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In this study, Ni-Al alloys and Ni-Al-Ti alloys were rapidly solidified by melt-spinning. The resulting ribbon microstructures were studied using light microscopy and transmission electron microscopy. By extrapolating experimental results obtained at low cooling rates, the cooling rates of ribbon by melt-spinning technique were approximately determined. The cooling rates of ribbon range from 9×10⁴K/s-1.2×10^6 K/s for 60-80 ㎛ thickness. Growth rates by thermal analysis vary from 0.18 ㎝/sec to 4.7 ㎝/sec in these alloys. The formation of γ` in the Ni-7.55 wt%Al, Ni-5.94 wt%Al-2.19 wt%Ti and Ni-8.30 wt %Al-2.85 wt%Ti alloys can be suppressed by melt spinning. The high hardness observed in these alloys after heat treatment is due to the subsequent development of small, ordered precipitations in the fine-grained matrix. And, grain boundary segregation could was suppressed, with increasing cooling rate.

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Various analytical techniques including scanning electron microscopy, transmission electron microscopy, micro hardness tester, electron probe micro analyzer, and X-ray diffractometer were used to understand the mechanical alloying process in a Ni-base superalloy. The mechanism of mechanical alloying process could be determined by observing the changes of powder morphology, microstructure, chemical homogeniety, degree of mechanical working during the mechanical alloying process. (Received August 14,1989)

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We have investigated the effects of the microstructure and mechanical properties with V content and heat treatment on the rapidly solidified V added Fe-13 Mn-1.2C steel strips which were produced by a twin roll melt-quenching method. Regardless of the V content, solidification process typically took place by primary austenite formation and then the residual liquid phase formed austenite by massive transformation. In melt-quenched, the austenite to ferrite transformation was suppressed, resulting in only austenite. As the V content increased, the equiaxed structure region increased and the secondary dendrite arm spacing became a little wider. With the increase in V content, the refining of austenite grain size after heat treatment was accelerated because the increase in the formation of carbides suppressed the grain growth. As 0.2%Y.S. was increased with increasing V content, a relation between it and grain size as a function of V content can be expressed as the following equation. 0.2% Y.S.(㎏/㎟)=22.05+7.4/√D The mechanism of the large strengthening effect of V on the 0.2% Y.S. was considered to be attributable to the grain refining by V addition and the large increase of Petch constan

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Optimization techniques were applied to reinforce experimentally determined phase diagram data of the binary systems, Na₃AlF_6-PbO and Na₃AlF_6-Cu₂O obtained by the differential thermal analysis(DTA). The technique is based on the least squares regression which permits the simultaneous analysis of diverse experimental binary thermodynamics and the phase diagram data. The optimization program was tested with the system Na₃AlF_6-Al₂O₃ obtained by Foster. The optimized Na₃AlF_6-Al₂O₃ phase diagram displayed excellent agreement with the experimentally determined original phase diagram, thereby proving the validity of the optimization methods applied in this study. Phase diagrams Na₃AlF_6-PbO and Na₃AlF_6-Cu₂O, experimentally determined by DTA, were optimized through the interactive computer-assisted analysis employing the technique. Eutectic points for the optimized Na₃AlF_6-PbO and Na₃AlF_6-Cu₂O phase diagrams occurred at X_(PbO)=0.307, T_E=1176.0 K and X(Cu₂O)=0.741, T_E=1146.7 K respectively.

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The reduction roasting behavior of chalcopyrite concentrate was investigated using various combination of a desulfurizing agent, such as CaO, Na₂CO₃, BaO and MgO under Hz and CO atomsphere. The laboratory reduction experiments were carried using thermogravimetric analysis(TGA), and the degrees of metallization were determined by both weight loss and chemical analysis methods. The rate of reduction was increased by increasing the temperature and the amount of desulfurizing agents. The effect of desulfurizing agents was in the order of Na₂CO₃, CaO, BaO and MgO. Using these combinations, the sulfur was removed from the mineral as CaS or Na₂S etc.. The apparent activation energy(ΔEa) for the reduction roasting of chalcopyrite is about 4.28 ㎉/㏖. Based on this magnitude of the activation energy, it is suggested that a diffusion reaction is the rate limiting step for the temperature range(500-900℃) investigated.

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YBa₂Cu₃O_7 thick films whose thickness varies from 150㎛ to 500㎛ were fabricated by a plasma spraying technique. It turned out that fresh films didn`t show the 123 superconducting phase by XRDP regardless of spraying conditions but annealed films showed high Tc under the particular spraying conditions. It was found that a particle size of the feeding powder and the annealing temperature after spraying play an important role on determining the superconductivity of the thick films. In this set of experiments, a particle size of over 100㎛ and the annealing temperature of 950℃ in O₂ ambient after the plasma spray turned out to be the best conditions to obtain a sharp Tc at around 85 K. Also it has been found that when H₂ gas instead of He was used as a secondary plasma gas superconduction phase formation was depressed and the reason has not been varified in this work.

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Microstructural observations and mechanical tests were carried out to clarify the causes of frequent crack occurrence of leaded nickel silver cast structure during cold rolling. Interdendritic micropores in cast structure are believed to be the major causes of the failures. High hydrogen gas absorbing power of this high nickel bearing alloy may lead to the formation of the micropores, when degassing is insufficient. This supposition was further supported by the comparison of tensile and impact test results of fully degassed specimens with those of insufficiently degassed specimens.

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Thermal effects on the initial permeability and the disaccommodation have been studied in amorphous (Co_(0.94-x),Fe_(0.06)M_x,)_(78)Si₄B_(18), alloys (M=Cr and Ta, x=0-0.08). Initial permeability decreases with increasing temperature, and this tendency reduces after heat treatment. Alloys containing Cr and Ta of x=0.04 with near zero magnetostriction show low decreasing rate of permeabilities at elevated temperature. The low temperature disaccommodation is smaller in alloys containing Ta which has larger atomic radius and strong chemical affinity with Fe than in Cr added alloys. The lowest disaccommodation at elevated temperature has been obtained in amorphous alloy containing Ta of x=0.04.

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Characteristics of subcritical cracking(SCC) in HY-steels were studied focussing on the effect of yield strength on SCC property. In order to accelerate the SCC phenomena and thus to shorten the test period, a new SCC test procedure, so called step-loading SCC test, was developed employing single-edge notched(SEN) tensile specimens. Applying this test procedure on the specimens having six different strength levels, it could be demonstrated that HY-steels having a yield strength over 111㎏/㎟(158 ksi) were highly susceptible to SCC but those having a yield strength lower than 83㎏/㎟(118 ksi) were substantially resistant to SCC. Such an improvement in SCC resistant with lower yield strength was accompanied with the change in failure mode from either quasi-cleavage or intergranular to micro-void coalescence fracture.

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The effects of alloying elements on the thermal stability of rapidly solidified Al-Fe based alloys have been studied. Various specimens which were heattreated in the temperature range of 200-550℃ for one hour were observed by optical and transmission electron microscopes. Lattice parameter change and precipitated phases were investigated by X-ray diffraction. Zone A region decreased in the specimen temperature at 300℃, 450℃ and 500℃ for the Al-Fe, for the Al-Fe-Mo and Al-Fe-Si, and for the Al-Fe-Mo-Si respectively. After one hour annealing at 550℃, second phases were precipitated in the zone B ; they were identified as Al₃Fe for the Al-Fe and Al-Fe-Mo and as Al₃Fe and Silicide for the Al-Fe-Si and Al-Fe-Mo-Si. Precipitates in the Al-Fe-Mo-Si were fine and spherical shape. The hardness of Al-Fe-Mo-Si alloys is higher than that of the other Al-Fe based alloys. The change of the lattice parameter was not observed from all the specimens annealed at 550℃ for one hour.

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