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The role of Mo in Zr-based alloys was studied in terms of the microstructure, texture and corrosion resistance. The base compositions of the experimental alloys were Zr-1Nb and Zr-1Nb-1Sn-0.1Fe to which Mo was added in varying amount up to 0.5%. Buttons of 300 ｇ in weight have been produced by plasma arc remelting (PAR) and processed in sequence by hot forging, hot rolling, beta annealing, cold rolling and recrystallization annealing. It was confirmed that Mo addition resulted in grain refinement: beta grains as well as recrystallized alpha grains. This, in turn reduced the formation frequency and the size of twins and relaxed of the surface normal preferred orientation, f_n. In the corrosion test in water containing 220 ppm LiOH (360℃, 17.9 ㎫), the alloys with up to 0.2% Mo showed a good corrosion resistance whereas that with 0.5% Mo showed a degraded resistance. Apparently, the corrosion resistance was related to the density and morphology of the second phase particles. Alloys containing fine and uniformly distributed β-Nb particles showed good corrosion resistance whereas those containing excessive number or undesirable distribution of particles particularly in uncrystallized region showed degraded corrosion resistance. Overall, the present study suggests that alloying of up to 0.2% Mo should be favorably considered for improving the mechanical properties without impairing the corrosion resistance of Zr-based alloys for nuclear core applications.

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High cycle fatigue tests were performed on both as-received material (SUS 304: an austenitic stainless steel) and the material with dispersed micro-sized α martensite particles by cooling at -78℃. In a high cycle regime, the fatigue life of the material with dispersed micro-sized α martensite particles was much longer than that of the as-received material. After high cycle fatigue test, dislocation structure and micro-structure (longitudinal section) observations were carried out using a TEM and a laser microscope, respectively. It was found that the re-arrangement of dislocations and the formation of large α martensite, which may cause fatigue crack initiation were suppressed in the material with dispersed micro-sized α martensite particles as compared to the as-received material.

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The effects of microstructural morphology on the quasi-static and dynamic deformation behavior of Ti-6Al-4V alloy were investigated in this study. Quasi-static and dynamic torsional tests were conducted using a torsional Kolsky bar for Widmansta¨tten, equiaxed, and bimodal microstructures, which were processed by different heat-treatments. And then the test data were correlated with microstructures, tensile properties, and fracture mode. The fracture behavior of the three microstructures was found to be independent of stress state and loading rate. Dynamic torsional behavior indicated that the equiaxed microstructure was prone to form adiabatic shear bands in comparison with other microstructures. The possibility of the shear band formation was analyzed from the energy required for plastic deformation and from the difference of the void initiation behavior under quasi-static and dynamic loading states.

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Al-Mg-Si base alloys have a good strength, formability, corrosion resistance and bake hardenability. But the bake hardenability of Al-Mg-Si-Cr-Be alloy is better than that of Al-Mg-Si alloy. Mn is added to improve the toughness in Al-Mg-Si alloys. But the precipitation behavior of Al-Mg-Si based alloys with large amount of Mn addition($gt;0.5 wt%) has not been reported yet. Therefore, the mechanical property and precipitation behavior of the Al-Mg-Si-Cr-Be alloy with 0.5-1.5%Mn was observed using the hardness and tensile tests, differential scanning calorimetry(DSC) and transmission electron microscope(TEM). The tensile strength of Al-Mg-Si-Cr-Be alloy increased with the increase of Mn content without decrease of elongation due to the increase of Mn dispersoid. Therefore, the combination of properties such as strength and ductility in the Al-Mg-Si-Cr-Be-Mn alloy was better than that in the Al-Mg-Si-Cr-Be alloy. The formation energy of β$quot; phase of Al-Mg-Si-Cr-Be alloy is decreased by the addition of Mn. But The precipitation of β` phase of Al-Mg-Si-Cr-Be alloy is little affected by the addition of Mn.

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This study is concerned with the development of low-alloy steels for nuclear reactor pressure vessel by investigating the effects of alloying elements on mechanical properties of base metals and heat affected zones(HAZ`s). Four kinds of steels with different alloying elements were fabricated by vacuum-induction melting and heat treatment, and their tensile properties and Charpy impact toughness were evaluated. Microstructural analyses indicated that coarse M₃C-type carbides and fine M₂C-type carbides were precipitated along lath boundaries and inside laths, respectively. In the steels having lower carbon content and higher molybdenum content, the amount of fine M₂C carbides greatly increased, while that of coarse M₃C carbides decreased, thereby leading to the improvement of tensile properties and impact toughness. Their simulated HAZ`s also had sufficient impact toughness after post-weld heat treatment. These findings suggested that low-alloy steels with high strength and toughness could be processed by reducing the amount of carbon and manganese and by raising the amount of molybdenum.

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Directional solidification process of Ti-43Al-3Si alloy has been studied. DS ingots having rotated columnar grains with respect to longitudinal axis as well as fully lamellar microstructure parallel to longitudinal axis were successfully obtained using a polycrystalline seed material. Successful seeding and growing require plane-front solidification condition during the entire DS process. (101)⊥ND texture by pole figure measurement was found to be dominant in the DS ingot grown at a velocity of 5 ㎜/hr. Fracture toughness of the DS alloys was superiors to that of the PST alloys, with a value of K_Q = 21.7∼31.7 ㎫(ｍ)^½ for the crack arrest/divide orientation and K_Q = 7.4∼9.0 ㎫(ｍ)^½ for the brittle orientation.

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In the present study, Al-SiC_p FGMs containing 10 layers with different volume fraction of Al and SiC_p, and different geometry in thickness (3 ㎜, 5 ㎜ and 7 ㎜) were fabricated by employing the pressureless infiltration technique. Optimum condition was established to obtain the enough strength of preforms by controlling the forming pressure and binder added. Al-10%Mg was properly infiltrated into the preforms at 920℃ with N₂ atmosphere within a tube furnace. Analysis of microstructure and mechanical properties revealed that Al-SiC_p FGMs were fabricated successfully. Curvature of Al-SiC_p FGMs plates was measured to predict the internal stress distribution via a theoretical model for the analysis of thermo-mechanical deformation.

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The microstructure and mechanical properties of AISI 304 wires reinforced AM60 Mg composites have been investigated in the present study. No reaction layers or reaction products are observed at the Fe/Mg interface. However, there are the presence of MgO thin layer (irregular thickness) along the Fe/Mg interface and fine Fe particles in the matrix. It has been suggested that MgO layer forms by reaction with oxide present at the wire surface and O₂ diffused from air. It has been suggested that the Fe particles farm by diffusion during squeeze casting. It also has been shown that there is a diffusion of Mg into Fe. Microcracks are nucleated mainly along the reinforcement/matrix interface, and composites are fractured finally by fiber fracture or pull-out. The manufactured composites show the strong potential for further improvement in strength and ductility. High strength is because stress level is nearly in accordance with the rule of mixture (ROM), and good ductility is related to the development of multiple-necking and ductile wire.

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The multiple-necking phenomenon and mechanism of reinforcement wires in AM60 Mg composites reinforced with AISI 304 wires has been investigated in the present study. AM60 Mg composite reinforced with AISI 304 wires exhibits ductility greater than the ductility of wire itself due to the occurrence of multiple-necking along the length of wire. As the volume fraction of AISI 304 wires decreases, the number of wire necking and the ductility of composites increase. It has been shown that the multiple-necking phenomenon is closely related to wires necking, interface reaction, and volume fraction of reinforcement. This phenomenon is also related to diameter of wires, strain rate, structure of metal, and interface strength. The multiple-necking results from local strain hardening of the matrix surrounding the neck, enabling the matrix to decrease neck growth by attraction force. Because multiple-necking plays an important role in the ductility and deformation behavior of composite, further works is needed to characterize the causes of this phenomenon in other metal/metal composites system.

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Densification behaviour of powder compacts during pressure-assisted compaction at an elevated temperature was investigated based on power-law creep. A constitutive equation for power-law creep which satisfies the uniaxial stress condition is proposed based on the plastic deformation theory for porous materials. The proposed power-law creep model is compared with Shima and Oyane`s model and Kim and Jeon`s model. Experimental data generated by hot isostatic pressing of 316 stainless steel powder compacts at a temperature of 1125℃ under different pleasures were compared with calculated results using the proposed constitutive model. The difference between the calculated results and the experimental values was discussed in conjunction with Ashby`s densification mechanism map or HIP diagram.

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