발간논문

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In this study, four API X80 linepipe steel specimens were fabricated with varying cooling rates and finish cooling temperatures, and their microstructures and crystallographic orientations were analyzed to investigate the effects of cooling conditions on their tensile and Charpy impact properties. All the specimens consisted of acicular ferrite, granular bainite, and secondary phases such as martensite and martensiteaustenite constituent. The volume fraction of secondary phases increased with increasing cooling rate, and the higher finish cooling temperature resulted in the reduction in volume fraction and grain size of secondary phases. According to the crystallographic orientation analysis data, the effective grain size and unit crack path decreased as fine acicular ferrites having a large amount of high-angle grain boundaries were homogeneously formed, thereby leading to the improvement of Charpy impact properties. The specimen fabricated with the higher cooling rate and lower finish cooling temperature had the highest upper shelf energy and the lowest energy transition temperature because it contained a large amount of fine secondary phases homogeneously distributed inside fine acicular ferrites, while its tensile properties well maintained.

keyword : API X80 Linepipe steel, cooing condition, tensile properties, charpy impact properties, effective grain size

Materials used for a reactor pressure vessel(RPV) are required high strength and toughness, which determine the safety margin and life of a reactor. Ni-Mo-Cr low alloy steel shows better mechanical properties than existing RPV steels due to higher Ni and Cr contents compared to the existing RPV steels. The present study focuses on effects of Ni, Cr contents on the cleavage fracture toughness of Ni-Mo-Cr low alloy steels in the transition temperature region. The fracture toughness was characterized by a 3-point bend test of precracked Charpy V-notch(PCVN) specimens based on ASTM E1921-08. The test results indicated that the fracture toughness was considerably improved with an increase of Ni and Cr contents. Especially, control of Cr content was more effective in improving fracture toughness than manipulating Ni content, though Charpy impact toughness was changed more extensively by adjusting Ni content. These differences between changes in the fracture toughness and that in the impact toughness were derived from microstructural features, such as martensite lath size and carbide precipitation behavior.

keyword : reactor pressure vessel, Ni-Mo-Cr Low alloy steel, cleavage fracture toughness, master curve, charpy impact toughness

Zr-based amorphous alloy matrix composites reinforced with tantalum continuous fibers were fabricated by the liquid pressing process, and their anisotropic mechanical properties were investigated by tensile and compressive tests of 0˚(longitudinal)-, 45˚-, and 90˚(transverse)-orientation specimens. About 60 vol.% of tantalum fibers were homogeneously distributed inside the amorphous matrix, which contained a small amount of polygonal crystalline particles. The ductility of the tantalum-continuous-fiber-reinforced composite under tensile or compressive loading was dramatically improved over that of the monolithic amorphous alloy, while maintaining high strength. When the fiber direction was not matched with the loading direction, the reduction of the strength and ductility was not serious because of excellent fiber/matrix interfacial strength. Observation of the anisotropic deformation and fracture behavior showed the formation of multiple shear bands, the obstruction of crack propagation by fibers, and the deformation of fibers themselves, thereby resulting in tensile elongation of 3%~4% and compressive elongation of 15%~30%. These results suggest that the liquid pressing process was useful for the development of amorphous matrix composites with excellent ductility and anisotropic mechanical properties.

keyword : composite, amorphous alloy, Liquid pressing process, anisotropic mechanical properties

In this study, bronze or SUS304 powders blended with 10 wt.% diamond particles were used to prepare metal/diamond composite materials deposited by cold spraying. The effects of matrix metal, diamond partical size, and the thickness of the Ni coating on the diamond were studied on Al 6061 substrate. The results showed that the hardness of the metal/diamond composite coating layers was higher than that of the same composite materials when using the sintering method. The fraction of diamond content in the coated layer increased when the metal matrix was soft. When the size of the diamond particles was reduced, the fraction of the diamond particles increased. In addition, in the case of diamond with a thicker Ni-coated layer, the fracturing of diamonds was mitigated in the composite coating layers.

keyword : cold spray, diamond composite, coating

The alloying effect of a small amount of nickel on low alloy steel for application to flue gas desulfurization(FGD) systems was studied. The structural characteristics of the rust layer were investigated by scanning electron microscopy(SEM). The electrochemical properties were examined by means of potentiostatic polarization test, potentiodynamic polarization test, and electrochemical impedance spectroscopy(EIS) in a modified green death solution of 16.9 vol.% H2SO4+0.35 vol.% HCl at 60℃ and an acid rain solution of 6.25×10(-5) M H2SO4+5.5×10(-3) M NaCl at room temperature. It was found that as the amount of nickel increased, the corrosion rate increased in the modified green death solution, which seemed to result from micro-galvanic corrosion between NiS and alloy matrix. In acid rain solution, the corrosion rate decreased as the amount of nickel increased due to the repulsive force of NiFe2O4 rust against Cl- ions by electronegativity.

keyword : Ni effect, Low alloy steel, acid rain environment, FGD environment, EIS, potentiostatic test, potentiodynamic test

The densification behaviors of mixtures of copper and steel powders during cold die compaction were investigated. We proposed the pressure-dependent yield function based on the rule of the mixtures of each yield function of a critical relative density type. The constitutive equations were implemented into a finite element program (DEFORM2D) to analyze the densification and deformation behavior of powder mixtures, and the simulated results are in good agreement with the experimental results in reference studies.

keyword : pressure dependent yield function, metallic powder mixture, cold die compaction, constitutive equation, critical relative density model, finite element method

Titanium/aluminum/stainless steel(Ti/Al/STS) clad materials have received much attention due to their high specific strength and corrosion-resisting properties. However, it is difficult to fabricate these materials, because titanium oxide is easily formed on the titanium surface during heat treatment. The aim of the present study is to derive optimized cladding conditions and thereupon obtain the stable quality of Ti/Al/STS clad materials. Ti sheets were prepared with and without pre-heat treatment and Ti/Al/STS clad materials were then fabricated by cold rolling and a post-heat treatment process. Microstructure of the Ti/ Al and STS/Al interfaces was observed using a Scanning Electron Microscope(SEM) and an Energy Dispersed X-ray Analyser(EDX) in order to investigate the effects of Ti pre-heat treatment on the bond properties of Ti/ Al/STS clad materials. Diffusion bonding was observed at both the Ti/Al and STS/Al interfaces. The bonding force of the clad material with non-heat treated Ti was higher than that with pre-heat treated Ti before the cladding process. The bonding force decreased rapidly beyond 400℃, because the formed Ti oxide inhibited the joining process between Ti and Al. Bonding forces of STS/Al were lower than those of Ti/Al, because brittle Fe3Al, Al3Fe intermetallic compounds were formed at the interface of STS/Al during the cladding process. In addition, delamination of the clad material with pre-heat treated Ti was observed at the Ti/Al interface after a cupping test.

keyword : Ti/Al/STS clad material, pre-heat treatment, bonding force, interface property, delamination

The effects of shear speed and tip height on the high speed shear test of Sn-3.0wt.%Ag-0.5wt.%Cu ball joints were investigated. Solder balls of 450 μm in diameter were reflowed at 245℃ on a FR4 PCB (Printed Circuit Board) in order to obtain a sample for the high-speed shear test. The UBM was comprised of Cu/Ni/Au, and the shear speed and tip height varied from 0.5 to 3.0 m/s, and from 10 to 135 μm, respectively. According to the experimental results, faster shear speed enhanced the shear strength of the solder joints, regardless of the tip height. The fraction of ductile (solder) fracture decreased when the shearing speed was raised from 0.5 to 3.0 m/s. With an increasing tip height from 10 to 50 and 135 μm, the fracture mode changed from pad lift to mixed (ductile and brittle) and ductile fracture, respectively, while the shearing energy also increased in the same order. The shear energy had a proportional relationship with the fraction of the solder fracture.

keyword : high speed shear test, solder ball, shearing tip height, shearing speed, shear strength

The effects of additives on the surface morphology and physical properties of copper electrodeposited on polyimide(PI) film were investigated here. Two kinds of additives, an activator(additive A) and a leveler(additive B), were used in this study. Electrochemical experiments, in conjunction with scanning electron microscopy(SEM), X-ray diffraction(XRD) and a four-point probe, were performed to characterize the morphology and mechanical characteristics of copper electrodeposited in the presence of the additives. The surface roughness, crystal growth orientation and resistivity could be controlled using various quantities of additive B. High resistivity and lower peel strength were observed on the surface of the copper layer electroplated onto the electrolyte with no additive B. However, a uniform surface, lower resistivity and high flexibility were obtained with a combination of 20 ppm of additive A and 100 ppm of additive B.

keyword : additive, plating, peel strength, resistivity, flexibility

TiNxOy/TiNx multi-layer thin films with a high resistance(~kΩ) were deposited on SiO2/Si substrates at room temperature by sputtering. The TiNx thin films show island and smooth surface morphology in samples prepared by α and RF magnetron sputtering, respectively. TiNxOy/TiNx multi-layer in has been developed to control temperature coefficient of resistance(TCR) by the incorporation of TiNx layer(positive TCR) inserted into TiNxOy layers(negative TCR). Electrical and structural properties of sputtered TiNxOy/TiNx multi-layer films were investigated as a function of annealing temperature. In order to achieve a stable high resistivity, multi-layer films were annealed at various temperatures in oxygen ambient. Samples annealed at 700℃ for 1 min exhibited good TCR value of approximately -54 ppm/℃ and a stable high resistivity around 20 kΩ/sq. with good reversibility.

keyword : thin film resistor, TiNxOy, TCR, thermal annealing, sputtering