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The precipitation reaction and mechnical properties of Fe-8Ni-3.5Mn-5Mo-1Ti alloys without and with 5% Cr addition were investigated. Two stages of age-hardening due to the precipitation of rod-type η-Ni₃Ti and spherical-type Fe₂Mo intermetallics were observed. The addition of chromium increased the age hardening rate and peak hardness of the base alloy. especially at the second hardening stage. From TEM analyses, it is concluded that chromium addition refines the precipitation, suppresses the coarsening of rod type precipitates and considerably promotes the formation of spherical type precipitates during the second stage of aging. The Fe-8Ni-3.5Mn-5Mo-1Ti alloy showed intergranular and cleavage fracture during first stage of aging at 520℃, but higher ductility with higher strength was achieved in the 5% Cr-bearing alloy. Lower grain boundary segregation of the embrittling alloying elements Mn and Ti, etc. in the Cr-bearing alloy was suggested. The optimun combination of strength and ductility was obtained in the Fe-8Ni-3.5Mn-5Mo-1Ti-5Cr alloy aged at 520℃ for 8 hours and obtained values were σ_(0.2) = 1611 MPa. σ_t = 1739 MPa and ε_f = 9.8%.

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The effects of addition of Mn on age hardening behaviors of Cu-6wt.%Ni-6wt.%Sn were investigated. The microstructural and hardness changes of alloys aged at 350 to 550℃ were studied by optical microscopy and X-ray diffraction techniques. The Mn free alloy decomposed spinodally by aging at 350℃ followed by the formation of metastable γ` phase which accompanies the increase of hardness. Further aging transformed metastable γ` phase to cellular structure of α+γ. When the specimens were aged at 450℃ and above, cellular structure of α+γ formed without spinodal decomposition and transition of metastable phase. The additions of Mn affected the sequences of precipitation. Additions of Mn suppressed spinodal decomposition of the alloy in the early stage of aging and exhibited a incubation period in hardness change. Hardening was accompanied by the formation of MnNi₂Sn phase.

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Residual stress in sputter-deposited molybdenum thin films was studied via a X-ray diffraction with PSD (position sensitive detector) and MCA (multi-channel analyzer). The average strain tensor of the film was determined from sin²Ψ method. Measuring time could be reduced significantly and average stress tensor of a thin film was evaluated easily using PSD and MCA. Instrumental errors due to beam optics and misalignment were corrected with a random molybdenum specimen. Orientation distribution function was calculated from the measured pole figures. Assuming plane stress state conditions average stress tensor of the film was evaluated from the volume averaged stiffness tensor. The measured residual stress from PSD was compared with the biaxial stress evaluated from substrate curvature method.

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The present study was made to obtain useful information for design of cobalt-based alloys with good high temperature strength properties by using the DV-X_α cluster method. The tendency of the local spin moment curve obtained from the present calculation was shown good agreement with the Slater-Pauling curve. The changes in the bond orders with alloying elements could be understood from the electron density difference maps. Using two calculated alloying parameters, the bond order(Bo) and the d-orbital energy level(Md) of transition alloying elements, the binary phase diagrams of the Co-M system could be classified according to the location in the Bo-Md map. These results will give some guide to design of cobalt-based alloys for high temperature applications.

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The present study is aimed at developing the high strength cold-rolled steel sheets with high ductility. The main emphasis of the present study has been placed on understanding the effects of Mn and Si on microstructure and properties of the cold-rolled steel sheets. Three kinds of 0.15C-Mn-Si cold-rolled steel sheets were prepared, and were intercritically annealed following isothermal treatment at bainitic region. Microstructural observation and tensile tests were conducted, and also the volume fractions of retained austenite were measured. In the case of the ECO-2 or ECO-3 steel sheet (high Mn content), the higher volume fractions of retained austenite were obtained than the ECO-1 steel sheet (low Mn content), but showed a dual phase stress-strain curve having continuous yielding behavior, high tensile strength over 1,000 MPa, and low elongation about 20%. The lower Si/Mn ratio, the steel sheets had the stronger band structure and the lower ductility. Therefore, Si/Mn ratio must be high for the increase of ductility.

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The superplastic deformation behavior of a Zn-0.3wt.%Al alloy has been investigated in this study, by employing a new thermomechanical treatment (TMT) process. The TMT process employed appears to produce a fine and equiaxed grain structure that is very stable exhibiting no further grain growth. A remarkable tensile elongation of nearly 1400% has been obtained at room temperature for a Zn-0.3 wt.%Al alloy with the average grain size of 1 ㎛. This elongation seems to be the largest reported so far in the open literatures for dilute Zn-Al alloys. Room-temperature superplasticity is very benificial for the direct investigation of superplastic deformation mechanisms by using in-situ TEM and SEM studies without severe experimental constraints. While the room-temperature superplasticity has been observed in several microduplex superplastic materials, only a few investigations have been reported for single phase alloys so far. Using the superplastic material obtained in this study, a more systematic study on the physicsal mechanisms of superplastic deformation in single-phase alloys appear to be possible through an in-situ TEM or SEM observation. The major deformation mechanism was found to be the grain boundary sliding for this alloy as reported for other single phase alloys. The strain rate sensitivity parameter of a specimen with 1 ㎛ grain size showed a somewhat lower value of 0.41 considering the large elongation of 1400%.

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The influence of nitrogen on the low cycle fatigue and creep-fatigue properties for Type 304L stainless steel was investigated with a strain rate of 4×10³/sec at 600℃ in air atmosphere. Under low cycle fatigue test, 304NG(i.e. N-alloyed steel) had shorter fatigue lives than 304L, which can be explained by lower ductility in 304NG due to the addition of nitrogen. When the creep deformation was introduced by imposing tensile hold time(20 min), result was completely reversed compared with the result of continuous fatigue. From the result of microstructural observation and Yoon-Nam`s life prediction model, the major damaging mechanism can be said to be the cavitation at grain boundary for two alloys during creep-fatigue test. And the reason why 304NG has longer creep-fatigue lives than 304L can be explained that 304NG has lower density of carbide that provides beneficial site for the cavity nucleation, which results in lowering cavity nucleation, because 304NG has a smaller content of carbon than 304L, which is compensated with the addition of nitrogen to maintain strength and the nitrogen is believed to retard carbide nucleation at grain boundary by inhibiting the diffusion of Cr to grain boundary.

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The crack growth process in rolling contact fatigue has been investigated on ring type plate specimens, in which and crack growth is observed on the side surface of the specimens. The results have shown that cracks in high carbon chromium bearing steel are initated from contact surface to the direction approximately normal to the contact surface. After some short length of growth. crack grows parallel to the contact surface and it grows until the separation of the surface layer, inducing flaking type failure. Silicon nitride shows a contact fatigue behaviour similar to that typically found in metallic material. From the fractographic and metallographic observations, it has been found that the crack initation in the silicon nitride subjected to rolling contact fatigue is to be induced by cyclic subsurface shear stress, as is known in high carbon chromium bearing steel.

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In the present study, tensile properties of continuous casted Cu-9Ni-6Sn alloy, with and without 95% swaging, were examined as a function of continuous casting rates. A large amount of Sn-rich phases were observed in the interdendritic regions of as-casted Cu-9Ni-6Sn alloy, regardless of casting rates. The size and distribution of these phases, however, differ greatly with different casting rates, as well as the amount of Sn in the phase. Slower casting rate tended to increase the size of the phase, but the amount of Sn in the phase increased with faster casting rate. During solutionization, most of Sn-rich phase appeared to decompose into the solid solution. After aging, a significant amount of coarse precipitates were observed in the alloy produced at a casting rate of 280 ㎜/min. It is believed that such coarse precipitates were formed around the Sn-rich phases due to relatively high Sn content after solutionization. SEM fractographic studies indicated that coarse precipitates, as well as weak Sn-rich phases, act as sources for microvoid. Consequently, both tensile strength and elongation of solutionized and aged Cu-9Ni-6Sn alloy increased significantly with decreasing casting rate. Cold swaging greatly improved the tensile properties of Cu-9Ni-6Sn alloy due to the reduced and refined Sn-rich phases and coarse precipitates.

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To find out the mechanism of the variation of δ-ferrite content through the thickness of continuously cast type 304 stainless steel slabs, the δ-ferrite morphology, solidification structure and secondary dendrite arm spacing were examined. And also the finite difference method was used for the analysis of heat transfer in a slab during continuous casting. In the 60 ㎜ thickness region below the surface of a slab, the δ-ferrite content increased with the depth. In this region the secondary dendrite arm spacing was the main factor determining the final δ-ferrite content. In the 60-90 ㎜ deep region, the δ-ferrite content increased owing to the effects of two factors; One was the decreased diffusion time and the other was different morphology originating from the equiaxed dendrite. In center region, the δ-ferrite content decreased due to the different solidification mode caused by the center segregation.

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