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It has been proposed that a primary water stress corrosion cracking (PWSCC) in pressurized water reactor (PWR) is governed by a lattice contraction due to a short range ordering reaction in Alloy 600. This leads researcher to think that the kinetics of lattice contraction may control a susceptibility of PWSCC in Alloy 600. A lattice variation with ordering treatment at 400 ℃ was systematically investigated using high resolution neutron diffraction(HRPD) in high temperature mill anneal (HTMA), low temperature mill anneal (LTMA), and sensitized (SEN) Alloy 600. The results showed that ordering treatment caused an isotropic lattice contraction due to short range ordering (SRO) reaction. The lattice contractions of (111) plane are saturated to be 0.04% in 4 to 1500 hours at 400 ℃ according to prior treatment condition. The lattice contraction in the magnitude of 0.03% of (111) plane in LTMA Alloy 600 is faster by 8 times and 66 times than that of SEN and HTMA, respectively. This fact may explain why the LTMA is most susceptible to PWSCC through of kinetics of lattice contraction in Alloy 600. Thus, it is possible to conclude that the susceptibility of Alloy 600 to PWSCC is governed by the kinetics of (111) lattice contraction. (Received August 11, 2020; Accepted October 7, 2020)

keyword : Alloy 600, PWSCC, mill anneal, neutron diffraction, lattice contraction, ordering reaction

To automatically reconstruct the prior austenite grains from as-quenched martensitic structure, we applied a deep learning algorithm to recognize the prior austenite grains boundaries hidden in the martensitic matrix. The FC-DenseNet architecture based on FCN (fully convolutional networks) was used to train the martensite and ground truth label of the prior austenite grain boundaries. The original martensite structures and prior austenite grain boundaries were prepared using different chemical etching solutions. The initial PAGS detection rate was as low as 37.1%, which is not suitable for quantifying the basic properties of the microstructure such as grain size or grain boundary area. By changing the weight factor of the neural net loss function and increasing the size of the data set, the detection rate was improved up to 56.1%. However, even when the detection rate reached 50% or more, the quality of the reconstructed PAGS was not comparable to the analytically calculated results based on EBSD measurements and crystallographic orientation relationships. The prior austenite grain size data sets were obtained from martensite samples via the FC-DenseNet method, and had a linear correlation with the mechanical properties measured in the same samples. In order to improve the accuracy of the detection rate using neural networks, it is necessary to increase the number of neural networks and data sets. (Received November 16, 2020; Accepted November 24, 2020)

keyword : A, I, artificial intelligence, artificial neural networks, deep learning, martensite, prior austenite grain boundary, PAGB

The effects of the cooling rate, after solution heat treatment, on the microstructures and mechanical properties of Fe-22Mn-8Al-0.8C-0.02Nb (low carbon) and Fe-20Mn-8Al-1.1C-0.1Nb (high carbon) light-weight steels were systematically investigated. The cooling process was controlled to achieve six different cooling rates, ranging from -0.016 to -465.1 ℃/s. Under the slowest cooling rate (furnace cooling), intra-granular and inter-granular precipitations of κ-carbides were observed throughout the austenite grains. The higher the C content was, the larger the size of the inter-granular κ-carbides was. The formation of κ-carbides resulted in an increase in yield strength, and a decrease in elongation and impact absorbed energy. In the Fe-20Mn-8Al- 1.1C-0.1Nb, the inter-granular precipitation of κ-carbides caused a drastic decrease in the impact absorbed energy and the inter-granular brittle fracture. To predict the distribution of yield strength and impact absorbed energy at production scale (a 10-ton scale slab), finite element analysis was conducted for water cooling and air cooling conditions. The average cooling rates at the center of the slab under water cooling and air cooling were predicted to be -0.126 and -0.031 ℃/s, respectively. Based on predicted cooling rates, the distribution of mechanical properties was determined. The prediction suggested that a large-scale slab of the light-weight steel with low C content would have good toughness at the center of the slab regardless of cooling condition. (Received November 2, 2020; Accepted November 23, 2020)

keyword : light-weight steel, cooling rate, κ-carbide, mechanical properties, finite element analysis

In this study, oxide (Y₂O₃)-embedded Ni particles were fabricated via a new process. The process involves the mechanical hydrogenation of Ni-Y alloy into Ni-YH₂ and the selective oxidation of Ni-YH₂ to Ni-Y₂O₃. The alloy powders were prepared for sintering by the mechanical alloying of a mixed powder. The powder was prepared with a desired composition (Ni-20wt.%Cr-1.2wt.%Y₂O₃) by adding Ni and Cr powders to the Ni-Y₂O₃ composite prepared by the new process. For comparison, a mixed powder with the same composite was prepared using a conventional mechanical alloying (MA) approach. Samples of the two powders were sintered by SPS at 850, 900, 950, and 1,000℃. The relative densities of all samples were higher than 99.7% at all sintering temperatures. The oxide particles and matrix grain sizes of the Ni-20Cr-1.2Y₂O₃ alloy prepared by this new process were finer than those in alloys fabricated by conventional MA processes. Therefore, the improvement in the mechanical properties of the Ni-20Cr-1.2Y₂O₃ alloy prepared by the new process was attributed to the refinement of the oxide particles. (Received September 14, 2020; Accepted October 7, 2020)

keyword : oxide-embedded Ni particles, oxide dispersion-strengthened alloys, selective oxidation, spark plasma sintering, mechanical properties, mechanochemical processing

Generally, molds are fabricated by the machining of massive billets of tool steels, such as AISI4140 or H13, but it has drawbacks, such as a large material loss and long-delivery time. The Wire-Arc Additive Manufacturing (WAAM) process could be an alternative fabrication method. It has the advantages of less material loss, short-delivery time, and the chance to make a reinforced mold using dissimilar materials. 5 Cr - 4 Mo steel wire has high potential to produce molds via the WAAM process. This is a commercial tool steel solid wire initially designed for the repair and modification of tools and molds that has superior hot wear resistance and toughness. However, no study has examined the WAAM of tool steels, even though it has high potential and advantages. Shielding gas has a significant effect on the performance of the WAAM process, which is based on gas metal arc welding (GMAW). Argon (an inert gas) and carbon dioxide (a reactive gas) are generally used for the GMAW of steel alloys, and they are frequently used as mixed gases at various ratios. Shielding gases have a significant influence on the arc stability, weld quality, and formation of weld defects. Therefore, using a proper shielding gas for the material and process is important to sound WAAM performance. This paper discusses the effect of the shielding gas on the additive manufacturability of tool steel, as a first step for the WAAM of die casting molds. The experiments were conducted with two different shielding gases, M21 (Ar + 18% CO₂) and C1 (100% CO₂). The use of C1 showed neither surface contamination nor internal defects, and resulted in a larger amount of deposition than the M21. (Received July 8, 2020; Accepted October 15, 2020)

keyword : GMAW, wire arc additive manufacturing, WAAM, tool steel, shielding gas, die casting mold

In this study, the effects of electrode surface design on the resistance spot weldability and degradation of the electrode following resistance spot welding (RSW) of aluminum 6014-T4 alloy were investigated. A new patterning method that can be produced through repetitive pressurization was applied to the electrode, producing a lattice-like pattern shape on the resulting patterned electrode. When RSW was performed using the lattice patterned electrode, the contact resistance decreased because of the effective removal of the oxide film from the surface of the aluminum alloy. As a result, heat generated by resistance on the E/S interface was reduced. Moreover, the growth rate of the weld nuggets formed with the patterned electrode in the thickness direction was lower than that of the nuggets formed with the as-received electrode, and there was comparatively less Cu-Al alloying of the patterned electrode. In addition, a continuous RSW process was performed on the alloy to observe the effect of the electrode surface design on electrode sticking. The results indicated that electrode surface shape can significantly influence resistance heat generation and electrode cooling effects, as well as produce welds with different weld morphology and microstructure. Finally, it was proved that the patterned electrode suffered less electrode degradation through EPMA on the electrode surface after the continuous RSW was completed. (Received September 14, 2020; Accepted November 12, 2020)

keyword : surface design, resistance spot welding, RSW, lattice-patterning, oxide film, cooling effects, electrode degradation

In this study, the weldability of the as-cast CM247LC superalloy for turbine blade applications was metallurgically evaluated in terms of its hot cracking behavior and susceptibility. For this purpose, a real blade was manufactured using a directional solidification casting process, and gas tungsten arc welding was performed at the tip and cavity of the upper blade. Hot cracking was confirmed in the heat-affected zone (HAZ) of gas tungsten arc welds, and the cracks were characterized as liquation cracks, since a cobble or dropletshaped crack surface consistent with a liquid film was clearly confirmed. Microstructural analysis of the cracking surface and thermodynamic calculations helped elucidate the metallurgical mechanisms of the liquation cracking. In other words, the cracking was attributed to liquation of the γ-γ’ eutectic colony and the constitutional liquation of the MC-type carbides: these phases existed in the as-cast microstructure. In particular, it was calculated that liquation of the γ-γ’ eutectic colony during welding occurs at least at 1488 K and that constitutional liquation of MC-type carbides begins at 1411 K, while the equilibrium solidus temperature of the CM247LC alloy is 1530 K. Finally, the liquation cracking susceptibility was quantitatively evaluated through a spot-Varestraint test, and it was confirmed for the first time that the higher susceptibility of as-cast samples can be suppressed by employing a pre-weld heat treatment such as solution treatment. (Received October 5, 2020; Accepted November 11, 2020)

keyword : turbine blade, CM247LC superalloy, welding, heat-affected zone, liquation crack, spot-Varestraint test

Spinel ferrites (Ni_0.5Zn_0.5)_1-x-yCo_xCu_yFe₂O₄, (x = 0 and y = 0, x = 0.2 and y = 0, x = 0.1 and y = 0.1, x = 0 and y = 0.2) were prepared by sol-gel method and post-annealed at 1100℃. The grain growth of the sample is very sensitive to the Cu substitution y. The average grain size of the non-doped sample (x = 0, y = 0) was ~400 nm and it increased to ~3 μm at the sample with x = 0 and y = 0.2. The real and imaginary parts of permittivities (ε', ε") and permeabilities (μ', μ") were measured on the spinel ferrite powder-epoxy (10 wt%) composite samples by a network vector analyzer in the frequency range of 0.1 ≤ f ≤ 15 GHz. The μ' and μ" depend on Co substitution x and the ε' is sensitive to Cu doping y. Reflection loss (RL), which implies electromagnetic (EM) wave absorption properties, were analyzed based on the complex permeability, permittivity spectra. In the RL map plotted as functions of sample thickness (d) and frequency (f), the intensive EM absorbing area (RL < -30 dB) shifted to a high frequency region with increasing Co substitution. This can be attributed to a permeability spectra shift, due to the increase in ferromagnetic resonance frequency produced by the Co substitution. The samples with x = 0.1 and y = 0.1, x = 0.2 and y = 0 also exhibited a very broad-ranged EM wave absorbing performance with a d < 3 mm, indicated by RL < -10 dB being satisfied in the frequency range 7~14 GHz. (Received September 8, 2020; Accepted October 18, 2020)

keyword : Ni-Zn ferrites, permittivity, permeability, Reflection loss, EM absorption

Nickel hexacyanoferrate (NiHCF) has a three-dimensional open framework structure, excellent long-cycling stability and rate performance as a cathode for aqueous sodium-ion batteries. However, the specific capacity of NiHCF is lower than that of present cathodes for aqueous batteries. A sodium-ion electrolyte was explored to achieve optimum capacity with NiHCF. Powder-type NiHCF was fabricated by coprecipitation with the atomic composition K_0.065Ni_1.44Fe(CN)₆·4.4H₂O. The presence of Fe vacancies in the atomic composition is attributable to the inclusion of coordinating and zeolitic water during coprecipitation. Two sodium-ion electrolytes, 1 M Na₂SO₄ and 1 M NaNO₃, were employed to analyze the electrochemical behavior of the NiHCF electrode. Identical redox potentials to 0.58 V (vs. NHE) were measured in both electrolytes. However, a lower overpotential was observed in NaNO₃ compared to Na₂SO₄ as a result of the smaller interfacial charge transfer resistance. The lower charge transfer resistance in the NaNO3 solution produced a higher specific capacity of 57 mAh g^-1 (1 C-rate) and the superior capacity retention of 46.6% at 20 C-rate. The anion in the aqueous electrolyte changed the charge transfer resistance at the electrode/ electrolyte interface, confirming the electrolyte anion has a crucial effect on the charge capacity and rate performance of NiHCF. (Received August 18, 2020; Accepted October 12, 2020)

keyword : nickel hexacyanoferrate, coprecipitation, electrolyte anion, charge transfer resistance, specific capacity, rate capability

Solar water splitting is an attractive method for producing hydrogen from renewable natural resources, and heterostructure photocatalysts have been widely investigated for photocatalytic applications. Hetero-component photocatalysts can reduce the charge recombination process by improving electron utilization, and are considered promising candidates for solar water splitting. Amongst various heterostructure systems, combinations of copper and zinc have been advantageous for constructing efficient band potential energy systems. In this work, Cu_xZn_1-xS composite photocatalysts were solvothermally prepared with various copper contents. The morphology of the Cu_xZn_1-xS photocatalysts was examined using scanning electron microscopy, and the crystalline structures were established with an X-ray diffractometer. Atomic analyses of the surface components of the photocatalysts were performed using X-ray photoelectron spectroscopy. UV-Vis spectroscopy and photoluminescence spectroscopy were also used to examine the efficiency of the photocatalysts’ light responses. Brunauer Emmett Teller analyses were employed to characterize the surface area and pore volume of the photocatalysts. Among the various Cu_xZn_1-xS compositions, the highest H₂ production rate was determined to be 1122 μmol g^-1 h^-1 from the Cu_0.03Zn_0.97S photocatalyst. This highest H₂ production rate is strongly related to the observed efficient light absorption, and its influence on charge generation. The improvement is mainly attributed to the optimized charge separation and utilization, high visible light absorption, and high surface area of the photocatalyst. (Received September 16, 2020; Accepted October 15, 2020)

keyword : water splitting, Cu_{xZn1-xS, photocatalyst, hydrogen production}