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The phase transformation of mixture of Y₂O₃, BaCO₃ and CuO powders with mole ratio of 1/2 : 2 : 3 and that of the superconducting Y₁Ba₂Cu₃Oy single phase were investigated by thermal gravimetric and differential thermal analysis between the temperature ranges of 20℃ and 1300℃. Above 1010℃, partial melting and phase decomposition of Y₁Ba₂Cu₃Oy single phase was detected and it was found that the new phases were (Y₂BaCuO_5) and BaCuO₂by SEM -EDX. By considering the change of the thermal expansion coefficient of the superconductor bar with temperature, it was concluded that superconducting Y₁Ba₂Cu₃Oy undergoes phase transformation at 400℃ and 700℃.

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The hydrogen entry into pulse-plated palladium deposits on copper and iron has been investigated as a function of pulse current parameters such as duty cycle and average current density. The maximum cathodic current efficiency of palladium plating on copper was found to appear at the duty cycle of 60% and the average current density of 2 ㎃ ㎝^(-2). Hydrogen extraction and permeation tests indicate that the amount of hydrogen absorbed into the palladium deposits on copper and iron decreased with decreasing duty cycle due probably to a decrease in the input hydrogen concentration at the deposit surface during the palladium plating. The palladium deposit on iron provides a catalytic effect rather than a barrier effect on the hydrogen entry into the deposit. From impedance measurements it is concluded that the resistance to hydrogen reduction is larger than that to hydrogen oxidation at the surface of the palladium deposit on copper. This is based upon hydrogen adsorption on the surface of the palladium deposits.

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Dynamic fracture toughness of welding structural steel including it`s weld zone has been estimated by employing instrumented impact testing method for small size specimens. Result showed that dynamic fracture toughness, K_(ID) was increased in the order of HAZ 4㎜, HAZ 6㎜, HAZ 1㎜, HAZ 2㎜, fusion line. Correlations between Charpy absorbed energy and critical COD have been investigated to get a useful method for the estimation of critical COD from Charpy V data. The correlations between vE(T+△T) and fe were taken into a consideration where vE(T + oT) is Charpy absorbed energy obtained by considering temperature difference between Charpy transition temperature and COD transition temperature. It was found that the correlations were establised over a wide temperature range including not only upper shelf range but also transition range.

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The magnetic properties of amorphous Co-Y thin films prepared by RF magnetron sputtering have been investigated. The saturation magnetization of Co-Y alloy is linearly decreased with increasing Y content, and the magnetic moments of Co-Y alloys larger than those of Friedel`s theory were interpreted by the atomic size effect. The coercive force is drastically decreased at about 8 at.% Y which is the minimum solute concentration to get the amorphous phase. The solute concentration of film is monotonously increased with increasing Ar pressure, and the coercive force shows the lowest values at the Ar pressure of 5 mTorr. Effective permeability in hard direction of thin film produced in magnetic field is lower at low Frequencies but higher at high frequencies above 100 ㎑ than those of easy direction. The rotating magnetic field annealing is effective to increase the effective permeability at overall frequency range.

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The effects of steam pressure and preoxidation on the nodular oxidation of Zircaloy -4 were investigated. The accelerated oxidation of nodular type occured at 500℃ above the threshold steam pressure and the oxidation rate increased with increasing the steam pressure. When the uniform preoxide layer, which had been formed at 500℃ near 1 bar, exceeded the size of intermetallic precipitate, the nodular oxidation was drastically reduced at 500℃ under 100 bar pressure. The preoxide thickness protecting the nodular oxidation increased as the steam pressure of preoxidation increased. It revealed that the nodular oxidation may be related to the hydrogen build-up at the sites of the intermetallic precipitates within the uniform oxide surface. The build-up of hydrogen at these sites may lead to stress generation, resulting the cracking of the protective oxide. The localized rapid oxidation with the surface cracking may have caused the nodular corrosion

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For practical application of cryogenic engineering, development and characterization of structural materials for use at low temperatures are essential. For these purposes, a system for mechanical testing at liquid helium temperature was developed and it was shown that the precision and accuracy of the system met the requirements of standards for materials testing machines. Using this system, tensile deformation behavior of AISI 304, 316 and 310S austenitic stainless steels at cryogenic temperatures was investigated. Tests were conducted on round, tensile specimens having a 6.25㎜ diameter at 4, 77, and 295 K and loading rate was 0.5㎜/min. Serrations were observed in all alloys at 4 K. The stress-displacement curves at 77 and 4 K showed different tendency from those at 298 K. As the testing temperature decreased, ultimate strengths of 304 and 316 were largely increased compared to the increase of yield strengths, but the increase of ultimate strength of 310S was almost the same to that of yield strength. Type 310S had the highest yield strength and the lowest tensile strength at all temperatures. These tensile characteristics were considered to be strongly affected by austenite stability.

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The stress-corrosion cracking (SCC) mechanism of Al-Zn-Mg alloy has been studied in terms of the relationship between pre-exposure embrittlement (PEE) and SCC. PEE and SCC susceptibilities were measured as a function of ageing state, pre-exposure time and applied potential by means of the slow strain rate method. The specimen pre-exposed to water vapour-saturated air, followed by reheat treatment (re-solutionized in vacuum and aged in dry air) showed a pronounced increase in fracture energy as compared to the pre-exposed specimen. This suggests that PEE is caused by reversible embrittlement, i.e., by hydrogen embrittlement. The similar trends in SCC susceptibility were obtained with respect to ageing state as well as to applied potential. This implies that the SCC mechanism remains unchanged regardless of ageing state and applied potential. When pre-exposure time was large enough than 120 h, the susceptibility to PEE and fracture surface of PEE were very similar to those associated with SCC. On the basis of experimental results, it is believed that SCC as well as PEE is caused by hydrogen embrittlement

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The conditions for the formation of Mn-Zn ferrite by oxygen oxidation in aqueous suspension were investigated with various pH, temperature, and Mn/Zn molar ratio and were compared with those of Mn ferrite and Zn ferrite. The oxidation products were then examined by XRD, chemical analysis, electron microscopic observation and magnetic property measurements. The most favorable pH at 70℃ to form the Zn and Mn ferrite and 9.0 and 12.0 respectively. In Mn-Zn ferrite formation, the reaction temperature affected the amount of Zn incoporated into ferrite than that of Mn. When changing the Mn/Zn molar ratio, it is possible to control the chemical composition of Mn-Zn ferrite. At temp. 70℃, M/Fe molar ratio 0.5, Mn/Zn molar ratio 1.5 and pH 11, Mn-Zn ferrite is obtained that has nearly stoichiometrical composition Mn_(0.45)Zn_(0.51)Fe_(2.04)O₄ and its saturation magnetization was 38 emu/g.

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In this study, Fe-3.6%C-Si alloy powders were made using high pressure gas atomizer and ribbons by melt spinner. The solidified structure and annealed structure of powders and/or ribbons were studied, then mechanical properties of its exturded bars were discussed. The solidified structure of coarse powders about 100㎛ was the mixture of ledebrite eutectic and dendrite, when Si content is 1∼2%, dendrite proved to be austenite and when 3∼4% to be cementite. The solidified structure of fine powders(20 ㎛) and ribbons was mainly ledebrite eutectic. The hardness of powders recorded 1000∼1500MVH(㎏/㎟) and increased as Si contents increased. After annealing, hardness decreased to 200∼300 MVH because graphite precipitated in powders. After extrusion, the matrix transformed to ferrite and graphite precipitated uniformly at prior particle boundary or inside of powders. The tensile strength increased with increasing extrusion rate and reached 65∼75㎏/㎟ when extrusion rate was 13. Toughness was 7∼7.5㎏·m/㎠ and hardness was 100∼110HR_8.

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Oxidation reaction behaviors of Cu(OH)₂ and Fe(OH)₂ coprecipitates by oxygen in aqueous solution were investigated in relation to the variation of initial Cu/Fe ratio, R (2(OH)-/SO₄^(2-)) value and temperature. When increase the Cu/Fe ㏖ ratio, initial pH decreased and final pH increased. Oxidation behavior monitored by ORP (Oxidation Reduction Potential) showed 2 distinct stages. The 1st stage was due to the oxidation from Fe(OH)₂ to Fe₃O₄, and the 2nd stage resulted from the transformation Cu(OH)₂ to CuO. In any conditions, temperatures ranging 50 to 80℃ and R value 1.0 to 1.4, Cu-ferrite did not formed. The final oxidation product was Fe₃O₄ covered with CuO. At Temp. 150℃, Cu/Fe= 0.5 and R=1.1, reaction product was CuO and Cu-ferrite. The Composition of Cu-ferrite was Cu_(0.65)Fe_(2.35)O₄ and its saturation magnetization was 28 emu/g.

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