발간논문

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Special specimen zigs and side grooves were used to prevent the apparent negative crack growth and to measure the crack length correctly in the 3-point bend J_(1C) test using unloading compliance method. All the specimens were Chapy impact types made of SA 533 B 1 steels, and the loading-unloading process was accomplished by the computer controlled MTS. The initial crack length and the crack growth could be measured within the error of 3% and 12% respectively. The apparent negative crack growth could be prevented to use the special specimen zigs and side grooves.

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Titanium nitride films were deposited onto the high speed steel substrates by reactive ion plating method The substrate temperature and ;partial pressure of the N₂gas were controlled as process variables. The nitride phases in the films were identified by X-Ray diffraction technique and were observed in a scanning electron microscope. The hardnesses of deposited films were also measured. The phases presented in the coatings could be predicted from nitrogen gas pressure. And the structure modification by ion bombardment during deposition was also observed. The maximum hardness values of 2800-3300 KHN were obtained at the nitrogen pressure of 0.8×10_(-3) torr.

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Stainless steel-copper-stainless steel composite sheets have been made by melting copper sandwiched between stainless steel sheets, followed by hot rolling. Effects of the melting time and temperature, and subsequent hot rolling on the mechanical properties of the sandwiched sheets were discussed on the basis of microstructures developed during fabrication. Flow curves or the composite sheets followed the rule of mixture. This was attributed not to the negligible transverse stresses compared to longitudinal stresses in the component layers, but to the fact that an increase in the longitudinal stress due to the tensile transverse stress developed in one component layer while a decrease in the longitudinal stress due to the compressive transverse stress in the other component layer.

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Change of microstructure and pore shape during sintering of Fe-1.9%C compacts were investigated. The compacts were isothermally sintered at 1140, 1150 and 1160℃ in Argon atmosphere with varing sintering time and cooling rate. It was found that total number of pore decreased with increasing sintering time and the rate of pore decrease was predominant within one hour of sintering. The number of pores smaller than 10μm size had been reduced mainly. whereas that of pores larger than 10μm appeared to be nearly unchanged during the sintering process. Average pore size had increased with increasing sintering time and this trend was significant when sintered less than 3 hours. The microstructure of as-quenched from 1 hour sintering at 1140℃ was completely fine penlite while that of as-quenched from 9 hours sintering at 1160℃ was made of net and lath shaped cementies in martensite matrix. The change of the microstructures may be associated with stabilization of austenite as the sintering time and temperature increases.

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The effects of partial substitution of cobalt for iron and small addition of Zr in the Nd-Fe-B system were studied. The results obtained were as follows: 1) The curie temperature increases linear with increasing cobalt content in the Nd-Fe-Co-B-Zr system. 2) In this system Nd₂(Fe.Co)_(14) B matrix phase, Nd rich phase and Zr rich phase were confirmed. 3) The Zr content in matrix is small regardless original addition of Zr in the system. 4) Magnetic properties such as (BH) max; 39MGOe, Br; 13.3 KG and iHc; 5.8 KOe were achieved in Nd-Fe-Co-B-Zr magnet.

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Vapor pressures of lithium over binary and ternary aluminium alloy melts were calculated on the basis of thermodynamic data for liquid binary aluminium and lithium alloys. According to the calculation the vacuum distillation method was found to be an inadequate way of elimination of lithium from aluminium melts. If the amounts of metallic impurities in the aluminium melts are less than 1 wt.%, they, on the contrary to hydrogen, do not change the vapor pressure of lithium significantly. The calculated interaction parameters, ε, which were used to obtain the vapor pressures of lithium over ternary alloys, showed a systematic dependence on the atomic number of respective i elements. It was assumed that this dependence is related to the properties of corresponding binary systems (i.e Al-Li. Li-i, Al-i), which form a ternary aluminium alloy(i.e.Al-Li-i).

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Effect of carbide precipitation on pitting behavior of 304 stainless steel was studied by a potentiostatic technique. Different amounts of carbide precipitation were achieved by senstifation treatments at 800℃ for 1, 3 and 5 hours, respectively after solution treatment in vacuum atmosphere of 10＾(-6) torr Corrosion behavior of these specimens were investigated by studying polarization curves in 1.0 N H₂SO₄solution. The specimens were pitted in 1.0N H₂SO₄+0.5N HCl solution by a potentiostatic method and the morphology of the pitts was observed by a scanning electron microscope. It was found that the as-solution treated specimen showed the widest passive region and the passive region became narrower as the amounts of carbide precipitation increased. As the potential increased and amounts of carbide increased, the induction time linearly decreased and pitting rate increased. Microstrucrures of the all specimens showed slow grain boundary corrosion during the induction time region. In the pit growth region, pits were observed in the grain interior in the case of the as-solution treated specimens whereas they appeared more in the grain boundary areas in the case of the carbides precipitated specimens. As corrosion time increased, the number and size of the piles also increased. Pits appeared remarkably around twin and grain boundary regions in the specimens sensitized for 3 hrs.

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The effect of nitrogen content on the susceptibity for stress corrosion cracking of AISI 304 weld metal in holing 42% MgCl₂ have been investigated using the slow strain rate technique. As the nitrogen content in weld metal increased the amount of retained delta ferrite was decreased And it was found that the nitrogen content has more effects than the amount of retained delta ferrite on the mechanical properties of weld metal. The increase in nitrogen content promoted the planar arrays of dislocations. This behavior, together with the strip step dissolution, could explain the increased tendency of transgranular stress corrosion cracking with the increased nitrogen content and decreased initial strain rate.

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