발간논문

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The behaviour and performance of complex metallic materials (e.g., advanced high strength steel and dual phase Ti alloys) are acknowledged to be multi-scale in nature and to rely on the details of the threedimensional morphology, chemistry and crystallography at the meso-scale level and on macro-scale level geometry and boundary and loading conditions. As those materials often contain a multi-phase and complicated microstructure, it is difficult to fully characterise a material’s deformation behaviour with conventional macro-scale testing. Recently, a small-scale testing method has been developed to investigate the local deformation behaviour of metallic materials based on the following core techniques: portable nanoindenation platform for providing a high resolved load-displacement system, focused ion beam (FIB) for fabricating a small-scale geometric specimen and electron backscatter diffraction (EBSD) for characterising microstructure and crystallographic orientation. This method has been combined with further high resolution techniques including HR-EBSD and X-ray micro-Laue diffraction, providing us a rich understanding of the fundamental behaviour of materials on the level of the individual microstructural constituents and potentially strengthening our understanding of materials by relating macro- and micro-scale behaviour. In this review article, we describe the small-scale testing method and further high resolution techniques used to studying complex metallic materials. (Received July 16, 2018; Accepted July 25, 2018)

keyword : metallic materials, deformation, micromechanics, small-scale testing, characterisation

In the present study, a choline chloride(ChCl)-based ionic liquid(IL) was used as an electrolyte to confirm the characteristics of electroless tin plating. The effect of the type of additive and reducing agent, as well as the addition of complexing agent and water, on the properties of the ChCl-based IL, including the freezing point, viscosity, conductivity, open circuit potential, and plating thickness, was investigated. The results showed that the crystallization temperature was 25 ℃ when the ChCl-thiourea molar ratio was 1:1, and it decreased to 10 ℃ upon adding the additive, while the conductivity increased and viscosity decreased. Dimethylthiourea was added to promote the electroless plating reaction. Electrolytic tin plating was performed at 70 ℃ using ChCl -thiourea-based IL as the electrolyte. The thickness of the tin-plated layer was 1.5 μm when malonic acid and dimethylthiourea were used. The addition of a small amount of water increased the plating speed and thickness, and the maximum thickness of 2 μm for the tin-plated layer was obtained after 30 min. These characteristics demonstrate the potential applications of these environmentally friendly electrolytes for fast, thick, and dense electroless tin plating using ILs at low temperatures. (Received May 4, 2018; Accepted July 2, 2018)

keyword : ionic liquid, Sn plating, electroless plating, choline chloride, deep eutectic

In this work, the growth of nanotubular oxide layers on Ti_1-xNi_x (x=0.49, 0.498, 0.511, 0.522, 0.525) shape memory alloys was investigated. All of the Ti-Ni alloys were fabricated by arc melting process under high vacuum atmosphere. The crystal structure and the growth of the nanotubular oxide layers were investigated by X-ray diffraction (XRD), field-emission scanning electron microscope (FE-SEM) and energy dispersive X-ray spectroscopy attached to a transmission electron microscope (TEM). The electrochemical anodization was carried out at various durations (5 min and 20 min) and applied voltages (20 V, 35 V and 50 V) at room temperature in ethylene glycol containing 0.06 M NH₄F and 1.5 wt% H₂O. Nanotubular oxide layers were formed on all Ti-Ni alloys by electrochemical anodization, but the morphology of the oxide layers was changed depending on the anodization duration and Ni concentration of the Ti-Ni alloy substrate. In the case of short duration (5 min), the nanotubular oxide layers were formed on all Ti-Ni alloys by electrochemical anodization, however, the morphology of oxide layer was changed from nanotubular structure to nanopore structure with increasing anodization duration. Moreover, it was cleared that the morphology variation of the nanotubular oxide layers was accelerated with increasing Ni concentration, from 49.0 at% to 52.5 at%, which is attributed to the formation of a Ni-rich layer on the Ti-Ni alloy surface by electrochemical anodization. (Received May 30, 2018; Accepted June 30, 2018)

keyword : shape memory alloy, Ti-Ni, nanotubular oxide layer, TiO₂, anodization

Titanium nitride (TiN) has been used extensively in coating materials and cutting tools because of its attractive properties, which include low density, high melting point, high hardness and thermodynamic stability. However, like many ceramic materials, its low fracture toughness limits wide industrial application. The methods generally utilized to improve its fracture toughness involve the fabrication of nanostructured materials and composites. In this paper, BN was evaluated as a reinforcing material in TiN ceramics by pulsed current activated sintering method. Highly dense nanostructured TiN and TiN-BN composites were achieved within 1 min at 1400 ℃. The effect of BN on the grain size, hardness and fracture toughness of TiN -BN composites was evaluated. The addition of BN to TiN simultaneously improved the hardness and fracture toughness of the TiN-BN composite by refining TiN and the deterrence of crack propagation by BN. (Received July 2, 2018; Accepted July 17, 2018)

keyword : nanostructured material, sintering, TiN, hardness, fracture toughness

This study examined the influence of nanosized aluminum nitride (AlN) powders (0, 0.01, 0.05, 0.10, 0.30, and 0.50 wt%), on an Al4047-based filler metal. Nanosized AlN powders were dispersed uniformly into the filler metal by a mechanical mixing and melting route using a stainless steel propeller. The influences of nanosized AlN powders on the melting behavior, microstructure, brazeability, and tensile properties were examined, and interfacial reactions between aluminum and copper were carried out. The experimental results showed that the thicknesses of Si and IMC keep decreasing with additions of nanosized AlN powders up to 0.10% in the brazing filler metal, which is attributed to the adsorption theory of nanosized AlN powders on the surface of Si and IMCs. In addition, the AlN-reinforced filler metal with 0.10 wt% samples showed a 2.61% enhancement in wettability due to the decreased surface tension in the filler metal matrix in the presence of nanosized AlN powders. Furthermore, aluminum to copper dissimilar brazing with 0.10 wt% AlN-reinforced filler metal showed a 16.07% improvement in the tensile strength compared to the 0% AlN filler due to the grain refinement of a filler metal matrix by the Orowan strengthening effect. (Received July 3, 2018; Accepted July 26, 2018)

keyword : brazing, AlN-reinforced filler, brazeability, tensile strength, grain refinement

Directional solidification experiments were carried out to estimate the effect of solidification velocity and temperature gradient on primary dendrite arm spacing (PDAS) in the Ni-based superalloy, CMSX-4 alloy. The specimens were solidified at velocities of 1, 5, 25, 50 and 100 mm/s under given temperature gradients of 12.5, 18, 23 K/mm. The models for predicting PDAS with solidification conditions had the form of λ∝V^-1/4G^-1/2, but the experimental results exhibited a discrepancy with the exponents of -1/4 and -1/2 in V and G, respectively. Therefore, a phenomenological formula based on the experimental results was proposed to predict PDAS, and it was found that the predicted PDAS under the experimental conditions was in good agreement with the experimental results. (Received July 12, 2018; Accepted August 7, 2018)

keyword : directional solidification, dendrite arm spacing, temperature gradient, Ni base superalloy

This study elucidates the effects of annealing on the properties of Fe-Si-Al flakes. The changes of the DO₃ phase, the lattice parameter and the grain size of the flakes were analyzed by X-ray peak broadening and shift. Static magnetic properties, namely, coericivity and saturation magnetization, were analyzed by a vibration sample magnetometer (VSM). The complex permeability spectra were measured by a precision impedance analyzer. The cocercivity and the real part of the complex permeability had the lowest and highest values after 1023 K annealing. The change in the magnetic properties with annealing temperature is successfully explained by relief of residual stress and the formation of DO₃ and B2 ordered phases. (Received July 5, 2018; Accepted July 25, 2018)

keyword : Fe-Si-Al flake, DO₃ hase, residual stress, complex permeability

Thermoelectric energy conversion has attracted much attention as an efficient and environment-friendly energy technology. Thermoelectric energy conversion performance is directly influenced by the figure of merit (ZT) of the thermoelectric material, however, most known thermoelectric materials with high ZT contain toxic and/or scarce elements. Herein, we report the thermoelectric properties of a polycrystalline p-type Si composite. Si is not only one of the most abundant elements in the earth’s crust and but also non-toxic. The composite was prepared by consolidating melt-spun B-doped Si ribbons using spark plasma sintering to achieve a nanocrystalline composite. However, unexpectedly, the resulting material was composed of micron-sized grains with submicron-sized pores. This microstructural character provided unexpected benefits from a thermoelectric point of view. First, the mobility of the composite was quite compatible with that of single crystalline B-doped Si wafer, as elucidated by our calculations based on Masseti's formula. Furthermore, the hole concentration was increased in the compound compared to the B-doped Si wafer, due to unintentional Cu-doping during the melt-spinning process, which resulted in enhanced electrical conductivity. Notably, the lattice thermal conductivity was significantly reduced by the existence of pores, while the electrical conductivity was enhanced. This phonon-glass electron-crystal (PGEC), realized in the polycrystalline p-type Si composite, could lead to an increase in ZT. (Received June 8, 2018; Accepted July 4, 2018)

keyword : thermoelectric, Si, melt-spinning, spark plasma sintering

Ge-doped higher manganese silicides (HMSs) MnSi_1.72-1.73:Ge_m (m = 0.01-0.04) were prepared and their thermoelectric properties were studied. HMS powders were synthesized by solid-state reaction of raw material powders (Mn, Si, and Ge) at 1273 K for 6 h in vacuum, and hot-pressed at 1173 K for 2 h under 70 MPa. Mn₁₁Si₁₉ or Mn₁₅Si₂₆ was the main HMS phase; only a small quantity of Si remained. The intermetallic compound MnSi was not produced. The lattice constants increased with the substitution of Ge at Si sites. With the increase in the Ge doping concentration, the electrical conductivity increased, while the Seebeck coefficient decreased. The power factor was significantly increased by the Ge doping, owing to the increase in the electrical conductivity. The Ge doping led to phonon scattering owing to the difference in mass between Ge and Si, leading to a reduced thermal conductivity. Therefore, the dimensionless figure of merit (ZT) was remarkably enhanced by the Ge doping. For MnSi_1.72:Ge_m, the maximum ZT of 0.44 was obtained at 823 K for MnSi_1.72:Ge_0.01, while for MnSi_1.73:Ge_m, the maximum ZT of 0.37 was achieved at 823 K for MnSi_1.73:Ge_0.03.

keyword : thermoelectric, HMS, higher manganese silicide, solid-state reaction, hot pressing