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The HANA alloys designed in KAERI were one of the newly developed materials having an improved corrosion resistance. The corrosion characteristics of Zr-based alloys were very sensitive to their microstructures which were determined by manufacturing process. The specimens of HANA-alloy were investigated in order to get the optimized manufacturing process such as the β annealing temperature and the cooling rate at the β region. From the microstructural analysis, the prior β grain size of HANA alloys was increased with increasing the β annealing temperature. The martensitic structure was formed by fast cooling rate of water quenching and the Widmanstatten structure was formed by slow cooling rate of air cooling. The corrosion behavior of HANA alloys was not affected by the β annealing temperature but that of HANA alloys was changed by cooling rate. The higher corrosion rate was shown at the martensite structure which the alloying elements of HANA alloy existed at supersaturated state.

keyword : Zr, Fuel cladding, HANA alloy, TREX, Heat treatment, Microstructure, Corrosion

The study was conducted to elucidate the dynamic strain aging behavior of Zr-1.5Nb-0.4Sn-0.2Fe alloy which was developed as an advanced nuclear fuel cladding material. Tensile tests were conducted in the range of 25℃ to 500℃ at the rate of 8.33×10^(-5)/sec and 1.67×10^(-2)/sec. The results showed that cladding exhibited the dynamic strain aging behavior above 200℃ which included the increase in the strength, work hardening exponent as well as the decrease in the ductility and strain rate sensitivity. The increase of heat treatment temperature from 470℃ to 510℃ shifted dynamic strain aging temperature to the higher range because initial dislocations causing dynamic strain aging were annihilated by the heat treatment. The analysis of the activation energy and diffusion model of solute atoms revealed that the mechanism of dynamic strain aging in Zr-1.5Nb-0.4Sn-0.2Fe alloy was due to the thermal migration of solute atoms such as oxygen with the help of the vacancy flow formed by substitutional atoms during plastic deformation.

keyword : Zirconium, Cladding, Dynamic strain aging, Solute atom, Diffusion

This study is concerned with the fabrication of surface composites by high-energy electron-beam irradiation of Fe-based amorphous alloy powders. Fe-based amorphous powders were deposited on a plain carbon steel substrate, and then electron beam was irradiated on these powders without flux to fabricate a one-layered surface composite. A two-layered surface composite was also fabricated by irradiating electron beam again onto the powders deposited on the one-layered surface composite. The composite layers of 1.3~1.9 mm in thickness were homogeneously formed without defects, and the two-layered composite contained a large amount (up to 48 vol.%) of hard and fine Cr2B crystalline particles in the Cr0.19Fe0.7Ni0.11 matrix. The hardness and wear resistance of the surface composite layers were directly influenced by hard Cr2B particles, and thus were about 2 to 3 times greater than those of the steel substrate. Particularly in the two-layered surface composite, high-temperature hardness was maintained over 300 VHN up to 750℃, which was better than that of a coating fabricated by high-velocity oxygen fuel spraying of Fe-based amorphous powders.

keyword : High-energy electron beam irradiation, Surface composite, Amorphous alloy powder, Wear resistance

Quasi-static and dynamic deformation behavior of a Zr-based amorphous alloy and an amorphous matrix composite containing dendritic β phases was investigated in this study. Quasi-static and dynamic compressive tests were conducted using a universal testing machine and a compressive Kolsky bar, respectively, and then the test data were analyzed in relation to microstructure and fracture mode. Quasi-static compressive test results indicated that the compressive strength of the amorphous composite was similar to that of the amorphous alloy while the ductility was better than that of the amorphous alloy. Under dynamic loading, the maximum shear stress and ductility of the amorphous alloy and composite were considerably lower than those under quasi-static loading because of the decreased resistance to fracture. The deformation and fracture behaviors occurring under quasi-static and dynamic loading conditions were explained by fracture mechanisms. Deformation under dynamic loading lowered strain and compressive strength because of the reduced fracture resistance, and the composite alloy showed more excellent compressive strength and ductility than the alloy. These findings consequently suggested that applying the evaluation criteria for mechanical properties measured under quasi-static loading could cause risks when Zr-based amorphous alloys were used under dynamic loading conditions.

keyword : Zr-based amorphous alloy, Ductile crystalline particle, Dynamic compressive Kolsky bar, Composite

Influences of molybdenum (Mo) substitution by tungsten (W) on the formation kinetics of secondary phases and the associated localized corrosion and embrittlement of Fe-29Cr-4Mo, Fe-29Cr-4W, and Fe-29Cr-8W ferritic stainless steels were investigated. Fine χ phase formed first in grain boundaries in an early stage of aging. and it was gradually substituted by IT phase with further aging. The precipitation rate of δ phase appears to be determined by both the diffusion rates of Wand Mo for the formation of the δ phase as well as by the affinity of χ phase, as a competitor, for the elements. Due to the high affinity of χ phase for W with a slow diffusion rate, the nucleation of δ phase was significantly delayed in Fe-29Cr-4W and Fe-29Cr-8W alloys compared with that in Fe-29Cr-4Mo alloy. In addition, the deterioration of ductility and localized corrosion resistance by the precipitation of secondary phases was significantly retarded in Fe-29Cr-4W alloy compared with that in Fe-29Cr-4Mo alloy, due to the delayed precipitation of secondary phases in Fe-29Cr-4W alloy. In particular, retardation of degradation in localized corrosion resistance by the formation of δ phase, which induced significant depletion of Cr and W (or Mo) around the phase, was prominent in the W-containing alloys. The W-containing alloys exhibited effective delay of δ phase formation.

keyword : Ferritic stainless steel, Tungsten, Molybdenum, Localized corrosion, Embrittlement, Sigma phase, Chi phase

In order to evaluate long term reliabilities of Sn-8 wt%Zn-3 wt%Bi (hereafter, Sn-8Zn-3Bi) solder, thermal shock (TS) test and high temperature humidity (HTH) test were applied to the solder joint. TS test was carried out up to 1,000 cycles between -40℃ to 80℃, and HTH was performed at 80℃ up to 1,000 hrs with relative humidity of 85%. The PCB (printed Circuit Board) pads for bonding substrates were finished by OSP (Organic Solderability Preservative), Sn and Ni/Au. A QFP (Quad Flat Package) was used for a bonding specimen with its lead of Cu being plated by Sn-3 wt%Bi. The joint strength of the QFP lead soldered on the pad was estimated by pull test, and Zn-phase behavior in solder joint was also examined. As experimental results, the pull strengths of Sn-8Zn-3Bi joints as soldered state were about 16 N with irrespective of pads coatings. However, the strength after TS test of 1,000 cycles decreased to around 14 N. Meanwhile, the pull strengths after HTH test to 1,000 hrs drastically deceased to around 6 N with irrespective of the pads coatings. The reason of drastic decrease of the pull strength after HTH test was proved to be a crack that was observed along the zinc oxide-phase in solder. The initial Zn-phase of acicular shape in the Sn-8Zn-3Bi solder changed to a line-upped branch type after HTH test of 1,000 hr.

keyword : Sn-8 wt%Zn-3 wt%Bi, Lead free solder, Zn-phase, Joint strength, High temperature humidity test

This paper presents the precipitation-stripping of cobalt from cobalt-loaded solvent (Cyanex 272 in kerosene) into an aqueous oxalate solution, where cobalt was precipitated as metal oxalate. The loaded solvent containing 2 g/ℓ cobalt was selected. The precipitation yield was tested as a function of several parameters such as pH of aqueous solution, the amount of oxalic acid, O/A ratio, temperature, and reaction time. The precipitation yield of cobalt greatly depended on pH of aqueous solution, and pH of 1.5 was optimum. Under optimum precipitation-stripping conditions (pH-1.5, oxalic acid-2 times of cobalt, O/A ratio-3, 25℃, 20 min.) about 95% efficiency was achieved.

keyword : Solvent extraction, Cobalt, Cyanex 272, Precipitation-stripping, Oxalic acid

Removal of metallic impurities from commercially pure Zr and Hf metals by Ar-H2 plasma arc melting has been examined. The adding H2 gas to the Ar plasma led to an enhancement of a great removal of Al, Cr, Mn, Fe, Ni, and Cu impurities which could not be much removed by only Ar plasma arc melting. As a result, most impurities in the Zr and the Hf metals by Ar-H2 plasma arc melting were removed down to a few mass ppm levels. It was found that impurities with higher vapor pressure than those of the Zr and the Hf metals were easily eliminated by Ar-H2 plasma arc melting. The dominant reason for the removal of the metallic impurities from the Zr and the Hf metals by the H2 addition is as follows: (a) elimination of the oxide layer on the molten surface of the metal by hydrogen atoms. (b) increase in the temperature of the molten metal due to the higher thermal conductivity of the hydrogen plasma arc melting, and (c) transfer enhancement of impurity vapors from the gaseous and molten surface boundary by hydrogen atoms.

keyword : Zirconium, Hafnium, Purification, Impurity, Hydrogen plasma arc melting