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Effect of tin and niobium content on the recrystallization behavior of Zr-0.8Sn-x%Nb ternary alloys were studied. The specimens with 0.2, 0.4, 0.8 and 1.0 wt.% of niobium were prepared under various annealing temperatures from 400℃ to 800℃ and times from 30 to 5000 minutes after vacuum arc remelting. The recrystallization behavior was observed by a polarized optical microscope, TEM and micro-Vickers hardness tester. The recrystallization temperature of the alloys slightly increased with niobium content due to increase of activation energy. The grain growth of the alloys with 0.2 and 0.4 wt.% niobium occurred rapidly, however, that of the alloys with 0.8 and 1.0 wt.% niobium were gradually retarded due to precipitation. The hardness of the alloy with a high niobium slightly increased by the precipitation of beta phase after annealing at 800℃.

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The microstructure of commercially used metastable β-Ti alloy was investigated at the as-quenched state of β-C (Ti-3Al-8V-6Cr-4Mo-4Zr) using a transmission electron microscopy. The microstructure of metastable β phase, so called $quot;tweed structure$quot;, shows fine streak lamella. The microstructure may have significant effect on mechanical properties of hardness, strength and toughness, because structure of the phase provides many heterogeneous nucleation sites during the aging. The commercially used β-Ti alloys do not undergo a martensitic transformation at the state of solution treatment and quenching because of the excess addition of β forming elements such as V, Cr and etc. However, β-C alloy shows the martensite phase transformation in the cases of subzero quenching after solution treatment and tensile test at room temperature after as-quenched state.

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The change of microstructures and mechanical properties in a Mn-Mo-Ni low alloy reactor pressure vessel steel with respect to intercritical heat treatment(IHT) conditions has been examined. The introduction of IHT after quenching step of commercial heat treatment process of the steel significantly improved the Charpy impact toughness and J-R fracture resistance. The IHT conditions showing improved toughness were the temperature range of 710-740℃ and the holding time of 4-8 hours. At the early stage of IHT, fine distribution of newly formed martensite (austenite in the intercritical temperature region) in tempered bainite matrix produced the fine microstructure with small rounded carbides after tempering, which resulted in the improved toughness. With the higher IHT temperature or longer time, however, the positive effect of IHT disappeared due to the coarsening of austenite. High toughness was obtained when the austenite volume fractions formed during heating to the intercritical temperature region were about 20-60%.

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The impact properties of high tensile strength 3Cr-1Mo-V steel were studied in relation to the cooling rate on continuous cooling after austenitization and the effective grain size. The effective grain size is defined as the parallel lath packet and/or the region which cleavage cracks propagate through in a nearly straight fashion. From the experiment it was found that fracture appearance transition temperature(FATT) of 3Cr-1Mo-V steel was linearly correlated with the inverse square root of the mean effective grain size, d^(-½). The mean effective grain size became finer as the cooling rate increased and the linear relation between d^(-½) and logarithm of the cooling rate was obtained. From the above results, dependence of FATT on the cooling rates could be expected by the following equation : FATT(℃) = -41.21·log υ + 16.58, where υ is cooling rate from 800 to 400℃ in ℃/min, 16.58 is constant which related with the mean effective grain size.

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Precipitation characteristics of the Mo-added Zr-alloy were studied by TEM after annealing at 590℃ up to 32 hours. Mo addition was found to decrease the size and to increase the volume fraction of precipitates, and to suppress the grain growth in Zr-1Nb-1Sn-0.1Fe alloy. (Zr,Nb)₄ Fe precipitate was observed in Mo-free Zr-alloy, while ZrNbMoFe type precipitate was found in Mo-added Zr-alloy.

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The main emphasis of the present study has been placed on understanding the effect of the tramp elements on formability of TRIP-aided cold-rolled steel sheets. Four kinds of 0.15C-1.5Mn-1.5Si-(Cu, Cr and Ni) cold-rolled steel sheets were prepared. These steel sheets were intercritically annealed at 780-790℃, and isothermally treated at 430℃. Tensile tests and limiting dome height(LDH) test were conducted, and the changes of retained austenite volume fractions as a function of tensile strain were measured using X-ray diffraction. The steel sheets containing Cu or Cu+Ni had higher volume fraction and stability of retained austenite, and formability than those of Cr containing steel sheets, due to high strain hardening rate maintained in the high strain region by the strain induced transformation of retained austenite to martensite. On the other hand, the addition of Cr resulted in forming martensite after heat treatment and showed a dual phase stress-strain curve behavior and low formability.

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A new interpretation for the anomalous yield phenomena has been suggested from an internal variable approach. The high temperature deformation behavior of β-CuZn alloy(B2 structure) has been investigated through a series of load relaxation tests and analyzed by the inelastic deformation theory. The high temperature deformation behavior of β-CuZn alloy is well described by inelastic deformation theory, consisting of three deformation modes, internal, plastic and dislocation creep deformation modes. Among the above three modes, inelastic deformation has proved to be the most important one to affect the anomalous phenomena. It has also been proposed that the anomalous phenomena can only occur when the reference strain rate(ε˙_o) at higher temperature falls into the lower strain rate and lower stress region. The activation energy for dislocation creep is obtained as 159 kJ/mole, similar to that for the self diffusion of Cu or Zn. It is therefore thought here that the decrease of yield stress above T_p in tensile test is mainly due to dislocation climb process.

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The neutron diffraction method is a powerful technique for the texture investigation owing to its high penetration capability, when compared with the X-ray diffraction method. The complete pole figures can be measured and the whole through-thickness texture can be analysed with the neutron diffraction method. Accordingly, the texture measured by means of the neutron diffraction is successfully applied to evaluate the anisotropic properties of the textured materials. Cold rolling of aluminum sheets was carried out with and without lubrication to investigate the formation of inhomogeneous textures as well as to compare the both measurement techniques (neutron and X-ray). The texture of the cold rolled aluminum sheets mainly resides along β-fiber. However, rolling without lubrication led to the formation of the {001}$lt;110$gt;-orientation at the surface layer which was clearly confirmed by X-ray diffraction. With neutron diffraction the whole through-thickness textures were measured. Accuracy of the measurement and texture analysis was estimated using such parameters as RP_(hkl), RP1_(hkl)-value, PF_(hkl)-dispersion and ΔC. It is proved that the texture analysis by the neutron diffraction is more accurate than that by the X-ray diffraction.

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A new type of ceramic short fibers(ASZ fibers) was developed to fabricate cost effective metal matrix composites having mechanical properties superior to those reinforced with current commercial fibers such as Saffil or SiO₂ short fibers. Effect of the preform sintering temperatures on the mechanical properties of the Al alloy composites reinforced with ASZ fibers were studied to evaluate the composites as automotive applications. Both the crystallization sequence of the ASZ fiber and morphological change of the hinder used to prepare the preform were studied to determine the appropriate sintering temperature suitable for the preform fabrication, thereby enhance the mechanical properties of the resultant composites. Mechanical properties of the squeeze cast composites were measured as a function of the preform sintering temperature. Observations using SEM were carried out on the fracture surface of the fibers to analyze the varying mechanical properties associated with the sintering temperature. Interfacial reaction between the fiber and the matrix was discussed based on the experimental results obtained from SEM and TEM.

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Effect of interfacial reaction on the mechanical properties of the AC8A Al alloy reinforced with ASZ short fibers(ASZ/AC8A composite) was studied. In the ASZ/AC8A composite, the interfacial reaction was observed to proceed between the SiO₂ binder layer and Mg of the matrix to form MgAl₂O₄ at the interface. Formation of MgAl₂O₄ was believed to enhance the interfacial bonding strength resulting in the improved composite strength. However, the interfacial reaction in the ASZ/AC8A composite always took place at the expense of Mg in the matrix, resulting in composite devoid of the precipitates containing Mg such as Al₂CuMg and Mg₂Si. Interfacial reaction mechanisms were investigated for composites containing various amount of Mg contents. The resultant mechanical properties of the composite were measured to determine the adequate amount of Mg contents within the composites. Microstructural changes of the composite were observed using TEM and DSC to provide qualitative analyses on the experimental observations.

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