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The effect of carbon content and isothermal heat treatment conditions on the microstructure evolution and mechanical properties of ultra-high strength bainitic steels was investigated. A reduction in carbon content from 0.8 wt% to 0.6 wt% in super-bainite steel with typical chemistry effectively improved not only the Charpy impact toughness but also the strength level. This suggests that reducing the carbon content is a very promising way to obtain better mechanical balance between strength and impact toughness. The higher Charpy impact toughness at a lower carbon content of 0.6 wt% is thought to result from a reduction in austenite fraction, and refinement of the austenite grain. The coarse austenite grains have a detrimental effect on impact toughness, by prematurely transforming to deformation-induced martensite during crack propagation. Mechanical properties were also affected by the isothermal treatment temperature. The lower isothermal temperature enhanced the formation of bainitic ferrite with a refined microstructure, which has a beneficial influence on strength, but reduces impact toughness. The lower impact toughness at lower isothermal temperature is attributed to the sluggish redistribution of carbon from the bainitic ferrite into the surrounding austenite. Higher solute carbon in the bainitic ferrite contributes to an increase of strength, but at the same time, encourages a propensity to cleavage fracture. (Received March 6, 2019; Accepted May 2, 2019)

keyword : high strength, bainitic ferrite, heat treatment, austenite, impact toughness

The incomplete elimination of oxide scale formed during the normalizing process can result in severe degradation of the surface and mechanical properties of ultra-strong steels. This study revealed that the formation of oxide scale was highly dependent upon the alloying elements Ni and Cu. Ni-bearing steel showed a much higher oxidation rate, resulting in much thicker scale than Cu-bearing steel. Uneven scale/steel interfaces and oxide penetration along the grain boundary of the steel were clearly observed in the Ni-bearing steel. The difference in the solid solubility limit of Ni and Cu to austenite could lead to the enrichment of Ni and precipitation of Cu at scale/steel interfaces in the two types of steels, respectively. This resulted in the different structure and properties of the oxide scales, which influenced the subsequent corrosion behavior in chloride containing conditions. Linear polarization resistance measurements showed that the addition of Ni and Cu to the steel had a beneficial influence on the corrosion resistance, by suppressing the anodic dissolution and cathodic reduction reactions of Ni and Cu-bearing steels, respectively. This study provided a proposed mechanism for the difference in corrosion behaviors between the two types of steel. (Received February 1, 2019, Accepted April 16, 2019)

keyword : nickel, copper, steel, oxidation scale, corrosion

Electrical steel is a soft magnetic steel material used in electric devices such as transformers and motors. The performance of these electric devices is primarily related to the magnetic properties of electrical steel, and the assessment of the magnetic properties of electrical steel has been considered an important topic. We use neutron grating interferometry, which is an imaging technique for visualizing the magnetic domain of electrical steel as the evaluation of magnetic properties. The dark-field image provided by neutron grating interferometry shows a sensitive contrast with respect to the magnetic domain of electrical steel due to the small angle neutron scattering generated at the domain wall. The Talbot-Lau interferometer was installed, and the feasibility test of high-resolution dark-field imaging was conducted at cold neutron imaging beamline of the NIST Center for Neutron Research. The dark-field image of electrical steel was compared with the magnetic domain image observed by the Bitter pattern based on the magnetic powder method to prove the validity of neutron grating interferometry. The dark-field image visualizes the magnetic domains of electrical steel, more detailed domain walls regardless of laser-irradiated lines than Bitter pattern result. (Received January 25, 2019; Accepted April 22, 2019)

keyword : magnetic materials, laser-irradiated electrical steel, magnetic properties, neutron diffraction/scattering, neutron imaging, grating interferometer

Li(Ni_1-x-yCo_xMn_y)O₂ (NCM) materials, which are the cathode active material for lithium ion batteries, have been developed and widely used as an alternative to lithium cobalt oxide, because of the high cost of cobalt. To synthesize high quality NCM materials, it is important to control the process manufacturing of the NCM precursor. We synthesized a precursor for LiNi_0.6Co_0.2Mn_0.2O₂ with high capacity through the co-precipitation method using the Taylor reactor. First, it was confirmed that the optimum concentration of ammonia water, which is complexing agent, was 2 M, for uniform particle contribution. The average particle size distributions of the synthesized NCM precursors, and the analysis of the crystal phase and the composition of the NCM precursor, were investigated using a Taylor reactor which is capable of a continuous production process. A reference sample fabricated at a stirring rate of 1,000 rpm showed a composition similar to the target NCM material. When the reaction time was more than 24 hours, the concentration in the Taylor reactor reached a constant steady state, and it was confirmed that continuous production is possible after a reaction time of 24 hours. The use of Taylor reactors can be an effective process because the NCM precursor can be continuously produced, and it is possible to reduce agitation time. (Received February 18, 2019; Accepted April 24, 2019)

keyword : NCM, taylor reactor, lithium ion battery, cathode active material

p-Type (La_1-zNd_z)_0.8Fe_4-xCo_xSb₁₂ skutterudites were prepared by partial double filling with La/Nd and charge compensation by substituting Co for Fe. The skutterudite phase was successfully synthesized using encapsulated melting, quenching, annealing and hot-pressing processes, but a small amount of the marcasite phase was identified. Partial filling within the filling fraction limit did not result in the formation of the (La,Nd)Sb₂ phases detected in the fully-filled La_1-zNd_zFe_4-xCo_xSb₁₂ because that phase is generated when the La/Nd partial filling exceeds the filling fraction limit. (La_0.25Nd_0.75)_0.8Fe₄Sb₁₂ showed the highest electrical conductivity of 2.32 × 10⁵ S m^-1 at 323 K, and (La_0.75Nd_0.25)_0.8Fe₃CoSb₁₂ exhibited the largest Seebeck coefficient of 167 μV K^-1 at 723 K. The power factors and thermal conductivities of all the specimens were in the range of 1.43-2.59 mW m^-1 K^-2 and 2.05-3.95 W m^-1 K^-1, respectively, in the temperature range of 323-823 K. (La_0.25Nd_0.75)_0.8Fe₃CoSb₁₂ showed the highest power factor of 2.59 mW m^-1 K^-2 at 823 K and the lowest thermal conductivity of 2.05 W m^-1 K^-1 at 323 K. Thus, (La_0.25Nd_0.75)_0.8Fe₃CoSb₁₂ exhibited a maximum dimensionless figure of merit (ZT_max) of 0.76 at 723 K. (Received April 4, 2019; Accepted April 24, 2019)

keyword : thermoelectric, skutterudite, partial double filling, charge compensation

During biomass combustion for renewable energy generation, combustion byproducts such as fly ash and metal chlorides cause serious erosion problems in boilers, as well as corrosion. To develop a highly wear-resistant composition of alloy for biomass boilers, we investigated how the addition of various metallic elements to BCC-Fe affected its elastic properties using first principles calculations. The added elements were Al, V, Cr, Mn, Ni, Nb, Mo, and W. These elements used to be included in T91 and T92 steels, which are considered suitable materials for biomass boilers. Our results revealed that except for Al and Ni, all of the other elements increased shear and Young’s moduli, implying high wear-resistant characteristics. In contrast, all of the added elements decreased bulk modulus. Furthermore, V, Cr, Nb, Mo, and W increased atomic bonding strength and thus the escape energy of Fe atoms, leading to hindrance of corrosion by metal chlorides, too. To design a highly wear-resistant BCC-Fe alloy for biomass boilers, the additive elements and their amounts are important, in the order of Mn, Cr, Mo, W > V, Nb >> Al, Ni. We expect that the present results will provide a basic guideline when developing Fe-based materials with superior elastic and mechanical properties for biomass renewable energy generation. (Received December 4, 2018; Accepted May 2, 2019)

keyword : biomass boiler, new and renewable energy, iron erosion, theoretical calculation, elastic property, BCC-Fe alloy

A study on shape optimization of a hot-stamped automotive door impact beam for lightweight design and improved crash performance was conducted using a static collision analysis based on FMVSS 214. An integral type one-piece door impact beam of bracket parts and pipe was realized using hot stamping technology. The method involves the heating in the austenite region and subsequent hot forming and quenching in a specially designed stamping tool with hydraulic system. A one-piece door impact beam is 6.8% lighter than a three-piece door impact beam because it eliminates two bracket parts. The one-piece door impact beam was hot-stamped with dies without cooling channels inside the dies, and revealed a Vickers hardness higher than 430 and a fully martensitic microstructure. Two one-piece door impact beams, with a straight pipe with circular cross section and a straight pipe with partially circular and partially elliptical cross section in the middle part of the pipe, were used for the collision analysis. The door impact beam with partially circular and partially elliptical cross section showed less plastic strain in the middle part of the pipe compared with the circular pipe, leading to better intrusion property. Intrusion analysis results also showed that the three-pieces door impact beam fractured at a 336 mm displacement due to excessive plastic strain in the welded part between the bracket and pipe. In contrast, the integral type one-piece door impact beam without welding did not show any fracture, even at a 457.2 mm displacement, confirming the better performance against lateral collision. (Received February 7, 2019; Accepted April 15, 2019)

keyword : automotive door impact beam, crash performance, lightweight design, hot stamping, one-piece door impact beam, pipe shape

We demonstrate a synthetic route to high-quality Sn microcrystals using thermal chemical vapor deposition (CVD) in a hydrogen-reducing atmosphere. During thermal CVD with SnO₂ powder, a mixture of Ar and H₂ (4%) gases was used as a carrier gas to maintain a hydrogen reduction environment. Hydrogen maintained a reduction environment throughout the thermal CVD process. Samples were prepared at temperatures of 800, 900, and 1000 °C. The structural properties of the Sn microcrystals were determined using synchrotron X-ray diffraction (XRD), transmission electron microscopy, and micro-Raman spectroscopy. In particular, the atomic position order in both the out-of-plane and in-plane directions was determined through high-resolution XRD measurement in a four-circle geometry. We found that at temperatures greater than 800 °C the supersaturation of Sn and O vapors induces the formation of microcrystals by self-assembly process without additional metallic seeds. As determined by the energy dispersive X-ray analysis, oxygen atoms and/or vapor were simultaneously reduced by hydrogen gas, resulting in the formation of Sn crystals rather than Sn oxide crystals. We also found that the Sn microcrystals were mostly strain-free singlecrystalline and of high quality with extremely low mosaicity. The order of the atomic positions in both the out-of-plane and in-plane directions was comparable to that of single-crystalline sapphire. (Received March 28, 2019; Accepted May 2, 2019)

keyword : tin, single crystal, thermal CVD, hydrogen reduction, x-ray diffraction

The effects of natural and pre-aging on the microstructure and mechanical properties of Al-8.0Zn-2.5Mg-2.0Cu aluminum alloy sheet fabricated by twin roll casting were investigated. Immediately after solution treatment, the Al-8.0Zn-2.5Mg-2.0Cu alloy sheet had a yield strength of 173 MPa and elongation of 19.5%. The hardness of the Al-8.0Zn-2.5Mg-2.0Cu alloy sheet increased sharply for 48 hours with natural aging and then increased gradually for up to 336 hours. After 1 week of natural aging, the yield strength was 371 MPa and the elongation was 12.5%. The yield strength after bake hardening treatment at 180 °C for 30 minutes was 519 MPa immediately after solution treatment, 534 MPa after natural aging. The fine GP zones generated by natural aging caused local deformation in the aluminum matrix, which reduced the elongation. In addition, they were mostly dissolved during the bake hardening treatment, and some GP zones served as the nuclei of the metastable η＇. Pre-aging at 120 °C for 10 minutes minimized the decrease in elongation by natural aging, and improved the yield strength after baking hardening. The elongation before bake hardening treatment increased to 21%, and the yield strength after bake hardening treatment increased to 573 MPa. (Received April 1, 2019; Accepted May 9, 2019)

keyword : Al-Zn-Mg-Cu alloy, twin roll casting, pre-aging, precipitation, mechanical properties