발간논문

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The influence of Mo substitution by W on impact toughness was investigated and related to the microstructure in duplex stainless steels. W had a lesser pronounced effect on promoting embrittlement than Mo. only sigma phase was observed in conventional alloy containing 3% Mo. However, as Mo was substituted by W chi and/or Laves phases were formed besides the sigma phase. Especially Laves phase was formed intragranularly within austenite grain. The sigma phase formation was also the main reason of embrittlement although Mo was fully substituted by W. However, the Mo substitution by W significantly delayed the nucleation of sigma phase and also decreased the number of its nucleus. Therefore, the increasing rate of sigma phase amount was remarkably slowed down. Chi phase nucleated prior to the sigma phase and occupied preferential nucleation sites, grain boundary triple junctions. One hour aging treatment, causing chi phase formation but virtually no sigma phase, resulted in a little reduction of impact toughness.

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The influence of carbon content on the austenite grain growth of Ti-microalloyed C-Mn steels during reheating to the austenite region has been studied using optical microscope and transmission electron microscope. The austenite grain coarsening temperature(GCT) particulary at the center of slab decreases as the carbon content increases from 0.09wt% to near peritectic composition(0.16wt.%). This is believed to arise from the effect of carbon content as the segregation behavior of the interdendritic region! The degree of segregation is distinctively small in low carbon steel because of two simultaneous effects, namely the faster diffusion rate and shorter diffusion length. This is because of dendrite formation mechanism differs depending on the carbon content. The austenite grain size during reheating is observed to be distinctively small in the case of low carbon steel. This is due to the fact that the upper temperature for the austenite single phase region is comparatively low in the case of low carbon steel.

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Precipitation behavior of θ(MgLi₂Al) and δ(AlLi) in squeeae cast Mg-8.5wt%Li-4.5wt5Al has been investigated by scanning electron microscopy(SEM), differential scanning calorimetry(DSC) and in-situ and ex-situ X-ray diffraction analyis. Microstructural and calorimetric analysis of as-cast Mg-8.5wt%Li-4.5wt%Al shows that the temperature range required for the proper solution treatment is 370-400℃. Both θ and δ precipitate in the b.c.c. β phase matrix of Mg-8.5wt%Li-4.5wt%Al after solution treatment at 400℃, forming two exothermic peaks at the temperature range of 130℃-180℃ and 236℃-280℃ with the peak temperatures of 153℃ and 262℃, respectively. θ and δ dissolves into the matrix forming an endothermic peak at the temperature range of 280℃- 352℃. The as-cast microstructure consists of α, β wand δ. δ dissolves into the matrix after solution treatment. During aging treatment at 50℃-100℃ θ forms at the early stage of aging, and δ begins to form after completion of θ precipitation. Activation energies for the formation of θ and δ are 97.3㎉/mole and 111.7㎉/mole, respectively.

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Indentation test has been used to assess the mechanical properties of various materials, especially locally embrittled materials. It has a potential to replace a conventional uniaxial tensile test, although the interpretation of experimental results has not been standardized due to complex stress fields beneath the indenter. We intend to analyze the indentation response of a material with an advanced technique, which records the indentation depth continuously as a function of indentation load. The response of materials to indentation can be elastic/plastic deformation and cracking, or their combination. For a spherical indenter, the representative strain of materials can be expressed by the ratio of plastic depth to contact radius, while deformation response can be divided into three stages as load increases; elastic, elastic/plastic and fully plastic stage. At the first stage, material response is reversible. The ratio of mean pressure to flow stress increases at the second stage and third stage is characterized by the constant ratio of mean pressure to flow stress. In each stage, the ratio of mean pressure to flow stress can be taken. Therefore, we can derive the true stress -true strain curve by analyzing the ratio of mean pressure to flow stress through continuous indentation test.

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Shear band formation during the tension of Al-3wt%Cu single crystals has been studied. A series of uniaxial tensile tests has been carried out for this purpose using the crystals with nominal solid solution and crystals containing θ′ precipitates at room temperature. The slip behavior and the band formation were continuously monitored with a video camera and a recorder. Optical microscopy, SEM, and TEM were then used to document the structure and micro-mechanism of localization processes. The localized shearing modes in single crystals were found to appear in two distinctly distinguishable processes, viz. coarse slip and macroscopic shearing. Coarse slip bands(CSB) were generally developed first on either the primary or conjugate slip planes, often accompanied by a significant load drop. Macroscopic shear band(MSB), on the other hand, occurred on an apparently non-crystallographic plane directly responsible for the final fracture of crystals. No clear indication of prior microfracture was detected Before shear band formation indicating that damage softening is not necessary for the shear localization. Slip planes seemed to strain harden continuously during the localized deformation in shear bands. TEM observation of CSBs in a solid solution crystal showed that the crystal lattice within the slip band was misoriented by less than 1˚ with respect to the matrix lattice. The experiments show that the geometrical softening, caused by non-uniform lattice rotation, has an important micromechanical influence on the localization process in single crystals.

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The effects of the microstructural factors such as hard carbides located along solidification cell boundary and the type of the martensite matrix on fracture property of nine high speed steel(HSS) rolls manufactured by centrifugal casting method were investigated in the present study. In situ fracture observation, fracture toughness measurement and fractographic observation were conducted on the rolls to clarify the microfracture process. From in situ observation, hard carbides located along cell boundary provided easily intercellular fracture sites under low stress intensity factor levels. In the rolls containing a small amount of intercellular carbides and lath-type tempered martensite matrix, fracture occurred along a tortuous transcellular path since carbides were well spaced, thereby leading to the enhancement of fracture toughness. For better microstucture, hardness and fracture property of the HSS rolls, it is suggested that the optimization of alloying elements is required to achieve the more homogeneous distribution of hard intercellular carbides.

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The energy absorption performance of extruded aluminum tube specimens for space frame was evaluated using computer-simulated compressive tests and quasi-static compressive deformation tests. Experimental deformation test and its simulation were conducted for seven extruded tube specimens on which various types of grooves were introduced, and the test data were investigated via observation of deformation mode, maximum resistant force, and absorbed energy. The results indicated that the computer simulation data correlated well with the compressive deformation behavior, indicating that the simulation was very useful for evaluation of the absorption energy performance. When the grooves were introduced at the folding sites pre-estimated by the computer simulation, the energy absorption performance could be improved, and the half-grooved specimens absorbed more effectively than the full-grooved specimens. On the other hand, when the grooves of the same interval were introduced without consideration of the folding sites, unhomogeneous deformation, together with bending, occurred, and deteriorated the energy absorption performance because energies were not as effectively absorbed as on folding.

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Effects of heat treatment on the mechanical properties in squeeze cast Mg-10wt%Li-3wt%Al and Mg-10wt%Li-5wt% Al alloys have been ivestigated. The as-cast microstructure consists of a mixture of α, βand AlLi. The best combination of hardness and tensile strength were obtained after solution treatment at 350℃ for 1 hour for Mg-10wt%Li-3wt%Al and 370℃ for 1 hour for Mg-10wt%Li-5wt%Al, respectively. Various microstructural features such as the ratio of α/βvolume fraction, the orientation relationship between α and β grains and shape change of α phase were observed after heat treatment. Strengthening of the Mg-Li-Al alloys is due to the precipitation of AlLi in the β phase matrix, dissolving of Al into the matrix during heat treatment and reprecipitation of α platelets during cooling after heat treatment.

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Effects of heat treatment on the mechanical properties in squeeze cast Mg-10wt%Li-3wt%Al and Mg-10wt%Li-5wt% Al alloys have been ivestigated. The as-cast microstructure consists of a mixture of α, βand AlLi. The best combination of hardness and tensile strength were obtained after solution treatment at 350℃ for 1 hour for Mg-10wt%Li-3wt%Al and 370℃ for 1 hour for Mg-10wt%Li-5wt%Al, respectively. Various microstructural features such as the ratio of α/βvolume fraction, the orientation relationship between α and β grains and shape change of α phase were observed after heat treatment. Strengthening of the Mg-Li-Al alloys is due to the precipitation of AlLi in the β phase matrix, dissolving of Al into the matrix during heat treatment and reprecipitation of α platelets during cooling after heat treatment.

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Hydride formation behavior and its effect on the mechanical properties of Zircaloy-4 nuclear cladding tubing produced by various manufacturers were studied. The hydride was formed by high temperature cathodic hydrogen charging method(HCM). TEM works revealed that the grain size of the commercial Zircaloy-4 cladding tubes was 2-7㎛ in which Zr(Fe, Cr)₂ was uniformly precipitated. δ(ZrH) spectra was mainly observed in the alloy after HCM, and, ZrH₂ and ZrO_(0.35) phases were hardly observed by X-ray analysis. The hydride was found to significantly influence the mechanical properties such as ductility, ultimate tensile strength, yield strength, the crack initiation and propagation behavior. The strain rate sensitivity and the activation volume of the cladding tubes containing the hydrides were observed to be not much different from those without hydrides, suggesting the deformation mechanism before crack initiation is not greatly influenced by the presence of hydrides. The rate controlling mechanism of the deformation was well explained by the dislocation trough model in which the segregation of alloying elements on dislocation determines the activation length.

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