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The effect of thermo mechanical treatment (TMT) process on thermal degradation characteristics in 11Cr ferritic steel was investigated. The alloy was manufactured by vacuum induction melting and processed through conventional heat treatment (CHT) and TMT. The major precipitates in this ferritic steel were M23C6 and MX. The amount of M23C6 precipitates was increased by the heterogeneous nucleation at dislocations in TMT sample. The strength of the TMT sample was higher than that of the CHT sample in the initial stage of aging. The Laves phase was additionally precipitated after 1,000 hours aging at 650℃. Once the Laves phase starts to precipitate during aging, the decrease of strength in TMT sample was larger than that in CHT sample. It is closely related to the shorter incubation period to nucleate the Laves phase in TMT sample.

keyword : 11Cr ferritic steels, TMT, precipitation behavior, Laves phase

Sticking phenomenon occurring during hot rolling of two ferritic stainless steels, STS 430J1L and STS 436L, was investigated in this study. A hot rolling simulation test was carried out using a high-temperature wear tester capable of controlling rolling speed, load, and temperature. The simulation test results at 900℃ and 1000℃ revealed that the sticking process proceeded with three stages, i.e., nucleation, growth, and saturation, for the both stainless steels, and that STS 430J1L had a smaller number of sticking nucleation sites than the STS 436L because of higher high-temperature hardness, thereby leading to a smaller amount of the sticking. When the test temperature was 1070℃, the sticking hardly occurred in both stainless steels as Fe-Cr oxide layers were formed on the surface of the rolled materials. These findings suggested that the improvement of high-temperature properties of stainless steels and the appropriate rolling conditions for readily forming oxide layers on the rolled material surface were required in order to prevent or minimize the sticking.

keyword : stainless steel, sticking, high speed steel roll, hot rolling

The present study aims at investigating the effects of eutectic Si particles on mechanical properties and fracture toughness of three A356 aluminum alloys. These A356 alloys were fabricated by casting processes such as rheo-casting, squeeze-casting, and casting-forging, and their mechanical properties and fracture toughness were analyzed in relation with microfracture mechanism study. All the cast A356 alloys contained eutectic Si particles mainly segregated along solidification cells, and the distribution of Si particles was modified by squeeze-casting and casting-forging processes. Microfracture observation results showed that eutectic Si particles segregated along cells were cracked first, but that aluminum matrix played a role in blocking crack propagation. Tensile properties and fracture toughness of the squeeze cast and cast-forged alloys having homogeneous distribution of eutectic Si particles were superior to those of the rheo-cast alloy. In particular, the cast-forged alloy had excellent hardness, strength, ductility, and fracture toughness because of the matrix strengthening and homogeneous distribution of eutectic Si particles due to forging process.

keyword : A356 Al alloy, eutectic Si particle, fracture toughness, microfracture process, shear band

The layered ternary carbides, Ti3AlC2, were synthesized via a powder metallurgical route, and their oxidation characteristics were investigated between 900 and 1,200℃ in air for up to 100 hr. The oxidation of Ti3AlC2 resulted in the formation of TiO2 and Al2O3. Up to 1,000℃, the scales formed were thin, compact and adherent. However, above 1,100℃, the Al2O3 oxides formed in the scales were neither dense nor continuous enough to protect Ti3AlC2from aggressive high-temperature oxidation, so that thick scales formed. At 1,200℃, thick scales formed were porous.

keyword : ternary carbides, Ti3AlC2, oxidation, TiO2, Al2O3

As an initial step to understand the forging technology of the Shilla era, the present study has examined metallurgical microstructures of 8 artifacts excavated from the Silla Wang-Gyong site in Gyeongju. As a result, important trends have been found in alloy compositions and also in manufacturing processes. In the design of alloys, the Sn content was apparently changing toward the peritectic point, 22 mass %, of the Cu-Sn phase diagram while the Pb addition was intentionally avoided. This trend in composition was found accompanied by the introduction, subsequent to casting, of such special thermo-mechanical treatments as quenching and forging in artifact manufacture. In addition, the Sn content in alloys containing a significant amount of As was relatively low and no evidence of forging was observed in them. This shows that artisans of that time had fully obtained the inter-relationship between the composing the alloy and method of manufacturing. Therefore, it could be understood as the bronze techonoligal system was at its mature level with application of this method in order to lower the brittleness of the material in bronze artifacts with high Sn contents. This paper will show that the Wang-Gyong era corresponds to that of innovations leading to the technical climax in Korean bronze tradition by providing various levels of bronze manufacturing technologies confirmed from the bronze artifacts of Wang-Gyoung remains.

keyword : bronze, technology, transition, silla wang-Gyong, microstructure

Nb and Ti were added to 316 stainless steel, and then heat-treatments and surface treatments were performed on the 316 stainless steel and the Nb- and Ti-added alloys. All samples indicated enhanced electrical conductivity after surface treatments, whereas they showed low electrical conductivity before surface treatments due to the existence of non-conductive passive film on the alloy surface. In particular, the Nb- and Ti-added alloys showed remarkable enhancement of electrical conductivity compared to the original alloy, 316 stainless steel. Surface characterization revealed that small carbide particles formed on the alloy surface after surface treatments, while the alloys indicated flat surface structure before surface treatments. Cr23C6 mainly formed on the 316 stainless steel, and NbC and TiC mainly formed on the Nb- and Ti-added alloys, respectively. We attribute the enhanced electrical conductivity after surface treatments to the formation of these carbide particles, possibly acting as a means of electro-conductive channel through the passive film. Furthermore, NbC and TiC are supposed to be more effective carbides than Cr23C6 as electro-conductive channels of stainless steel.

keyword : PEMFC, bipolar plate, contact resistance, 316 stainless steel, carbide

Polythiophenes, oligothiophene, and their other derivatives have been paid attention to for organic display applications or bio-sensing device applications since they show very interesting electrical and optical properties as well as excellent environmental stabilities. However, they are very expensive, and hence instead of directly depositing them, 2,5-diiodothiophene monomer has been alternatively deposited and polymerized to polythiophenes during device processes for applications. In this study, gold substrate was deposited with 2,5-diiodothiophene, and Temperature Programmed Desorption was carried out increasing temperature from the liquid nitrogen temperature to 800K to characterize its chemical desorption reactions on the substrate. 2,5-diiodothiophene was characterized by its 82, 83, 127, 164, 209 and 210 atomic mass unit(amu) during desorption, and in this study, 82 amu was especially paid attention to because of showing typical characteristics of 2,5-diiodothiophene. 82 amu developed two desorption peaks at 220K and 260K competitively. 260K peak increased as Langmuir increased, while 220K peak showed an incubation period of Langmuir amount to start to appear as Langmuir increased. Hence it was concluded that 260K peak was related to the adsorption of 2,5-duodothiophene on pure gold surface, and that dose for depositing one monolayer of 2,5-diiodothiophene was greater than 0.5 Langmuir.

keyword : organic display, thiophene, desorption, Langmuir, evaporation

The effect of Ge addition on the thermoelectric properties of ReSi1.75 single crystals has been investigated in the temperature range between 323 and 1073 K. The value of the electrical resistivity along [100] of the ternary alloys is lower than that of the binary in the whole temperature range, while the value along [001] of the ternary approaches to the value of the binary as the temperature increases. The band-gap along [001] for the ternary alloys is smaller than that of the binary. The ternary alloys show the large values of the Seebeck coefficient along [001] at 323 K The maximum dimensionless figures of merit of 0.45 at 923 K and 0.8 at 1073 K are obtained for ReSi1.73Ge0.02 samples along [100] and [001], respectively.

keyword : ReSi(1.75), thermoelectric properties, seebeck coefficient, electrical properties

Encapsulated induction melting was attempted to prepare the Sn-doped CoSb3 skutterudites (CoSb(3-y)Sn(y)) and their thermoelectric properties were investigated. Single phase δ-CoSb3 was successfully obtained by the encapsulated induction melting and subsequent annealing at 773K for 24 hours. However, if the Sn content (y) was larger than 0.4, δ-CoSb3 was decomposed to γ-CoSb2 and Sn. Sn atoms successfully acted as election acceptors by substituting Sb atoms. Seebeck coefficients of the Sn-doped samples showed positive values, and electrical resistivity was remarkably reduced by Sn doping. Thermal conductivity was considerably reduced by doping due to the phonon scattering. Thermoelectric properties were improved by the appropriate post-annealing and Sn doping, and they were closely related to phase transitions and dopant activation.

keyword : thermoelectric, skutterudite, doping, induction melting