발간논문

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Material degradation of Cr steels in boiler tubes was accompanied by the microstructural changes including carbide behavior and crack formation. The microstructural change and the mechanical behavior of hardness and creep properties in accelerated heat-treatments were studied in order to identify the material degradation of the X20 Cr steel. The degradation behavior was occurred in the hardness increasing followed by decreasing due to carbide dissolution and precipitation.

keyword : material degradation, crsteel, carbide behavior, accelerated heat-treatment

We observed the in-situ stress behavior of Cu and Ag thin films during deposition using a thermal evaporation method. Multi-beam curvature measurement system was used to monitor the evolution of in-situ stress in Cu and Ag thin films on 100 Si(100) substrates. The measured curvature was converted to film stress using Stoney formula. To investigate the effects of the deposition rates on the stress evolution in Cu and Ag thin films, Cu and Ag films were deposited at rates ranging from 0.1 to 3.0A/s for Cu and from 0.5 to 4.0A/s for Ag. Both Cu and Ag films showed a unique three stress stages, such as ``initial compressive``, ``a tensile maximum`` and followed by ``incremental compressive`` stress. For both Cu and Ag films, there is no remarkable effect of deposition rate on the thickness and average stress at the tensile maximum. There is, however, a definite decrease in the incremental compressive stress with increasing deposition rate.

keyword : in-situ stress, thin film, deposition rate, thermal evaporation

The effect of the alloy elements(Si/Mn) ratio on the coating quality including wettabilty with molten zinc, galvannealing kinetics and crater has been investigated in interstitial-free high strength steel(IFHSS) containing Si and Mn. When the Si/Mn ratio was below 0.75, IF-HSS exhibited a good wettability leading to a good galvannealed coating quality after annealing at 800℃ for 40s in15%H2-N2 mixed gas with dew point ?60℃. In contrast, the wettability and galvannealed coating quality were deteriorated in the Si/Mn ratio above 0.75. It is shown that they have relevance to oxides forms by selective oxidation on the steel surface. The oxide particles dispersed on the steel surface with a surface coverage of below 40% resulted in good wettability and galvannealed coating quality. The oxide particle is mainly consisted of Mn2SiO4 with low contact angle in molten zinc. On the other hand, the continuous oxide layer on the steel surface, such as network- and film-type,caused to poor wettability and galvannealed coating quality. The coverage of oxide layer was above 80%, and its chemical species was SiO2 with high contact angle in molten zinc. Consequently, the Si/Mn alloy ratio played an importance role in galvannealed coating quality of IF-HSS.

keyword : hot-dip galvannealed steel(GA), wettability, galvannealing kinetic, crater, selective oxidation, Si/Mn ratio, IF high strength steel containing Si and Mn

Hydrogen permeation through dense palladium-based membranes has attracted the attention of many scientists largely due to their unmatched potential as hydrogen-selective membranes for membrane reactor applications. Although it is well known that the permeation mechanism of hydrogen through Pd involves various processes such as dissociative adsorption, transitions to and from the bulk Pd, diffusion within Pd, and recombinative desorption, it is still unclear which process mainly limits hydrogen permeation at a given temperature and hydrogen partial pressure. In this study, we report an all-electron density-functional theory study of hydrogen permeation through Pd membrane (using VASP code). Especially, we focus on the variation of the energy barrier of the penetration process from the surface to the bulk with hydrogen coverage, which means the large reduction of the fracture stress in the brittle crack propagation considering Griffith`s criterion. It is also found that the penetration energy barrier from the surface to the bulk largely decreases so that it almost vanishes at the coverage 1.25, which means that the penetration process cannot be the rate determining process.

keyword : hydrogen permeation, pd membrane, hydrogen embrittlement, penetration energy barrier, Ab inito study

Magnesium alloys containing Mg2Si particles, as a promising cheap heat-resistant magnesium alloy for automobile power train parts applications, are attracting more attention of both material scientists and design engineers. Modification of the Chinese script shape Mg2Si particle is a key for using this alloy in sand or permanent mould casting. In the present work, the modification effect of Sr and Sb on the corrosion properties of the Mg-5Al-2Si alloy was investigated. Sr or Sb addition promoted the formation of fine polygonal shape Mg2Si particles by providing the nucleation sites. Sr was more effective element than Sb for shape modification of Chinese script shape Mg2Si. Such improved microstructure of the modified alloy resulted in large improvement in corrosion resistance as compared to unmodified Mg-5Al-2Si alloy.

keyword : modification, heat-resistant Mg, chinese script Mg2Si, polygon Mg2Si, corrosion

Electromigration test of flip chip solder bump is performed at 140℃ and 4.6×104A/㎠ conditions in order to compare electromigration with thermomigration behaviors by using electroplated Sn-3.5Ag solder bump with Cu under-bump-metallurgy. As a result of measuring resistance with stressing time, failure mechanism of solder bump was evaluated to have four steps by the fail time. Discrete steps of resistance change during electromigration test are directly compared with microstructural evolution of cross-ectioned solder bump at each step. Thermal gradient in solder bump is very high and the contribution of thermomigration to atomic flux is comparable with pure electromigration effect.

keyword : electromigration, thermomigration, joule heating, flip chip Pb-free solder bump, Sn-3.5Ag solder

The effect of Mn additive on the thermoelectric properties of Fe-Si alloys prepared by a RF inductive furnace was investigated. The electrical conductivity and Seebeck coefficient were measured as a function of temperature under Ar atmosphere to evaluate their applicability to thermoelectric energy conversion. The electrical conductivity of the specimens increased with increasing temperatures showing typical semiconducting behavior. The electrical conductivity of Mn-doped specimens are higher than that of undoped specimens and increased slightly with increasing the amount of Mn additive. This must be due to the difference in carrier concentration and the amount of residual metallic phase ε-FeSi(The ε-FeSi was detected in spite of 100 h annealing treatment at 830℃). And metallic conduction increased slightly with increasing the amount of Mn additive. On the other hand, Mn-doped specimens showed the lower Seebeck coefficient due to metallic phase. The power factor of Mn-doped specimens are higher than that of undoped specimens and would be affected by the electrical conductivity more than Seebeck coefficient.

keyword : Fe-Si alloy, seebeck coefficient, electrical conductivity, power factor

Cu based amorphous (Cu54Zr22Ti18Ni6) coating was produced by cold spraying as a new fabrication process. The microstructure and macroscopic properties of amorphous coating layer was investigated and compared with those of cold sprayed pure Cu coating. Amorphous powders were prepared by gas atomization and Al 6061 was used as the substrate plate. X-ray diffraction results showed that Cu based amorphous powder could be successfully deposited by cold spraying without any crystallization. The Cu based amorphous coating layer (300~400 μm thickness) contained 4.87% porosity. The hardness of Cu based amorphous coating represented 412.8 Hv, which was correspond to 68% of the hardness of injection casted bulk amorphous material. The wear resistance of Cu based amorphous coating was found to be three times higher than that of pure Cu coating. The 3-point bending test results showed that the adhesion strength of Cu based amorphous coating layer was higher than that pure Cu coating. It was also observed that hard Cu base amorphous particle could easily deform soft substrate by particle collisions and thus generated strong adhesion between coating and substrate. However, the amorphous coating layer unexpectedly represented lower corrosion resistance than pure Cu coating, which might be resulted from the higher content of porosity in the cold sprayed amorphous coating.

keyword : Cu based bulk amorphous, cold spraying, amorphous coating, porosity, micro-hardness, were resistance, adhesion, corrosion resistance