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The effects of Sn addition on the internal oxidation of Ag-Cd alloys were studied. Four types of Ag-15Cd alloys with p.3, 0.5, 0.8, snd 1.5 wt% Sn were prepared by induction melting in Ar atmosphere. Alloys were internally oxidized at 677, 735 and 782 C with 1.0-3.0×10^5 Pa oxygen partial pressure. Internal oxidation increased the hardness of all alloys by 65-100%. Alloys with higher Sn content showed higher hardness. Oxidation condition proved to have significant influence on the oxidation microstructure and wear resistance. The oxides observed after internal oxidation were identified as SnO₂, Cd₂SnO₄ and CdO. Cd₂SnO₄ particles were embedded within CdO particles. The mechanism of oxidation is considered as follows. First. Sn is oxidized to form SnO₂, then it transforms to Cd₂SnO₄ by furthier reaction with Cd and O. Finally CdO grows around the Cd₂SnO₄ particles. Because Cd₂SnO₄ particles act as nuclei far CdO precipitation, finer dispersion of oxides occurs in higher Sn content alloys resulting in higher hardness and improved wear resistance of the alloy.

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The present work has been carried out focusing on changes of aging response by misch metal(Mm) addition in Al-Mg-Si alloys with excess Mg. With addition of Mm, precipitation of (Mg₂Si) phase and formation of clusters decreased and delayed. However, thermal-stability of precursor phases increased, which is considered as reasons for delayed aging response. Although strengthening precipitate decreased in Mm added Al-Mg-Si alloy with most excess Mg content, slight improvement of tensile properties was observed and it may be estimated that additional solid solution strengthening by excess Mg worked on strength in Mm added Al-Mg-Si alloys.

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The purpose of this work is to investigate .the effects of reduction ratios and temperature during hot rolling on the mechanical properties of Zr-Nb alloys. Based on the results of prior research work, Zr-1Nb-0.5Sn-0.5Mo-0.1Fe was selected as a basic alloy. The selected alloy was hot rolled with 25% and 50% reduction ratios at 700℃, 800 and 900℃. and microstructure, mechanical properties and texture were evaluated. From the results of DTA, it was found that β phase started to tranform into α phase at 960-950℃, and all the β phase transformed into α phase above 650-660℃. Also Kearns number, tensile strength and elongation increased with increasing reduction ratios of hot rolling. With increased hot rolling temperature, tensile strength increased. but elongation and Kearns number decreased.

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The effect of nitrogen on the precipitation characteristics of superaustenitic Fe-22Cr-21Ni-6Mo-(0.3N) stainless steels was investigated by means of X-ray diffraction and microstructural observations such as SEM and TEM. The sigma phase and M_(23)C_6 carbide were recognized` in all specimens regardless of their heat treating condition. In case of nitrogen-free specimen, four phases such as σ, M_(23)C_6, M_6C and χ phases were formed by aging treatment after solution treatment. However, two more nitrides such as Cr₂N and AlN were recognized by long time aging in the nitrogen containing specimen. The addition of nitrogen into superaustenitic Fe-22Cr-21Ni-6Mo steel markedly retarded the coarsening of precipitates and promoted homogeneous distribution of precipitates during isothermal aging. The orientation relationships between the matrix and the precipitates formed during aging treatment were also elucidated.

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The effects of thermal treatment on the microstructure evolution of Inconel 690 alloy were investigated. The carbides were precipitated mainly at grain boundaries by thermal treatment at 720℃. The intergranular carbidas were identified as M_(23)C_6 type and have cube-to-cube orientation relationship with the matrix. The size and shape of intergranular carbides were sensitively dependent on the thermal treatment time at 720℃. The average length and thickness of intergranular carbides increased with increasing thermal treatment time. The intergranular carbides showed inhomogeneous distribution after thermal treatment for 1hr, and were changed into semicontinuous after thermal treatment for 10hrs. The average distance between intergranular carbides showed the minimum value after thermal treatment for 10hrs due to the change of carbides from semicontinuous into discontinuous morphologies. The morphology of intergranular carbides did not affect the mechanical properties, such as yield strength, ultimate tensile strength and elongation. The solid solution temperature of intergranular carbides was measured as 1120-1140℃. The intergranular carbides hindered the grain growth during thermal treatment below the solution treatment temperature due to the Zener dragging of grain boundaries.

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The dynamic strain aging behavior during tensile tests of Inconel 690 alloy has been investigated in temperature ranged 25-700℃ under strain rate ranged 10^(-5)-10^(-3)s^(-1). Characteristic serrations in stress-strain curves were observed in temperature between 200 and 600℃, and negative strain rate sensitivities were observed in temperature between 300 and 500℃ Four different types of serrations, identified as A1, A2, B, C type serrations, were observed depending on the temperature, strain rate and strain. A1 type serration, a periodic rise and drop of stress with small amplitude, was observed in temperature ranged 200-245℃ A2 type serration, a rise of stress followed by a drop of stress, was observed at a higher temperature ranged 245-400℃. B type serration, a successive oscillation of stress, was observed in temperature ranged 245-500℃. C type serration, characterized as abrupt irregular stress drops, was observed at 600℃. The activation energies for the onset of serration were calculated as 122, 72 and 132kJ/㏖ for A1, A2 and B type serration, respectively, from the analysis of the critical strains for the onset of serrations. The rate controlling mechanism for dynamic strain aging is suggested as the migration of substitutional atoms for A1 type serration, the carbon diffusion through dislocation core for A2 type serration and the carbon diffusion through lattice far B type serration.

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In this study, microstructures of the heat affected zone (HAZ) of SA 508 steel were identified by Mo¨ssbauer spectroscopy in conjunction with microscopic observations, and were correlated with impact toughness. Specimens with the peak temperature risen up to 1350℃ showed mostly martensite, but the microstructures were not changed, much from the base metal because of the minor tempering effect when it was risen to 650℃ or 700℃. With the peak temperature risen to 900℃ over the A₃ temperature, the martensite fraction was reduced, while bainite or martensite island were formed because of the slow cooling from the lower austenite region. As the martensite fraction present inside the HAZ increased, hardness and strengths tended to increase, whereas impact toughness decreased. However, impact toughness of the subcritical HAZ with the peak temperature risen to 650℃-700℃ was seriously reduced after post-weld heat-treatment (PWHT) since carbide particles were of primary importance in initiating voids. Thus, the most important microstructural factors affecting impact toughness were the martensite fraction before PWHT and the carbide fraction after PWHT.

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An investigation was conducted into the effects of microstructural parameters on the yield strength drop occurring during spiral piping of two high-strength API-X70 steels processed with titanium and molybdenum additions. Compression-tension tests were conducted as well as tensile tests in order to investigate the Bauschinger effect during piping, and then the test data were compared via microstructures, tensile properties, textures, and yield point phenomenon. The tensile results indicated that the yield strength of the Ti-containing steel dropped, together with yield point phenomenon, after piping, whereas this drop did not occur in the Mo-containing steel. It appeared that competing mechanisms existed in the steels studied herein; the yield strength decrease due to the Bauschinger effect accompanied by the yield strength increase due to the strain hardening effect. In the case of the Ti-containing steel, the yield strength drop was associated with the small strain hardening overrode by a large amount of the Bauschinger effect. For the Mocontaining steel. the large strain hardening, commonly associated with continuous yielding behavior, overrode the Bauschinger effect, and finally caused the increase in yield strength after piping.

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The study focuses on the basic deformation mechanism of ECAP (Equal Channel Angular Pressing) which gives a large shear deformation without change in the reduction of area. A model deformation in which a plasticine is pressed through a transparent die shows a typical shear deformation in detail. ECAP was performed on a commercial pure aluminum and an Al-7%Si alloy in order to investigate the change in the microstructure after ECAP. Friction force between specimen and die acts to prevent uniform shear deformation of a specimen. ECAP significantly reduces grain size of both aluminum alloys below the order of micronmeter by giving a large shear deformation, which provides a new method for grain refinement. A drastic increase in the yield strength combined with a small increase in the strain hardening is observed in both ECA pressed aluminum alloys. The ECAP is believed to be very effective in the grain refinement.

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