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Since reconstruction of old town in Korea requires high-rise and seismic design construction, many attentions have been paid to high strength seismic reinforced steel bar. In the present paper, technical developments and trends are summarized for developing next-generation seismic reinforced steel bar of grade 700 MPa. Steelmaking process requires high energy efficiency and refining ability. Effects of alloying elements are explained, and alloy design based on computational thermodynamics is introduced. On the other hand, it is considered that grain size refinement by the controlled rolling and low temperature transformation structures formed by the accelerated cooling are effective to obtain acceptable mechanical properties with high strength. Finite element simulation analysis is also useful to understand plastic deformation by rolling, internal and external heat transfer, and corresponding phase transformation of austenite phase to various low-temperature transformation structures. †(Received September 20, 2016; Accepted September 26, 2016)

keyword : metals, thermomechanical processing, strength, computer simulation, seismic design

The deformation mechanism at room temperature in water quenched (WQ) and furnace cooled (FC) mild steels was investigated in a plane tilted 45° from the tensile direction, using electron back scattered diffraction, XRD, and TEM. The occurrence of (211) twinning was confirmed by pole figures and the slip systems were analyzed by variation in full width half maximum during tensile deformation. In WQ steel the deformation occurred through both (211) twinning and (110) slip in the low strain region, and by (211) slip in the high strain region. However, in FC the deformation was produced by both (211) twinning and (211) slip in the low strain region and (211) twinning and (110) and (211) slip in the high strain region in FC. †(Received May 27, 2016; Accepted June 23, 2016)

keyword : alloys, deformation, twinning, XRD, deformation mechanism

The effects of alloying elements (Mn, Cr, and C) on the corrosion resistance of steel were examined using weight loss test, polarization test in a sour environment, analysis of rust, and the prediction of stable corrosion product. Carbon was found to improve corrosion resistance after 7-day immersion, while manganese and chromium negatively affected the corrosion resistance of steels during the entire immersion time. The negative effect of manganese and chromium on the corrosion resistance in the sour environment was explained, in that both manganese and chromium were stable as ionic species were dissolved in the sour solution, which was proved by the prediction of stable phases based on phase equilibria. The positive effect of carbon, on the other hand, was due to the formation of amorphous carbon distributed near the metal/rust interface, which may improve the adhesion of the corrosion product to the matrix. This is consistent with not only the formation of adhesive rust on the high carbon-containing steels but also the change in the corrosion behaviour of the high carbon-containing steels, from linear to parabolic behaviour, with time. †(Received May 23, 2016; Accepted June 11, 2016)

keyword : alloys, hot rolling, corrosion, TEM, E-pH diagram

The low mechanical properties of SiC limit its wide application despite many attractive properties. To improve the mechanical properties, a second phase has been generally added to make a nanostructured composite. In this respect, graphene was evaluated as the reinforcing agent of SiC. SiC-graphene composites were sintered within 2 min by high-frequency induction heating under a pressure of 80 MPa. The rapid consolidation method allowed the retainment of the nano-scale microstructure by inhibiting the grain growth. The effect of graphene content on the microstructure and hardness of the SiC-graphene composite was investigated using FE-SEM and Vickers hardness tester. †(Received June 1, 2016; Accepted July 18, 2016)

keyword : sintering, composites, nanostructured materials, mechanical properties

Open cell Cu foam, which has been widely utilized in various industries because of its high thermal conductivity, lightweight and large surface area, was successfully joined with Cu plate by diffusion bonding. To prevent excessive deformation of the Cu foam during bonding process, the bonding pressure should be lower than 500 kPa at 800 ℃ for 60 min and bonding pressure should be lowered with increasing holding time. The bonding strength was evaluated by tensile tests. The tensile load of joints increased with the bonding pressure and holding time. In the case of higher bonding pressure or time, the bonded length at the interface was usually longer than the cross-sectional length of the foam, so fracture occurred at the foam. For the same reason, base metal (foam) fracture mainly occurred at the node-plate junction rather than in the strut-plate junction because the bonded surface area of the node was relatively larger than that of the strut. †(Received May 4, 2016; Accepted June 2, 2016)

keyword : foams, diffusion bonding, mechanical properties, tensile test, fractograph

The reliability of printed circuit boards (PCB) has emerged as a critical concern as the size of solder bump decreases and current density to solder bump increases by fine pitch formation. The main failure mode of solder bumps is open-circuits due to void formation as intermetallic compounds (IMC) grow, mainly due to electromigration. This study modeled IMC growth by electromigration in Sn-0.7Cu solder bumps. The IMC produced in the reflow process grew again significantly due to electromigration upon application of electric current. The thickness of the IMC under electromigration increased as the current increased from 1 A (current density: 1.3 × 104 A/cm2) to 1.5 A (current density: 1.9 × 104 A/cm2). For the current density applied in the study, IMC growth of Cu6Sn5 was faster than that of Cu3Sn. The Nernst-Einstein relation was used to model the IMC growth induced by electromigration. The modeling results of Cu3Sn and Cu6Sn5 thickness showed good agreement with the experimental observations of IMC growth under electromigration. Specifically, a good prediction for Cu3Sn growth was derived for the current density of 1.3 × 104 A/cm2. However, the modeling values of 1.9 × 104 A/cm2 and Cu6Sn5 thickness showed a minor difference as compared with the experimental IMC thickness results. As the current density increased from 1.3 × 104 A/cm2 to 1.9 × 104 A/cm2, the solder bump probably evolved under heat generation, and the further effects of aging and thermomigration should be incorporated in the IMC growth. †(Received April 7, 2016; Accepted June 17, 2016)

keyword : electromigration, Sn-0.7Cu solder, intermetallic compounds, current density, modeling

In this work, niobium (V) fluoride was selected as an additive to heighten the hydrogen sorption rates of Mg. Specimens of pure Mg, 5 wt% niobium fluoride－added Mg, and 10 wt% niobium fluorideadded Mg were prepared by planetary ball milling in hydrogen. The hydrogen sorption properties of the specimens were then examined. An Mg－based hydrogen－storage alloy with an effective hydrogenstorage capacity of about 5.5 wt% was developed. At 593 K in 12 bar hydrogen at the first cycle (Cn = 1), the 5 wt% niobium fluoride－added Mg stored 4.37 wt% hydrogen in 5 min and 5.50 wt% hydrogen in 30 min. At 593 K in 1.0 bar hydrogen at Cn = 1, the 5 wt% niobium fluoride－added Mg released 2.11 wt% hydrogen in 10 min, 4.66 wt% hydrogen in 30 min, and 5.43 wt% hydrogen in 60 min. The planetary ball milling of Mg with NbF5 in hydrogen, which generated MgF2, NbH2, and NbF3, is believed to have produced imperfections both on the surface and in the interior of the Mg particles, created clean surfaces, and diminished the particle size of the Mg. The 5 wt% niobium fluoride－added Mg specimen stored a larger quantity of hydrogen in 30 min and a larger quantity of hydrogen was released in 60 min compared with the 10 wt% niobium fluoride－added Mg, or the pure Mg. †(Received April 5, 2016; Accepted June 27, 2016)

keyword : hydrogen absorbing materials, mechanical milling, microstructure, X-ray diffraction, niobium (V) fluoride-added Mg alloys

The aim of this study is to optimize the microstructure and mechanical properties of Ti-39Nb-6Zr (TNZ40) for bio-implant applications. TNZ40 was designed to have a low elastic modulus (~40 GPa) and good biocompatibility. However, the alloy shows relatively low strength compared to other titanium alloys for bio-implant. In the present study, we tried to obtain the proper combination of elastic modulus and strength by tailoring the direct aging conditions after severe plastic deformation. The mechanical properties are closely linked to characteristics including the distribution and volume fraction of precipitates. †(Received November 6, 2015; Accepted May 19, 2016)

keyword : implant, titanium alloy, low elastic modulus, direct aging, microstructure, mechanical properties

In this study, we established a correlation between the electrical conductivity and catalytic properties of vanadium borophosphate glass containing Fe2O3. Glasses were synthesized in the 78V2O5- 15P2O5-7B2O3 ternary-system containing x mol% Fe2O3 (x = 1, 5, 7.5, and 10). Electrical conductivity was measured at 45 ℃ for samples heat-treated at 340 ℃ for 12 h. To examine catalytic effects, we used TGA measurements of a mixture of fatty acid and sample powders. Fourier transform infrared spectroscopy (FTIR) was used to analyze the structural changes after crystallization, while X-ray photoelectron spectroscopy (XPS) analysis indicated changes with different valence state ions. X-ray diffraction (XRD) analysis of the structure array verified these inferred changes. Conductivity and catalytic effects are discussed in relation to the migration of vanadate ions and iron ions with different valence states due to the increase crystallinity. †(Received May 10, 2016; Accepted June 7, 2016)

keyword : crystallization, electrical conductivity, catalytic property, polaron

Korea is famous for its long history of manufacturing high tin bronze objects of near peritectic composition by the application of forging and rapid cooling at approximately 700 °C. Not much is known, however, of the origin and developmental processes of this technique. Fortunately, valuable evidence for assessing the associated technological evolution was found by examining the microstructure and chemistry of bronze objects excavated in the National Gyongju Museum construction site. Initial awareness of the technique was clearly visible in the selection of unleaded high tin bronze alloys of varying tin levels and the application of special thermo-mechanical treatments involving forging and quenching from the α-β or α-γ phase field of the Cu-Sn system. This observation along with the excavation contexts suggests that the high tin technology was gradually established in Korea by a long period of experimentation, starting from the 8th century AD or earlier. †(Received May 27, 2016; Accepted June 16, 2016)

keyword : Korea, high tin bronze, technological evolution