발간논문

Home > KJMM 논문 > 발간논문

This study proposed a constitutive equation to predict the change in yield strength according to the behavior of β″ metastable precipitates, which have a profound effect on strength among materials precipitated during the T6 heat treatment of Al-Mg-Si alloy. The β″ precipitate is a metastable phase before it becomes a β (Mg2Si) precipitate, and is distributed in the form of nano-scale rods in the aluminum alloy matrix. Existing precipitation strengthening models assume the shape of the precipitate to be spherical, and in that case the equation that depends on the Orowan mechanism with the average precipitate size and distribution should dominate. However, precipitates are formed in various shapes and sizes by anisotropic growth. In particular, rod-shaped precipitates are not suitable for the existing precipitation strengthening model. In this study, an Al-Mg-Si alloy was fabricated by gravity casting followed by T6 heat treatment. The new precipitation strengthening effect equation proposes that the β″ precipitate affects yield strength during plastic deformation of the Al-Mg-Si alloy. The proposed precipitation strengthening effect equation probabilistically considers the Critical Resolved Shear Stress (CRSS), which varies depending on the angle between the dislocation and the precipitate, when the dislocation passes through a rod-shaped precipitate. (Received April 1, 2021; Accepted May 11, 2021)

keyword : Al-Mg-Si alloy, precipitation, probability dependence, yield strength model

The microstructure, mechanical properties and residual stress of flux-cored arc welded ASTM A516-70N carbon steel using a Mn-based low-temperature transformation (LTT) welding consumable were investigated. Microstructural analysis with X-ray diffraction, an electron backscattered diffractometer and a field-emission scanning electron microscope showed that the LTT weld metal was made up of ferrite, austenite, martensite, and bainite with phase fractions 50.5%, 0.2%, 40.2% and 9.1%, respectively. The increase in hardness and the decrease in absorbed impact energy of the LTT weld metal compared with conventional consumable welds were confirmed to be due to the relatively high fraction of martensite phase in the weld metal. The welding residual stress distributions in three coupons (LTT, conventional and postweld heat-treated conventional weld) were compared by the results using instrumented indentation testing. The LTT weld coupon showed compressive residual stress distributed in the weld metal and heat-affected zone (HAZ), confirming previous studies in which this residual stress was attributed to a martensitic phase transformation at relatively low temperature. PWHT in the conventionally welded coupon considerably reduced the tensile residual stress distributed in the weld metal and HAZ. The LTT consumable, however, showed a significant advantage in welding residual stress, even compared with the heat-treated conventional consumable. (Received April 6, 2021; Accepted April 29, 2021)

keyword : low transformation temperature, LTT consumable, flux-cored arc welding, mechanical properties, residual stress, indentation

The effects of adding CO₂ to low level H₂S containing aqueous environment on the corrosion and hydrogen penetration behaviors of high-strength steel were evaluated using a range of experimental and analytical methods. The corrosion rate of the steel sample exposed to a low level of H₂S dissolved in an aqueous solution was comparatively higher than the one exposed to a mixture of low concentrations of H₂S with CO₂ dissolved in the aqueous solution. The higher corrosion resistance of the steel in the mixture of low concentrations of H₂S and CO₂ was attributed primarily to the three-layer structure of corrosion scale, comprised of an outer Fe-oxide, middle FeS_1-X, and inner FeCO₃, which formed on the steel sample. In particular, the formation of a thin FeCO₃ layer with protective and non-conductive nature may serve as an effective barrier against the penetration of aggressive ionic species in solution, as well as hydrogen atoms formed by cathodic reduction or hydrolysis reactions. Consequently, the hydrogen permeation level, which was measured in a mixture of low-level H₂S and CO₂, was controlled to a comparatively lower value. Nevertheless, the higher level of hydrogen permeation in a mixture of low levels of H₂S and CO₂ at the early corrosion stage might increase the potential risk of pre-mature failure by hydrogen-assisted cracking. (Received May 7, 2021; Accepted May 18, 2021)

keyword : high strength steel, corrosion, H_{2S, CO2, hydrogen permeation}

Transparent and conductive Ti-doped In₂O₃ (TIO)/Ag/Ti-doped In₂O₃ (TAT) multilayer films were deposited on colorless poly imide (CPI) substrates by direct current (DC) and radio frequency (RF) magnetron sputtering at room temperature. During deposition the thickness of both the top and bottom TIO layer was fixed at 30 nm, while the thickness of the Ag interlayer was varied, to 5, 10, and 15 nm, to enhance the optical, electrical and mechanical properties of the films. In the XRD analysis the TIO films did not show any characteristic peaks in the diffraction pattern. The 10 nm thick Ag inter layer showed some characteristic peaks of Ag (111), (200), (220) and (311), respectively, and the grain size of the Ag interlayer enlarged as Ag thickness increased. To investigate the most efficient Ag interlayer thickness, a figure of merit (FOM) based on the opto-electrical performance of the transparent conducting films was compared. The films with a 10 nm thick Ag interlayer exhibited a higher FOM of 1.71 × 10^-2 Ω^-1 than the other films. When the radius of the film's curvature was reduced to 1.7 mm, the TIO single layer films showed a 13 times increase in sheet resistance, while the TAT (30/10/30 nm) films showed an insignificant change in sheet resistance. From the observed results, it was concluded that the Ag interlayer in the TAT multilayer films enhanced the opto-electrical performance of the films and also acted as a potent bridge which assured the high flexibility endurance of the films. (Received April 1, 2021; Accepted April 29, 2021)

keyword : TIO, Ag, X-ray diffraction, figure of merit, flexible endurance

Sintered M-type hexaferrites with the chemical formula of Sr_0.3Ca_0.4La_0.3Fe_9.8Co_0.2-xMn_xSi_0.135O_19-d (x = 0, 0.05, 0.1, 0.2) and Sr_0.3Ca_0.4La_0.3Fe_9.8-yCo_0.2Mn_ySi_0.135O_19-d (y = 0.05, 0.1, 0.2) were prepared by conventional solid station reaction routes. A high sintering density of more than 95% of the theoretical density was achieved in all hexaferrite samples when calcination was carried out at 1100 ℃ for 4 h, followed by sintering at 1230-1250℃ for 2 h without the use of sintering additives. High saturation magnetization and coercivity were achieved simultaneously at the x = 0.05 composition, where Mn replaces part of the Co. The secondary phase Fe₂O₃ generated by the initial addition of SiO₂ was gradually reduced when the Fe contented was decreased in the Sr_0.3Ca_0.4La_0.3Fe_9.8-zCo_0.15Mn_0.05Si_0.135O_19-d samples, and a single M-type hexaferrite phase was confirmed in the Sr_0.3Ca_0.4La_0.3Fe_8.3Co_0.15Mn_0.05Si_0.135O_19-d (z = 1.5) sample, which also exhibited optimized hard magnetic properties, with a saturation magnetization of 4581 G and coercivity of 4771 Oe. Anisotropic sintered magnets were fabricated using the optimized composition, and showed excellent hard magnetic properties, with a remanent magnetic flux density of 4400 G and intrinsic coercivity of 4118 Oe, and a maximum energy product of 4.72 M·G·Oe. This result is very promising because high magnet performance can be achieved with a single batch process without the need for sintering additives during the process. (Received February 25, 2021; Accepted March 16, 2021)

keyword : M-type hexaferrites, coercivity, saturation magnetization, permanent magnet

Tetrahedrite is a promising thermoelectric material mainly due to its low thermal conductivity, a consequence of its complicated crystal structure. However, tetrahedrite has a high hole concentration; therefore, optimizing carrier concentration through doping is required to maximize the power factor. In this study, Te-doped tetrahedrites Cu₁₂Sb_4-yTe_yS₁₃ (0.1 ≤ y ≤ 0.4) were prepared using mechanical alloying and hot pressing. The mechanical alloying successfully prepared the tetrahedrites doped with Te at the Sb sites without secondary phases, and the hot pressing produced densely sintered bodies with a relative density >99.7%. As the Te content increased, the lattice constant increased from 1.0334 to 1.0346 nm, confirming the successful substitution of Te at the Sb sites. Te-doped tetrahedrites exhibited p-type characteristics, which were confirmed by the positive signs of the Hall and Seebeck coefficients. The carrier concentration decreased but the mobility increased with Te content. The electrical conductivity was relatively constant at 323-723 K, and decreased with Te substitution from 2.6 × 10⁴ to 1.6 × 10⁴ Sm^-1 at 723 K. The Seebeck coefficient increased with temperature and Te content, achieving values of 184-204 μVK^-1 at 723 K. The thermal conductivity was <1.0 Wm^-1K^-1, and decreased with increasing Te content. Cu₁₂Sb_3.9Te_0.1S₁₃ exhibited the highest dimensionless figure of merit (ZT = 0.80) at 723 K, achieving a high power factor (0.91 mWm-1K-2) and a low thermal conductivity (0.80 Wm^-1K^-1). (Received March 9, 2021; Accepted May 6, 2021)

keyword : thermoelectric, tetrahedrite, mechanical alloying, hot pressing, doping

The bonding properties of adhesives are mainly affected by surface roughness, topography and chemical adsorption. In this paper, we studied the effects of surface pretreatment of Al 2024-T3 (bare) in terms of surface roughness, topography and surface free energy. Surface pre-treatment included solvent cleaning, FPL etching, PAA and CAA treatment. The surface energy and roughness of the aluminum surface were significantly increased by the anodizing treatment. Single lap shear and fatigue tests were performed to investigate bonding properties and durability. The evaluation revealed that the surface energy and surface roughness resulting from the aluminum surface treatment had a significant impact on bonding properties and durability. PAA treated surfaces had the highest bonding strength, and CAA treated surfaces had superior bonding retention performance in hot water or salt spray environments. The results of the fatigue test most clearly demonstrated how the surface pretreatment of the aluminum alloy differently affected bonding performance. (Received March 27, 2021; Accepted May 10, 2021)

keyword : adhesive, bonding strength, anodizing, surface roughness

In recent years, human-convenient smart wearable devices have attracted considerable attention as emerging applications in smart healthcare systems, soft robotics, and human-machine interfaces. In particular, resistive film heaters with mechanical flexibility and excellent mechanical and electrothermal performance have recently been widely explored for wearable thermotherapy applications. Here, we present a simple and efficient way of fabricating highly flexible and stretchable resistive film heaters based on a patterned silver nanowire (AgNW)/polymer composite structure. The AgNW/polymer composite electrodes were successfully prepared using a photolithographically patterned polymer mold based selective transfer of a AgNW percolation network. The photolithographic mold patterning process allows the heater fabrication to be precise and reproducible. The mesh-patterned AgNW/polymer composite heater exhibited the excellent electrothermal performance of ~46.7 ℃at 3 V. This low-voltage operation is highly desirable in practical wearable device applications. Moreover, the AgNW/polymer heater can be stretched up to 20% without significant degradation in electrothermal performance thanks to its open-cell architecture, suggesting that the device can stably transfer heat to the skin after being attached to various body parts with curvilinear surfaces. The experimental results suggest that the mesh-structured AgNW/polymer composite heaters are highly feasible for use as a wearable thermotherapy tool in many emerging applications. (Received April 7, 2021; Accepted May 13, 2021)

keyword : metal nanowire, lithographic mold patterning, selective transfer, stretchable film heater

The effect of the heat treatment on the Mg₂Si phase in Al-Mg-Si alloy was investigated by a laser flash apparatus (LFA), Differential scanning calorimetry (DSC) and corrosion test. The alloy samples were solution treated at 590 ℃ for a half hour followed by warm water quenching, and then aged in air at 180, 200 and 240 ℃ for 5 hours. The results showed that the corrosion resistance of the solid solution treated sample was more improved than the as cast sample. Aging treatment also helped increase corrosion resistance at room temperature. It is thought that the fine Mg₂Si precipitation phase on the grain had a more positive effect on improving corrosion resistance than crystallization of the Mg₂Si phase on the grain boundaries. Corrosion rate also decreased with increasing aging treatment. The corrosion rate of AT240 was reduced to 1.16 MPY compared with the AT180 test piece, which had a corrosion rate of 3.79 MRY. The solution treated sample also showed lower thermal diffusivity than the aged samples. The thermal diffusivity increased as the solute concentration of Mg and Si in the a-Al matrix rapidly decreased during aging treatment. On the other hand, the thermal diffusivity of the aged samples, in which precipitation was completed by the aging process, decreased as the temperature rose. The thermal conductivities of all samples were similar near 250 ℃ when the β'' phase and β' precipitation was completed. (Received February 4, 2021; Accepted May 27, 2021)

keyword : Al-Mg-Si, thermal diffusivity, corrosion, precipitation, dissolution