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Precipitation behavior of solution heat treated Alloy 690 tubes (60Ni-30Cr-10Fe: UNS N 06690) has been studied. Carbide precipitates along grain boundary were revealed as M_(23)C_6 type carbide having semi-coherent interface with one adjacent grain. Also drastic change of precipitate density was observed on a twin intercepted grain boundary, i.e, abundant precipitates on general grain boundaries, but no precipitate on boundary segments intercepted by twins. This selective precipitation phenomenon was attributed to the different orientation relationships of grain boundary segments, i.e. different misorientation characteristics, and eventually drastically different boundary energy. Different misorientation characteristics of interfaces were investigated through Kikuchi pattern analyses in TEM. The grain boundary with precipitates was random high angle grain boundary and the clean segment of boundary was CSL (Coincidence Site Lattice) special boundary (e.g. ∑=15), judged by Brandon`s criterion. Of course coherent twin boundary with ∑=3 were free of precipitate. (Received April 2,1997)

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Nitrogen effect on sensitization behavior of type 316L stainless steel was investigated in view of thermal precipitation. The investigated alloys had a variation in carbon and nitrogen contents. The sensitization resistance was improved with decreasing of carbon content and increasing of nitrogen content. From a microstructural observation on the sensitized alloys, it was found that the addition of nitrogen up to 0.15% increased sensitization resistance through suppressing the initiation and growth of M_(23)C_6 type carbide. M_6C type carbide was only formed at the alloy with medium nitrogen content. Besides carbides, Mo-rich precipitates such as intermetallic η and χ phases, and five-fold symmetry phase were observed at the grain boundary. These phases were more popular in the medium nitrogen alloy than in low nitrogen alloy at the same aging condition. From a TEM-EDS(energy despersive spectroscope) analysis, it was found that these Mo-rich phases had not a significant deterimental effect on the sensitization resistance.

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In order to investigate the effects of Cr and Si additions in γ-TiAl alloy, the microstructure and mechanical properties before and after heat treatment were studied. Simultaneous additions of Cr and Si were more effective in stabilizing the lamellar structure of TiAl alloys after heat treatment. The stoichiometric composition of the precipitates formed in the alloy with 0.5% Si addition was estimated to be Ti_5(Si, Al)₃, and that with 3%Cr0.5%Si addition to be(Ti, Cr)_5(Si,Al)₃. Good mechanical properties could be achieved by simultaneous addition of Cr and Si. Tensile tests before and after heat treatment showed that maximum yield strength was obtained from the alloy with 1%Cr0.5%Si addition and while the highest elongation was observed by 1%Cr0.25%Si addition before heat treatment. However, the alloy with 2%Cr0.5%Si addition showed the highest elongation after heat treatment.

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To determine the optimum thermochemical processing (TCP) methods, A Ti-6Al-4V alloy was hydrogenated with 60% hydrogen and 40% argon mixture gas at 760℃ and 830℃ for various times. After holding the hydrogenated Ti specimens at 760℃ and 830℃ for 30 minutes, they were pre-treated with various cooling rate for providing the nucleation sites for hydrides. And then they were aged at 500℃ for 2-10 hours for the formation of hydrides. Finally, Ti specimens were dehydrogenated at 700℃ and 830℃ for 2-30 hours to degas and refine microstructures via recrystallization. Specimens were tested with Instron machine to determine the mechanical properties. Also, the superplastic deformation tests were carried out at 840℃, 880℃ and 927℃ to measure the superplsticity of the specimens. As a result of TCP, Ti specimens exhibited fine equiaxed grains of 2㎛. Grain refinement with TCP induced the increase of mechanical properties and superplastic characteristics.

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The effect of grain size on the overall elastic and plastic properties of nanocrystalline materials has been evaluated using the elasto-plastic finite element method. The tensile deformation behavior of nanocrystalline copper(based on a unit cell model comprising 2 to 100 ㎚-sized copper crystallites and 1 ㎚-wide grain boundaries) has been analyzed. Assuming that the material is elastic-perfectly plastic (because of the very fine grain size), it has been shown that the yield stress increased and the elastic modulus decreased with a decrease in grain size. These results are consistent with the reported experimental observations. This analysis can be effectively used to predict the mechanical behavior of any type of nanocrystalline material.

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Effects of the W addition on the microstructure evolution and mechanical properties of high strength Si-Cr spring steels have been investigated. The W addition would reduce the coarsening rate of cementite due to the lower diffusion rate, resulting in a smaller size of cementite particles. The sag resistance depends on the distribution and the size of precipitates. Therefore, an abundance of precipitates at 350℃ tempering exhibits the maximum hysteresis loop area, i.e. the sag resistance. At the tempering temperature of 450℃-500℃, the W addition resulted in the improvement of the sag resistance through the suppression of the cementite coarsening.

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Hot workability and static softening of the 10vol.% SiC_p reinforced Al-12Si matrix composites and Al-12Si matrix alloy were studied by hot torsion tests at temperatures from 270 to 520℃, and strain rates from 1.2×10^(-3) to 2.16×10^(-1)/sec, respectively. Detailed analyses of flow curves and deformed microstructures were made to identify the hot restoration mechanism of the composites and the critical strain (ε_0) for the DRX (dynamic recrystallization) was evaluated from the work hardening rate and peak stress. The critical deformation condition for minimizing damage of the SiC_p reinforcements and Si precipitates of the composites could be obtained from the relationship between Zener-Hollomon parameter (Z=εexp(Q/RT) and peak stress (σ_p). Isothermal interrupted deformation was also performed to investigate the effect of static softening on the hot ductility of the composites. During the isothermal interrupted-deformation, the ductility of the composites was more enhanced by introducing the SRV (static recovery) than SRX (static recrystallization) and grain growth.

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MoSi₂ is one of the most promising candiate for high-temperature structural materials because of its high melting point and excellent oxidation resistance. However, the low room temperature fracture toughness and ductillity, etc. of MoSi₂ still remains as major obstacles to potential high temperature structural applications. The fracture toughness of brittle matrix ran be improved by the incoporation of a ductile second phase such as metallic Nb. The purpose of this study is to improve the room temperature fracture toughness and ductillity of sintered MoSi₂ and MoSi₂/Nb composites respectively by densification, refining of microstructure and minimizing of interfacial reactions between matrix and Nb. These compound and composite are prepared by mechanical alloying(MA)-spark plasma sintering process(SPS). The microstructures and material properties of these sapmle were investigated by optical microscopy, SEM, XRD, EPMA, microhardness measurement and 4-point bending test. The Vickers hardness, bending strength and fracture toughness increased with the incorporation of metallic Nb.

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Present study was carried out to investigate the effect of alumina content of sintered ore on the degradation. In order to investigate the effect of the crystallization depending on alumina contents, temperature and cooling rate, the tablets composed of first grade reagent were sintered in a electric furnnace. RDI(Reduction Degradation Index) was based on JIS(Japanese Industrial Standard) measured as a degree of reduction degradation. Optical microscopy, XRD, EDX, SEM and microhardness test were used to investigate the morphologies and properties of the iron ore sinter. Iron oxide transformed to another phases such as calcium-ferrite which resulted in the deterioration of RDI when the alumina contents was increased. With the results of SEM and EDX, the microcracks were found in glassy silicate when SFCA(Silico-Ferrites of Calcium and Aluminum) was crystallized. It was found that the more alumina contents was increased, the more microcracks were observed.

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In the study, effects of alloying elements of AISI 304 austenite stainless steels on the behavior of ion nitrding were investigated using plasma ion nitriding system. The characteristics of ion nitriding, their microstructures, and physical properties were investigated as a function of process parameters. In the case of AISI 304 stainless steels, constant concentration profiles on N and alloying elements such as Cr and Ni are observed through nitride layers. Nitride layer consists of the upper white layer having various nitride phases((Cr,Fe)₂N_(1-x_, Fe₄N, CrN, Fe_(2-3)N) and the underneath diffusion layers(CrN+Fe(2-3)N). Diffusion coefficients are 16.8kJ/mole value at 10 mTorr and 171.1kJ/mole value at 0.4mTorr.

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