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This study is concerned with the effects of microstructure on Charpy V-notch (CVN) impact properties of a high-toughness API X70 pipeline steel. Six kinds of steel specimens were fabricated by varying hot-rolling conditions, and their microstructures and CVN properties were investigated. In addition, their effective grain sizes were characterized by the electron back-scatter diffraction (EBSD) analysis. The Charpy impact test results indicated that the specimens rolled in the single phase region had the higher upper shelf energy (USE) and the lower energy transition temperature (ETT) than the specimens rolled in the two phase region because their microstructures were composed of acicular ferrites and fine polygonal ferrites. The decreased ETT in the specimens rolled in the single phase region could be explained by the decrease in the overall effective grain size due to the presence of acicular ferrite having smaller effective grain size. On the other hand, CVN properties of the specimens rolled in the two phase region were deteriorated as they contained a small amount of martensites or cementites.

keyword : API X70 pipeline steel, Rolling condition, Charpy V-notch, CVN, impact test, Upper shelf energy, Energy transition temperature

In the present study, effects of microstructure on drop-weight tear test (DWTT) properties and inverse fracture occurring in the hammer-impacted region of high-toughness API X70 pipeline steels were investigated. Six kinds of steel specimens were fabricated by varying hot-rolling conditions. The DWTT was conducted on the rolled steel specimens, and the results were discussed in comparison with the Charily V-notch (CVN) impact test data. The DVVTT results indicated that all the specimens showed excellent DWTT properties as the percent shear area well exceeded 85%, irrespective of microstructures, although the inverse fracture occurred at test temperatures below -20℃. The specimens rolled in the single phase region had the lower 85% shear appearance transition temperature (SATT) than the specimens rolled in the two phase region, which was similar to the CVN test results. This 85% SATT had a good correlation with the energy transition temperature (ETT) obtained from CVN test.

keyword : API X70 pipeline steel, Drop-weight tear test, Charpy V-notch, CVN, impact test, Inverse fracture

Correlation of Charpy V-notch (CVN) impact properties, drop-weight tear test (DWTT) properties, and crack tip opening angle (CTOA) values of a high-toughness API X70 pipeline steel was investigated in this study. Two-specimen CTOA test (TSCT) was conducted on the rolled steel specimens to measure CTOA, and the test results were related with the CVN and DWTT data to find correlations between them. The CVN total energy density showed almost 1:1 linear correlation with the DWTT initiation energy density. The TSCT test results indicated that the specimens rolled in the single phase region had the larger CTOA as well as the higher CVN and DWTT energies than those rolled in the two phase region because their microstructures were composed of acicular ferrites and fine polygonal ferrites. The measured CTOA showed better correlation with the DWTT propagation energy than with the CVN or DWTT total energy as it was related with the crack propagation speed. Since the value of Sin(2CTOA) was linearly proportional to the DWTT energy, it could be used as a useful parameter to reliably estimate the CTOA.

keyword : API X70 pipeline steel, Crack-tip opening angle, Drop-weight tear test, Charpy V-notch, CVN, impact test

High temperature deformation behavior of AZ31 Mg alloy was investigated in this study on the basis of a processing map (ε = 0.6). To construct a processing map, compression tests were carried out at wide range of temperatures and strain rates (T=250-500℃, ε˙=10^(-4)-100/s). Two regions of high deformation efficiency (η) were identified as: (1) a dynamic reciystalization (DRX) domain at 250℃ and 1/s and (2) a superplasticity domain at 450℃ and 10^(-4)/s. The average grain size of the specimens deformed in the DRX region was smaller than those of deformed in any other regions. In the superplastic condition, tensile elongation to failure approached to 1040%. The results of compression tests showed that 10 of log Z (Zener-Hollomon parameters) was the boundary of flow softening and hardening. At the high Z (above 10^(10) of Z) regions, flow softening occurred accompanying the dynamic recrystallization, while at the low Z (below 10^(10) of Z) regions, flow hardening occurred due to the grain growth. Possible deformation mechanisms operating at high temperature were discussed in relation to the activation energy.

keyword : AZ31 Mg alloy, Formability, Processing map, Superplasticity, Dynamic recrystallization

This study is concerned with the fabrication of two-layered amorphous/Cu surface composites by high-energy electron-beam irradiation. The mixture of Zr-base amorphous powders and LiF＋MgF₂ flux was deposited on a pure Cu substrate, and then electron beam was irradiated on these powder mixture to fabricate a one-layered surface composite. A two-layered surface composite was also fabricated by irradiating electron beam again onto the powder mixture deposited on the one-layered surface composite. The composite layers of 1.6~1.7 mm in thickness were homogeneously formed without defects, and the two-layered composite contained a few fine crystalline phase particles in the amorphous matrix. This fabrication could be achieved by (1) the use of flux which protected the oxidation environments, (2) the second electron-beam irradiation which melted already-formed crystalline particles, and (3) the fast cooling rate of the irradiation method which provided the high amorphous forming ability. Such a development of the fabrication technique of amorphous alloy surface composites presented new applicability of amorphous alloys to structures and components requiring excellent mechanical properties.

keyword : Two-layered surface composites, Electron beam irradiation, Amorphous alloy, Crystalline phase

Bulk amorphous Ni_(57)Ti_(18) Zr_(20)Si₃Sn₂ feedstock was sprayed using a high velocity oxy-fuel spraying process. In order to evaluate the amorphous formability ign view of chemical instability, the oxygen to hydrogen gas flow rate ratio was changed at the constant total gas flow rate. Flame gas enthalpy was increased with the O₂/H₂ gas ratio. In-flight particle melting state was improved with the increase of oxidizer flow rate. Fthe amorphous phase fraction within an as-sprayed coating was decreased with the oxygen flow rate. From the thermal analysis and x-ray diffraction, the effects of 0₂/H₂ gas ratio on the crystallization of HVOF sprayed bulk amorphous Ni_(57)Ti_(18) Zr_(20)Si₃ could be divided into two categories. One is due to the solid-state crystallization during flight along the low gas enthalpy The other results from the of bulk amorphous feedstock due to the chemical composition change resulting from in-flight particle oxidation at the higher oxygen gas flow rate.

keyword : Bulk amorphous material, High velocity oxy-fuel, In-flight particle oxidation, Phase evolution

The effect of Nb and Fe addition ratio on the microstructure, corrosion behavior of Zr-based alloys has been performed. The Nb/Fe ratio was controlled to be 0.6 to 7.0 maintaining the same Nb + Fe content in each alloy. The microstructural analysis on the grain size and the precipitate properity such as area fraction, mean diameter and crystal structure was used for interpreting the corrosion kinetics. The grain size and the precipitate area fraction were not largely affected by the Nb/Fe ratio, but the precipitate mean diameter and the precipitate crystal structure were changed with those of ratios. The corrosion resistance of ternary Zr-Nb-Fe alloys was improved when the Nb/ Fe ratio that controlled the precipitate crystal structure was lower than 1.0. Therefore, it seems that the corrosion behavior was largely affected by the crystal structure of precipitate among the various microstructural characteristics.

keyword : Zr, Fuel cladding, Zr-Nb-Fe, Nb/Fe ratio, Precipitate, Crystal structure, Corrosion

Sputter-deposited Al-doped zinc oxide (ZnO:AI) is an attractive transparent conductive oxide material for application as transparent electrode in various electronic devices. In this work, the ZnO:AI films are prepared by the capacitively cooped DC magnetron sputtering on glass substrates and the influence of the substrate temperature and working gas pressure on the electrical, optical and morphological properties are investigated. We find that the ZnO:AI film with the optimum deposition parameters shows the resistivity of 0.96×10^(-3) Ω㎝ and the transmittance of average 90% in the wavelength range of the visible spectrum.

keyword : ZnO:Al, Resistivity, Transmittance, DC magnetron sputtering

The morphology and thermal stability of the interfacial phases in the joints between lead free solder (Sn3.5AgO.7Cu) and electroless Ni-P (12 at%P) and electro Ni under bumper metallizations(UBM) were investigated as a function of thermal aging. Samples were prepared by reflowing at 250℃, N₂ atmosphere and aging in the range of 125 to 170℃ for up to 1000h. After reflow, in the electrolytic Ni/solder joint, the interfacial IMC was mostly Nirich (Ni,Cu)₃Sn4 with an addition of about 8 at% Cu while in electroless Ni-P/solder joint it showed both Cu-rich (Cu,Ni)_(6)Sny and Ni-rich (Ni,Cu)₃Sn4. After relatively short aging time, the interfacial IMC on electrolytic Ni layer has developed into the dual-layed structure of (Ni,Cu)₃Sn₄(bottom) and (Cu,Ni)_(6)Sny(top). But IMC on electroless Ni-P layer were (Cu,Ni)_(6)Sn_(6) and changed into the dual structure only after long aging time of 500h. The growth rate of IMC with aging was higher on electrolytic Ni layer than on electroless Ni-P. The different behavior of electroless Ni-P/SAC joint came from Ni₃P formation between IMC and electroless Ni-P layer which retards diffusion of Ni and Sn to the opposite direction. The thickness of Ni₃P layer was propositional to that of IMC. For longer aging time, NiSnP layer was found at the interface between IMC and Ni₃P layer. Interestingly, in spite of low consumption rate of electroless Ni-P than electrolytic Ni layer by suppressing IMC growth, the interface of Ni-P/IMC was locally hollowed by the accelerated reaction of the Ni-P layer through local thickness reduction of Ni₃P and NiSnP layers. This reduced the effective remaining thickness of Ni-P layer.

keyword : Lead-free solder, IMC, Intermetallic compound, Electroless plating, BGA, Ball grid array, UBM, Under bump metallization, Solder joint

In this study, for the applications of Fe-Ni alloys into MEMS technique through a UV-LIGA processing, the electroplating conditions of the Fe-Ni alloys on the Cu plate as well as micropatterned mold have been studied. The compositional changes of the deposited layer have been related to the electroplating conditions including metal-ion ratio, current density, rotating speed, pH, temperature and pulse current. The effects of every condition were investigated by electrofoming in RDE(rotating disk electrode) type plating cell and then the compositions of deposit were examined. It has been found that current density and metal ion ratio were the most influential factor on the compositional changes in deposited layer and the other conditions were also discussed in the view of anomalous behaviour of electrodeposition of Fe-Ni alloy.

keyword : Fe-Ni alloy, Alloy-plating, UV-LIGA, MEMS, Pulse-plating, SU-8