발간논문

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Various Cu-Zr bearing monolithic amorphous alloys, exhibiting different stress-strain behaviors, were prepared to investigate the nature of the plasticity shown by theses alloys. Microscopic observation of the quasi-statically compressed Cu_(50)Zr_(43)A1_(7) specimen revealed abundant precipitation of nanocrystallites, while insignificant changes in the microstructure were observed from the Cu_(64)Zr_(36) alloy. Finite element analysis was conducted in order to better understand the role of nanocrystallites in an amorphous matrix on the plasticity of the amorphous alloy. The activation energies for crystallization of the amorphous alloys were measured and correlated with the plasticity of the amorphous alloys. Herein, we report for the first time that the origin of the plasticity of monolithic amorphous alloys lies in the activation energy for crystallization. (Received February 14, 2005)

keyword : Deformation-induced nanocrystallites, Shear bands, Amorphous alloy

In this investigation, effects of nitrogen on the precipitation behavior of Fe-Cr-Mo ferritic steels were studied. Three alloys having different nitrogen content were manufactured by vacuum induction melting in N₂ gas atmosphere. Major precipitates in these steels containing 88 ppm.~682 ppm. nitrogen were M_(23)C_(6) and fine MX. Also, FeW₂B₂ and Cr₂C phases were precipitated in as-tempered condition. The precipitation rate of M_(23)C_(6) and MX were accelerated by addition of nitrogen. However, formation of the FeW₂B₂ phase was suppressed with increase in nitrogen contents. This is due to the increase of W solubility in the matrix by higher stabilization of austenite with increase in nitrogen contents. Small amounts of Laves and CrN phases were additionally precipitated in specimens tempered at 650℃ for 3 hours and aged at 700℃ for 1000 hours. (Received November 22, 2004)

keyword : Fe-Cr-Mo ferritic steels, Precipitation behavior, FeW2B2 phase, Nitrogen effect

The microstructural characteristics of the various low-temperature transformation ferrites such as quasi-polygonal ferrite (α_(B)), granular bainitic ferrite (α_(B)) and bainitic ferrite (α˚_(B)) have been investigated in an ultra-low carbon high strength cold-rolled steel. It was found that a grain of the aB and α˚_(B) comprised several subgrains with orientations of ND//<111> or ND//near <111>, but the aq grain consisted of several subgrains with orientations of ND/ /<001> or ND//near <001>. The EBSD result showed that the α˚_(B) and α_(B) were characterized by low angle grain boundaries whose misorientation angles are smaller than 15˚Even the very low angle grain boundaries with misorientation angle of 2-5˚or less were substantially observed in the α˚_(B) and a_(B) grains, resulting in the not well-etched grain boundaries in optical and scanning electron microscopy. The grain boundaries of the a_(q) were characterized by the high angle grain boundary with a winding shape. (Received September 23, 2004)

keyword : Ultra-Low carbon steel, IF(Interstitial Free) steel, Cold-rolled sheet steel, Low temperature transformation products, Polygonal ferrite, Quasi-polygonal ferrite, Granular bainitic ferrite, Bainitic ferrite, EBSD

In the present study, the macroscopic formability of a Zr_(41.2)Ti_(13.8)Cu_(12.5)Ni_(10)Be_(22.5) bulk metallic glass was examined through an extrusion tests in a laboratory scale within undercooled liquid state after rapid heating. From the result of compressive deformation tests, basic processing map based on dynamic materials model (DMM) could be constructed to evaluate feasible forming conditions. The formability determined by macroscopic failure matched a good correspondence with the iso-efficiency contour in processing map. Empirical deformation map providing the boundaries among three different deformation modes could be overlapped the processing map and found to present a proper forming condition. In addition, the effect of specimen size and heating rate on the optimal forming condition of this metallic glass system could be discussed.

keyword : Bulk amorphous alloy, Extrusion, Formability, Processing map, Deformation map

The object of the present study is to investigate the microstructural development of the adiabatic shear band formed by dynamic torsional experiments of ultra-fine-grained low-carbon steels fabricated by equal channel angular pressing (ECAP). Dynamic torsional tests were conducted on four steel specimens, two of which were annealed after ECAP, using a torsional Kolsky bar, and then the test data were compared via microstructures, tensile properties, and fracture mode. The ECAP`ed specimen consisted of very fine, equiaxed grains of 0.2 !Am in size, which were slightly coarsened and had an equiaxed shape after annealing. Some adiabatic shear bands were observed at the gage center of the dynamically deformed torsional specimen, and their width was smaller in the ECAP`ed specimen than in the 1-hr annealed specimen, although they were not found in the 24-hr annealed specimen. Ultra-fine, equiaxed grains of 0.05~0.2 μm in size were formed inside the adiabatic shear band, and their boundaries had characteristics of high-angle grain boundaries. These phenomena were explained by dynamic recovery and recrystallization due to the highly localized plastic deformation and temperature rise occurred within the shear band. The temperature rise in the shear band formation process was inferred to be above 540℃ from the observation of spheroidization behavior of cementites in pearlite grains. (Received February 7, 2005)

keyword : Dynamic torsional test, Ultra-fine-grained Low-carbon steel, Equal channel angular pressing, ECAP, Adiabatic shear band, Annealing

The objective of this study was to investigate the correlation between microstructure and the wear resistance of stainless steel (STS) 316 blend coatings applicable to cylinder bores. STS 316 spray powders were mixed with 0, 10, 20, and 30 wt.% of Al₂O₃_(-)ZrO₂ powders, and then were sprayed on a low-carbon steel substrate by atmospheric plasma spraying. Microstructural analysis of the coatings showed that iron oxides were formed in the austenitic matrix by oxidation during spraying for the STS 316 coating, while Al₂O₃_(-)ZrO₂ oxides were additionally formed in the matrix for the blend coatings. The hardness and wear test results revealed that the blend coatings showed the better hardness and wear resistance than the STS 316 coating and that the hardness and wear resistance increased with increasing volume fraction of total oxides. However, the main wear mode changed from abrasive wear to delamination wear because of cracking of oxides and separation of oxide/matrix interfaces as the volume fraction of total oxides increased. This led to the slight enhancement of wear resistance in the blend coating having the highest volume fraction of total oxides, although its hardness was much higher than that of the other blend coatings. (Received December 28, 2004)

keyword : Atmospheric plasma spraying, STS 316 blend coating, Wear resistance, Al2O3-ZrO2 oxide

Three different fatigue tests for 316 and 316LN stainless steels were conducted at RT, 600℃ and Ast= 1% for low cycle fatigue, at 200~400℃, 300~600℃ and Δε_(t) = 0% for thermal fatigue, and at 300~600℃ and Δε_(t) = 2% for thermomechanical fatigue. Low cycle, thermal and thermomechanical fatigue lives increased with the addition of nitrogen. Low cycle, thermal fatigue strength was increased but thermomechanical fatigue strength was almost same with the addition of nitrogen. Thermal fatigue life was decreased but thermal fatigue strength was increased at the higher temperature range. Thermomechanical fatigue life of in-phase was longer than that of out-of-phase. Fracture mode was transgranular for all tests. (Received September 3, 2004)

keyword : 316 Stainless steel, Thermomechanical fatigue, Nitrogen

The chlorination reaction of tantalum pentoxide is one of the important intermediate steps to produce high purity tantalum metal by a chlorination method. The results of experiments on the kinetics of the reaction between tantalum pentoxide and gaseous carbon tetrachloride are presented in this paper. The experiments were carried out in the temperature range 723 and 798 K, using a thermogravimetric analysis technique. The solid reactant was powder of tantalum pentoxide (-44 μm). A shrinking core model was found to fit well the chlorination reaction rate over the entire temperature range. The chlorination reaction was controlled by surface chemical reaction, and an activation energy of 46.2 kJ/mol (11.0 kcal/mol) was obtained. (Received February 1, 2005)

keyword : Chlorination reaction, Tantalum pentoxide, Carbon tetrachloride, Shrinking core model

Solvent extraction experiments have been performed to separate Gd and Sm from chloride solution with PC88A and saponified PC88A. The effects of extraction conditions on the distribution coefficients of Gd and Sm were investigated. The distribution coefficients of Gd were higher than those of Sm in the experimental ranges of experimental conditions of this study. A solvent extraction model was developed to predict the distribution coefficients of metals and separation factor between Gd and Sm from the initial extraction conditions. The measured distribution coefficients of Gd and Sm with PC88A and saponified PC88A showed good agreements with the predicted values. (Received March 3, 2005)

keyword : GdCl3, SmCl3, PC88A, Saponification, Separation

Solvent extraction of copper with LIX973N in kerosene was carried out to separate copper from the sulfate leaching solution of matte which is the intermediate product of manganese nodules. It was found that LIX973N had high loading capacity for Cu and separation factors with nickel, cobalt and iron compared to those of LIX84I. Extraction percentage of copper greatly increased with increasing extractant concentration while a little increase in the extraction of the other metals was observed. The two stage counter-current batch simulation study showed that extractions of copper, nickel, cobalt and iron were 96.1%, 2.2%, 0.5% and 0.0% respectively. (Received January 8, 2005)

keyword : Solvent extraction, LIX973N, Copper, Counter-current batch simulation