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Zirconium-based alloys, widely used for nuclear fuel cladding, are highly susceptible to hydrideinduced embrittlement. The degree of the hydride embrittlement is strongly affected by not only its concentration but also its morphology. The hydride can drastically reduce the mechanical properties of the cladding, especially when it is radially precipitated. Recently it has been reported that circumferential hydrides can re-precipitate into radial hydrides if applied hoop stress in the cladding exceeds a certain limit, so-called threshold stress, during the long-term dry storage of spent nuclear fuel. This hydride reorientation phenomenon is now considered as one of the most critical issues in the nuclear fuel industry, threatening cladding integrity during the steady state storage period as well as transportation of spent nuclear fuel. In fact, the phenomenon is known to be quite complex, and affected by numerous factors. In this paper, the major influential factors affecting reorientation issues, namely temperature, stress, number of thermal cycles, cooling rate, and hydrogen concentration, are reviewed. In addition, recent studies on the synergistic effects of the factors on the threshold stress triggering the reorientation are discussed in detail. Finally, summarizing conclusions for mechanistic understanding are drawn and recommendations for future work are made. (Received October 30, 2017; Accepted November 27, 2017)

keyword : zirconium alloy cladding, zirconium hydride, hydride reorientation, threshold stress

The toughness and creep strength of Mod. 9Cr-1Mo steel, a ferritic heat resistant steel, are closely related to the amount of delta ferrite remaining in the weld. This study compared the amount of retained δ ferrite in the laser beam (LB) weld with low heat input and the gas tungsten arc (GTA) weld with high heat input. A greater amount of retained δ ferrite in both the LB and GTA welds was found in the fusion zone (FZ) interface than in the FZ center and coarse-grain heat affected zone (CGHAZ). LB welds had more retained δ ferrite than GTA welds. Thermal history of the welding process and location in the welds were calculated by finite element analysis. By simulating the thermal history, calculated at the FZ interface for each welding process, it was confirmed that the LB welds had more retained δ ferrite than the GTA welds in the specimens. The amount of retained δ ferrite increased as the time spent in the δ ferrite phase region increased and as the cooling rate increased. The effect of the cooling rate on the amount of retained δ ferrite was greater than the time spent in the δ ferrite phase region upon heating. (Received September 29, 2017; Accepted November 28, 2017)

keyword : Mod. 9Cr-1Mo, δ ferrite, thermal history, finite element analysis, Gleeble test

This research focused on the development of the part unit evaluation for an organic coated automotive lower arm using the electrochemical cyclic corrosion test (E-CCT). The E-CCT accelerates both the coating degradation and the perforation corrosion. The applied potential for the coating degradation was determined to be -500 mV_SCE for anodic polarization and -900 mV_SCE for cathodic polarization. The applied current for the perforation corrosion was equivalent to the applied anodic potential. The E-CCT was conducted for the duration of 8 cycles, which is considered to be equivalent to 4 years of driving. After the E-CCT, the coating degradation was evaluated by electrochemical impedance spectroscopy (EIS) measurements. Especially, the Z_{100 mHz} value (impedance value at a frequency of 100 mHz) of the part unit represented the amount of corrosion damage of the lower arm parts. The Z_{100 mHz} of the degraded coating area had a decisive effect on the Z_{100 mHz} value of the entire lower arm. This result was caused by electrochemical interaction between the non-degraded coating area and degraded coating area. The non-degraded coating area and the degraded coating area of the lower arm formed the parallel circuit which provides a satisfactory result. (Received July 24, 2017; Accepted December 11, 2017)

keyword : metals, oxides, coating, corrosion, electrochemistry

WC-Co coatings manufactured by high velocity oxy-fuel (HVOF) spraying have a superior hardness and wear resistance. In this study, HVOF-sprayed WC-12Co coatings from three different feedstock powders were evaluated in terms of microstructural features, carbide decomposition and erosion resistance. The fusedcrushed and the sintered-crushed WC-12Co powders had an angular shape, a dense internal structure and a high apparent density, whereas the agglomerated-sintered powder was found to have a spherical morphology with a porous internal structure and a relatively low apparent density. These differences in powder characteristics significantly affected porosity, carbide decomposition, microhardness and erosion resistance in the WC-Co coating. The fused-crushed coating contained a large amount of intersplat pores and a low fraction of carbide, resulting in the lowest microhardness. The erosion wear rate of the fused-crushed coating increased sharply with increasing impact angle, due to the brittle damage mechanism. The agglomerated-sintered coatings had a low porosity and a high microhardness, and consequently exhibited superior erosion wear resistance with no dependency on impact angle. (Received October 11, 2017; Accepted November 14, 2017)

keyword : WC-Co, HVOF coating, powder processing, porosity, erosion

This study manufactured Fe-Cr-Ni-based alloy (HK30) using the metal injection molding (MIM) process and investigated the microstructure and room·high temperature mechanical properties of the material. The effect of hot isostatic pressing (HIP) on properties was also examined. The average grain sizes were 24.1 μm for an MIM specimen and 29.1 μm for an MIM+HIP specimen. Microstructure and phase analysis results confirmed that the matrix mainly consisted of γ-Fe, and C_r23C₆ phase. To evaluate the mechanical properties of the material, tensile tests were performed at room temperature and 900 ℃. The tensile tests at room temperature indicated that the MIM and MIMTHIP specimens showed tensile strength of 609.6 MPa and 645.0 MPa, and elongation of 32.7% and 43.7%, respectively. In both strength and elongation, the MIM+HIP sample had greater performance. The tensile results at 900 ℃ also showed that the HIP+MIM specimen had higher strength and elongation. This was attributed to fewer macro cracks, denser structures and finer distribution of Nb based carbide due to the additional HIP. This study also discussed the deformation behavior of MIM material at room·high temperatures. (Received August 21, 2017; Accepted November 16, 2017)

keyword : metal injection molding, Fe-Cr-Ni based alloy, hot isostatic pressing, microstructure, high temperature, mechanical properties

An excellent grain-oriented electrical steel is composed of grains with exact Goss orientation. In order to produce this, it is important to control the Goss-oriented grains during the cold-rolling process. In this study, textures measured by X-ray diffraction (XRD) and electron backscatter diffraction (EBSD) were analyzed for 1, 10, and 36-pass cold-rolled sheets at a constant total thickness reduction rate of 89% in Fe- 3.2 wt% Si grain-oriented electrical steel. After cold rolling, strong α-fiber textures and relatively weak γ-fiber textures were developed in all specimens, regardless of the number of rolling passes. On the surface of the 36-pass rolled specimen, a weak α-fiber texture and a relatively strong η-fiber texture developed compared with the other specimens. The Goss-oriented grains were mainly found in the shear band or micro band, which show that their fraction increases as the number of rolling passes increases, especially within the surface layer. As the number of Goss-oriented grains increased, the exactness of their orientation improved. It was determined that it is possible to obtain a grain-oriented electrical steel with a high-quality Goss orientation using a process such as subsequent recrystallization. (Received August 24, 2017; Accepted December 8, 2017)

keyword : electrical materials, cold-rolling, EBSD, texture

The effects of submerged-arc welding consumables, wire and flux, on solidification cracking in the weld metal of medium-carbon steel were investigated. It showed that weld metal with a greater amount of C, S, and P had a wider brittleness temperature range, BTR, which resulted in longer total and maximum crack lengths in Varestraint tests. To determine the effect of flux on the amount of C, S, and P in the weld metal, transfers of the elements between the slag and the weld metal were evaluated. While C and S were removed from the weld metal to the slag, P was added from the slag to the weld metal. The amount of each element removed or added, however, was dependent on the basicity of the flux used. As the flux became more basic, more S was removed and less P was added, resulting in less S and P in the weld metal. In contrast, with increasing flux basicity, less C was removed, leaving a larger amount of C in the weld metal. Therefore, to minimize the amounts of these elements in the weld metal, in addition to the wire used, the flux basicity should also be carefully considered. (Received September 7, 2017; Accepted November 6, 2017)

keyword : metals, welding, solidification, SIMS, element transfer

Solvent extraction of La, Ce, Pr, Nd and Sm from chloride solutions was studied using PC88A and Cyanex 572 as the extractants. Our results indicated that the acidity of the aqueous solution and saponification degree had some effect on the percentage extraction of metal ions. The distribution coefficient of the metal ions increased with increasing aqueous solution pH in both of PC88A and Cyanex 572. But the pH₅₀ value was lowered by about 0.05~0.34 in PC88A. In extraction with saponified Cyanex 572 with NaOH, the distribution coefficients of the metal ions were independent of the degree of saponification, and did little to improve the separation behavior of these metal ions. With Cyanex 572 as the extractant, the separation factor of the binary La/Ce and Nd/Sm was enhanced about 5.0, but Ce/Pr and Pr/Nd showed no difference in separation factor as compared with PC88A. The group separation factor of La/(Pr, Nd, Sm) and (Pr, Nd)/Sm was increased by 3.0~8.0, but for (La, Ce)/(Pr, Nd) and Ce/ (Pr, Nd) it was only increased by 0.2~0.7 in the Cyanex 572 system. (Received October 18, 2017; Accepted November 15, 2017)

keyword : rare earth metal, solvent extraction, Cyanex 572, PC88A, separation factor

In this work, MgH₂, Mg₂Ni, and Ni were added to Mg in order to improve the hydrogen-storage properties of Mg. A 84 wt% Mg + 10 wt% MgH₂ + 5 wt% Mg₂Ni + 1 wt% Ni sample (designated the Mg-based sample) was prepared by milling under a hydrogen atmosphere in a planetary ball mill for 5 h. The hydrogen absorption and release properties of the prepared sample were investigated and compared with those of a 94 wt% MgH₂ + 5 wt% Mg₂Ni + 1 wt% Ni sample (the MgH₂-based sample). The Mg-based sample had larger quantities of hydrogen absorbed and released for 60 min than the MgH₂-based sample. The Mg-based sample had an effective hydrogen-storage capacity (the quantity of hydrogen absorbed for 60 min) of near 5.7 wt%. At n = 2, the Mg-based sample absorbed 5.18 wt% for 2 min, 5.57 wt% for 10 min, and 5.67 wt% for 60 min at 648 K. At n = 1, the Mg-based sample released 0.083 wt% for 2 min, 5.13 wt% for 10 min, and 5.49 wt% for 60 min at 648 K. The addition of MgH₂, Mg₂Ni, and Ni to Mg had stronger effects than the addition of Mg₂Ni and Ni to MgH₂. It is believed that the effects of the reactive mechanical grinding (reducing the Mg particle size, making clean surfaces, and creating defects) are stronger in the Mg-based sample than in the MgH₂-based sample. (Received August 16, 2017; Accepted September 29, 2017)

keyword : hydrogen storage, magnesium, mechanical milling, X-ray diffraction, MgH₂ Mg₂Ni or Ni addition

We report the effects of interface control using reduced graphene oxide (RGO) on the thermoelectric transport properties of polycrystalline SnSe compounds. Polycrystalline SnSe-RGO composites were prepared by the consolidation of SnSe powder coated with the RGO by hot pressing, and the RGO content was controlled to be 0, 0.2, 0.5 and 1 wt%. The incorporation of RGO did not induce any structural changes in the crystal structure of SnSe, and no additional secondary phase could be observed. The electrical conductivity of the composites increased with increasing RGO content. This increase was largely the result of increased hole concentration due to the hole Fermi liquid phenomenon at the SnSe-RGO interface, and it was elucidated by the reduced Seebeck coefficient with increasing RGO content. However, this tendency could only be clearly observed at relatively low temperatures, and the incorporation of the RGO did not lead to a significant increase in the power factor. The thermal conductivity of the composites decreased with the increase in RGO content, and revealed that there was additional phonon scattering at the grain boundary interfaces due to the existence of the RGO. These results, demonstrated that independent control of charge and thermal transport, i.e., phonon glass-electron crystal, could be realized by interface control using RGO in polycrystalline SnSe compounds. (Received October 18, 2017; Accepted November 6, 2017)

keyword : thermoelectric, SnSe, reduced graphene oxide, hot pressing, interface control