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In the present study, new Mg-based bulk metallic glasses have been developed in a Mg-Cu-Gd- Nd based system using a Mg-Nd byproduct obtained from the recycling process of NdFeB magnet scrap through liquid metal extraction. The Mg-Cu-Gd-Nd bulk metallic glasses with a diameter of 2 mm are fabricated by injection casting using Mg-Nd by-product. The compressive fracture strength and total strain are 850 MPa and 1.8%, respectively, for the Mg_66.5-xCu_28.5GdxNd₅(x=8, 10 at%) bulk metallic glasses. The activation energy of the Mg_56.5Cu_28.5Gd₁₀Nd₅ bulk metallic glass using Mg-Nd byproduct is similar to that of conventional Mg_56.5Cu_28.5Gd₁₀Nd₅ bulk metallic glass using pure elements, indicating that the thermal stability in the supercooled liquid region is similar. From the results, the recycling process for NdFeB magnet scrap can be considered a cost effective method for the formation of Mg-based bulk metallic glasses. It can therefore be concluded that the newly developed Mg-Cu-Gd-Nd bulk metallic glasses using Mg-Nd byproduct obtained from the recycling process of NdFeB magnet scrap have potential for industrial application. (Received July 31, 2019; Accepted August 27, 2019)

keyword : Mg, bulk metallic glass, NdFeB, recycling, scrap

High-Mn steels developed for offshore industries require good resistance to stress corrosion cracking (SCC) in seawater. Elements like Cr and N are often added to improve the resistance to SCC. In this study, the SCC behavior of Fe18Mn3Cr0.1N and Fe19Mn19Cr0.6N steels in artificial seawater was examined. Slow strain rate tests were conducted at a nominal strain rate of 10-6/sec in air and artificial seawater under anodic and cathodic applied potentials. The tensile ductility drop in artificial seawater was compared to air and evaluated as the resistance to SCC. It was found that both specimens showed intergranular cracking in artificial seawater under both anodic and cathodic applied potentials. The intergranular SCC was more severe under anodic applied potential than cathodic applied potential. However the sensitivity to SCC in artificial seawater was substantially reduced in Fe19Mn19Cr0.6N specimen with higher Cr and N content, as compared to the Fe18Mn3Cr0.1N specimen under both applied potentials. Potentiodynamic tests in artificial seawater showed an increase in pitting corrosion potential, rather than corrosion potential, with increasing Cr and N content in high-Mn steel. The SCC behavior of high-Mn steels with different Cr and N contents was discussed based on micrographic and fractographic observations. (Received July 3, 2019; Accepted August 12, 2019)

keyword : stress corrosion cracking, high-Mn steel, artificial seawater

The microstructural evolution and corresponding mechanical properties of 5052 Al alloys with different deformation temperatures and post-deformation annealing conditions were investigated. The warmrolled alloy showed higher strength and elongation than the cold-rolled and cryo-rolled alloys. The improved strength and ductility of warm rolled alloys is attributed to the formation of fine precipitates and a higher degree of recovery during rolling. The formation of precipitates and the occurrence of dynamic recovery during the warm-rolling process were confirmed by the absence of the first two peaks in DSC curves of warm-rolled alloys. In particular, it was found that the application of cryo-rolling combined with warm-rolling at 448K increased tensile strength and yield strength without a decrease of ductility. This notable increase of strength is attributed to the increased dislocation density during cryo-rolling and the subsequent formation of fine precipitates at dislocations during warm-rolling. The contribution of fine precipitates and fully recovered microstructures, during cryo-rolling, warm-rolling, and subsequent annealing enhances the effective combination of strength and elongation. The ultra-fine grained 5052 Al alloy with high strength (405 MPa) and ductility (11.4% elongation) could be achieved by a combination of cryo-rolling with warm-rolling (448 K) and subsequent static annealing at 448 K. (Received August 12, 2019; Accepted August 29, 2019)

keyword : cryo-rolling, warm-rolling, 5052 Al alloys, annealing

Electroplating chromium films in a trivalent chromium bath has been extensively investigated as a replacement for the conventional hexavalent chromium bath. However, commercialization of the trivalent chromium method has been hindered because the resulting chromium films exhibit inferior mechanical properties compared to hexavalent chromium coated film. In this study, we enhanced the properties of trivalent chromium electroplated film using a pulse-reverse (PR) method. Firstly, the cathodic current density needed to produce a shiny surface was optimized using direct current (DC) electroplating. After optimizing the cathodic current density, the appropriate anodic current density for PR electroplating needed to achieve optimum crack density and corrosion resistance was investigated. The chromium coating prepared using PR electroplating exhibited higher corrosion resistance than that prepared by DC electroplating, because it suppressed crack formation. Concerning the electrochemical corrosion behavior in 3.5 wt% NaCl solution, the optimized sample (PR2) electroplated at an anodic current density of 0.04 A cm^-2 showed a corrosion potential shift of 129.9 mV to the anodic direction, and an increase in electrochemical corrosion resistance which was bigger than the chromium film prepared using DC electroplating. This optimization of electrochemical variables opens a new way to obtain improved electrodeposited film with low crack density and high corrosion resistance. (Received May 14, 2019; Accepted August 21, 2019)

keyword : trivalent chromium, crack, pulse-reverse electroplating, anodic current, corrosion resistance

Cast Ti-44Al-6Nb-2Cr-0.3Si-0.1C alloy having good air-oxidation resistance was corroded under 1 atm of flowing 0.2%SO₂/Ar gas at 900-1100 oC for 50-300 h in order to examine its corrosion behavior in a SO₂-containing environment, because SO₂ is an industrially important by-product in the field of combustion and processing of fossil fuels. The corrosion rate in 0.2%SO₂/Ar gas was faster than that in 0.2%O₂/Ar gas, suggesting that sulfur deteriorated the corrosion resistance of the alloy. The scale consisted primarily of an outer TiO₂ oxide layer with some α-Al₂O₃ islands, and an inner α-Al₂O₃-rich, TiO₂-deficient oxide layer. Ti and Al, having strong oxygen affinities, preferentially oxidized to highly stable rutile-TiO₂ and α-Al₂O₃ in the oxide scale. Small amounts of Cr and (Nb, Cr) dissolved in the outer and inner oxide layer, respectively. The outer oxide layer formed mainly by outward diffusion of Ti and Al, along with a lesser amount of Cr. The inner oxide layer formed by inward diffusion of oxygen and sulfur. The consumption of oxygen in the oxide scale facilitated the formation of Ti₂S and Nb_1-xS sulfides in the subscale. Oxidation and sulfidation prevailed in the oxide scale and the subscale, respectively. Sulfur migrated across the oxide scale into the subscale, where Nb, Cr, and Si accumulated to a certain extent due to their thermodynamic nobility when compared to Ti and Al. Although the alloy displayed good corrosion resistance in 0.2%SO₂/Ar gas, microcracking and void formation in the oxide scale and subscale were unavoidable. (Received July 8, 2019; Accepted August 27, 2019)

keyword : TiAl, intermetallics, oxidation, sulfidation

Electrodialysis was used to develop a highly efficient process for concentrating and recovering Li from the Li-containing waste solution that is generated after recovering Co and Ni from a leach solution of waste cathode materials of Li ion batteries. The optimum conditions were investigated by comparing the concentration of Li in the concentrated solution, water recovery rate, energy consumption and average flow rate. Limiting current density was also checked. The results showed that the concentration ratio of Li was enhanced up to 133.33% at an applied voltage of 10 V and volume ratio(V< Sub >D< /Sub >/V< Sub >C< /Sub >) of 4/1. In order to investigate the concentration ratio of Li based on large volume ratio, the total volume of the Li-containing solution was fixed, and the volume ratio of the dilute solution to the concentrate solution was adjusted to 1/1 to 10/1. The concentration ratio of lithium was increased in the initial stage and then decreased due to the osmotic phenomenon. Finally, an the analysis of the experimental results, including concentration ratio, average flux, and energy consumption, determined that the volume ratio(V< Sub >D< /Sub >/V< Sub >C< /Sub >) of 3/1 at 10 V of applied voltage was most efficient and the concentrate ratio of Li increased up to 160.61%. (Received April 18, 2019; Accepted August 21, 2019)

keyword : electrodialysis, lithium, lithium ion battery, separation, concentration

The dehydrogenation rates of graphene-added Mg (named Mg-5graphene) were very low at 573 K and 593 K. Ni was added to increase the dehydrogenation rates of graphene-added Mg. Samples (designated as Mg-2.5Ni-2.5graphene) with a composition of 95 wt% Mg + 2.5 wt% Ni + 2.5 wt% graphene were prepared by milling in hydrogen (reactive ball milling). Mg-2.5Ni-2.5graphene had significantly higher initial hydrogenation and dehydrogenation rates and much greater amounts of hydrogen absorbed and released after 60 min, H_a (60 min) and H_d (60 min), than Mg-5graphene. The addition of Ni lowered the magnesium hydride decomposition temperature from 683 K to 581 K. The activation of Mg-2.5Ni-2.5graphene was finished after the second hydrogenation-dehydrogenation cycle (n=2). Mg-2.5Ni-2.5graphene had a very high efficient capacity of stored hydrogen (the amount of hydrogen absorbed after 60 min) higher than 7 wt% (7.07 wt% at 573 K in 12 bar H2 at n=1). At n=1, the quantities of hydrogen released by Mg-2.5Ni-2.5graphene were 0.26 wt% H after 2.5 min and 3.34 wt% H after 60 min at 573K in 1.0 bar H₂. (Received June 28, 2019; Accepted August 20, 2019)

keyword : hydrogen absorbing materials, reactive ball milling, scanning electron microscopy (SEM), X-ray, diffraction, graphene and Ni addition

Herein we report the effect of Cu/In doping on the electronic and thermal transport properties of Bi-Sb-Te thermoelectric alloys. To closely examine the role of each doping element when incorporated in a Bi_0.4Sb_1.6Te₃ alloy, different groups of samples were prepared and characterized, including undoped Bi_0.4Sb_1.6Te₃, In single-doped samples and In and Cu doped ones. It was observed that Cu and In had different impacts on the thermoelectric properties of the Bi_0.4Sb_1.6Te₃. For example, the Hall carrier concentration of Bi_0.4Sb_1.6Te₃ was increased by Cu doping and decreased by In doping while maintaining Hall mobility, which suggests that the physical parameters related to the thermoelectric transport can be carefully controlled by doping with Cu and In. In addition, we found that the electronic structure of Bi_0.4Sb_1.6Te₃ can be modified by Cu/In doping. The density of states effective mass (m*) value of the Cu-doped sample (1.09 m₀) was increased. However, the m* of the In-doped sample (0.85 m₀) was decreased compared to the pristine sample (0.97 m₀). Cu single-doped Bi_0.4Sb_1.6Te₃ exhibited the maximum thermoelectric figure-of-merit because of the complexity of substitutional doping on Bi/Sb sites. Our results indicate that to enhance the performance of thermoelectric materials by doping with more than one element a well-designed doping strategy is required. (Received July 31, 2019; Accepted August 14, 2019)

keyword : Bi_0.4Sb_1.6Te₃, Cu/In doping, thermoelectric transport, electronic structure, density-of-states effective mass

Open-pored metal foams have been partly used for orthopedic or dental applications because of their good bone ingrowth behavior and low elastic modulus. In this paper, we report a method that can produce open-pored Ti foams using a technique that combines the powder injection molding process and feedstocks containing expandable polystyrene beads as a space holder. The expandable beads in the feedstocks undergo swelling during the foaming process, which causes the spaces occupied by the expandable beads to directly contact each other and become interconnected. This behavior results in the formation of open-pored Ti foams with high porosity (equivalently, low Young’s modulus) and large faces (greater than 140 μm). Compared to metallic foams prepared by conventional methods, Ti foams produced in this study have significantly larger face sizes (140-170 μm) and lower Young’s modulus (~17 GPa). These characteristics of the developed foam materials can promote fast bone growth into open-pored porous structures and permit improved fixture installation stability. This paper discusses the mechanism for the formation of the openpored porous structure, and analyzes the effect of various processing conditions on the face size, porosity, and Young’s modulus of Ti foams. (Received May 21, 2019; Accepted August 13, 2019)

keyword : porous titanium, implant, powder injection molding, space holder method, mechanical property