has been developed to fabricate a core frame structure with joint nodes out of tubes, for the development of a titanium high performance bicycle. The hydroforming process has been applied for bulging of a tube in the superplastic condition before, and during the diffusion bonding process. In this experiment, a commercial Ti-3Al-2.5V tube was selected as raw material for the study. The forming experiment has been performed using a servo-hydraulic press with a capacity of 200 ton. Next, nitrogen gas was used to acquire necessary pressure for the bulging and bonding of the tubes to fabricate the joint nodes. The pertinent processing temperature was $870^{\circ}C$ for the superplastic hydroforming/diffusion bonding (SHF/DB) process, using the Ti-3Al-2.5V tube. The bonding quality and the progress of bulging and diffusion bonding have been observed by the investigation of the joining interfaces at the cross section of the joint structure. The control of the nitrogen pressure throughout the SHF/DB process, was an important factor to avoid any significant defects in the joint structure. The whole progress stage of the diffusion bonding could be observed at a joint interface. A core structure with 5 joint nodes to manufacture a titanium bicycle could be obtained in a SHF/DB process.

• Metals and materials international
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• 제24권6호
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• pp.1403-1411
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• 2018

Graphene (multilayer graphene) was chosen as an additive to improve the hydrogen uptake and release properties of magnesium (Mg). Five weight percent of graphene was added to pre-milled Mg by milling in hydrogen (reaction-involving milling). The hydrogen uptake and release properties of the graphene-added Mg were investigated. The activation of Mg-5graphene, which was prepared by adding 5 wt% graphene to Mg pre-milled for 24 h, was completed after the second cycle (cycle number, CN=2). Mg-5graphene had a high effective hydrogen-storage capacity (the quantity of hydrogen absorbed for 60 min) of 6.21 wt% at CN=3 at 593 K in 12 bar $H_2$. At CN=1, Mg-5graphene released 0.46 wt% hydrogen for 10 min and 4.99 wt% hydrogen for 60 min. Milling in hydrogen is believed to create defects (leading to facilitation of nucleation), produce cracks and clean surfaces (leading to increase in reactivity), and decrease particle size (leading to diminution of diffusion distances or increasing the flux of diffusing hydrogen atoms). The added graphene is believed to have helped the sample have higher hydrogen uptake and release rates, weakly but partly, by dispersing heat rapidly.

• 소성∙가공
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• 제28권5호
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• pp.247-256
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• 2019

In this study, the development of annealing textures in cold rolled and annealed tantalum sheets was analyzed using electron backscatter diffraction. At $900^{\circ}C$, the textures of the recrystallized grains in the partially and completely recrystallized microstructures displayed significant similarities. The average diameter of the recrystallized grains with ${\gamma}-fiber$ orientations exceeded that of grains with different orientations, and the average growth rates were unrelated to the orientations after an initial stage of recrystallization. Additional cold rolling and annealing was done for controlled initial microstructures and textures inherited from various processes of prior cold rolling and annealing. This second cycle of the process resulted in stronger textures with major ${\gamma}-fiber$ orientations as a result of the enhanced ${\gamma}-fiber$ orientations in the preceding textures. A coarse-grained prior microstructure resulted in a weaker annealing texture than a fine grained one regardless of the stronger previous texture, which was occasioned by the sub-structures of the minor orientations at local deformation inhomogeneities such as sharp in-grain shear bands.

• 대한금속재료학회지
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• 제50권10호
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• pp.769-774
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• 2012

Mg-x wt% $Fe_2O_3-y$ 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 $Mg-5Fe_2O_3-5Ni$ was completed after one hydriding (under 12 bar $H_2$) - dehydriding (in vacuum) cycle at 593 K. At n = 2, $Mg-5Fe_2O_3-5Ni$ 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 $Mg-10Fe_2O_3$ 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 $H_2$. Activated $Mg-10Fe_2O_3-5Ni$ 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 $H_2$.

• 대한금속재료학회지
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• 제49권1호
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• pp.52-57
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• 2011

Nickel ferrite ($NiFe_2O_4$) powder was prepared through the ceramic route by calcination of a stoichiometric mixture of nickel oxide (NiO) and iron oxide ($Fe_2O_3$). The pressed pellets of $NiFe_2O_4$ were isothermally reduced in pure hydrogen at 800, 900, 1000 and $1100^{\circ}C$. Based on thermogravimetric analysis, the reduction behavior and the kinetic reaction mechanisms of the synthesized ferrite were studied. The initial ferrite powder and various reduction products were characterized by XRD, SEM, reflected light microscope and VSM to reveal the effect of hydrogen reduction on the composition, microstructure, magnetic properties and reaction kinetics of the produced Fe-Ni alloy. Complete reduction of the $NiFe_2O_4$ was achieved with synthesis of homogeneous nanocrystalline Fe-Ni alloys. Arrhenius equation with the approved mathematical formulations for a gas-solid reaction was applied for calculating the activation energy ($E_a$) values and detecting the controlling reaction mechanism.

• 대한금속재료학회지
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• 제47권11호
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• pp.699-706
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• 2009

Deformation behavior of pure aluminum during equal channel angular pressing (ECAP) was simulated using a three-dimensional version of the finite element method in conjunction with a constitutive model based on the dislocation density and cell evolution. The three-dimensional finite element analyses for the prediction of microstructural features, such as the variation of the dislocation density and the cell size with the number of ECAP, are reported. The calculated stress and strain and their distributions are also investigated for the route Bc ECAP processed pure aluminum. The results of finite element analyses are found to be in good agreement with experimental results for the dislocation cell size. Due to the accumulation of strain throughout the workpiece and an overall trend to saturation in cell size, a decrease of the difference in cell size with the number of passes (1~4) was predicted.

• 대한금속재료학회지
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• 제49권5호
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• pp.367-373
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• 2011

This paper studies the experimental method that uses the homogeneous precipitation method to prepare mica flakes coated with anatase-type titania pearlescent pigment with urea as precipitant. The optimum technology parameters, the chemical composition, the microstructure, and the color property of resulting pigments are discussed. The coating principle of mica coated titania with various coating thickness is analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy(TEM) and tested by spectrophotometer analysis. The colored nanocrystalline pigments with different morphology and coating thickness 45-170 nm were prepared by homogeneous precipitation treatment of $TiOSO_4$(titanum oxysulfate) aqueous solutions. Characterizations on the pigments show that the pearlescent effects of the pigments depend mainly on mica size, thickness of the metal oxide deposit, its chemical composition, and crystal structure.

• 대한금속재료학회지
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• 제47권1호
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• pp.1-6
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• 2009

Magnesium alloys are drawing a lot of attention and have been extensively studied. The major obstacle to the practical application of the alloys is the poor formability at room temperature, originating basically from the insufficient number of slip system. Development of a proper texture is one promising solution to improve the formability. In the present work, after extrusion and full annealing, microstructures, texture developments and tensile properties of AZ31 Mg alloys are studied. After full annealing strong <1010>||ED fiber texture and weak <1120>+<1230>||ED fiber texture (c-axes in the radial direction) were developed. The textures are distinctly influencing the tensile properties, which can be understood in terms of the activation of basal slip modes. With the random orientation, which is developed in the $45^{\circ}$ sample to the extrusion direction, the better workability can be achieved at room temperature.

• 대한금속재료학회지
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• 제46권7호
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• pp.403-411
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• 2008

Zr-based amorphous alloy matrix composites reinforced with tantalum continuous fibers were fabricated by liquid pressing process, and their microstructures and mechanical properties were investigated. About 60 vol.% of tantalum fibers were homogeneously distributed inside the amorphous matrix, which contained a small amount of polygonal crystalline particles. The ductility of the tantalum-continuous-fiber-reinforced composite under tensile or compressive loading was dramatically improved over that of the monolithic amorphous alloy, while maintaining high strength. The consequential observation of the tensile deformation and fracture behavior of the composite showed the formation of multiple shear bands and multiple necking, crack deflection in the amorphous matrix, and obstruction of crack propagation by ductile fibers, thereby resulting in very high tensile elongation of 7.2%. These findings suggested that the liquid pressing process was useful for the development of amorphous matrix composites with improved ductility.