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The behavior of crack nucleation and propagation during low cycle fatigue of ductile the cast irons with different nodule counts and similar nodule size have been compared. In spite of similar nodule size, the fatigue life of ductile cast iron with lower nodule count was longer than that with higher one. This fact shows that not only the nodule size but also the nodule count is the factor governing the fatigue life. Meanwhile, the crack nucleation density of the former was notably increased in the early stage of stress cycling, but the increasing rate was diminished greatly in the middle one. Therefore, difference in fatigue life of these specimens could not be explained with the crack nucleation behaviors only. The difference in fatigue life of these specimens can be explained by the easiness of crack coalescence which depends upon the nodule count. Consequently, the effect of the nodule count on the fatigue life was significant because of the importance of crack coalescence and propagation.

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Because duplex stainless steel shows the good strength and corrosion resistance properties, the necessity of duplex stainless steel, which has long life in severe environments, has been increased with industrial development. The fatigue crack propagation behavior of heat affected zone(HAZ) has been inrestigated in super duplex stainless steel. The fatigue crack propagation rate of HAZ of super duplex stainless steel was faster than that of base metal of super duplex stainless steel. We also analysed acoustic emission signals during the fatigue test with time-frequency analysis method. According to the results of time-frequency analysis, the frequency ranges of 200-400 kHz were obtained by striation and the frequency range of 500 kHz was obtained due to dimple and separate of inclusion.

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Commercially pure Al alloy sheets were processed using high strain rate dissimilar channel angular pressing(DCAP) as many as 100 passages through the channel with φ= 120˚ in order to investigate the work hardening and the work softening behaviors caused by extremely high strain level (~58). The hardness of the alloy increased significantly by a factor of 2 at strains less than ~2.0, while a gradual decrease in the hardness was observed at strains greater than ~2.0. The work hardening and the work softening behaviors observed from the Al alloy were analyzed by correlating the measured properties with microstructural evolutions observed by transmission electron microscopy(TEM). A detailed microstructural evolution sequence occurring at successive strain stages was also demonstrated.

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This study is concerned with the effect of alloying elements on mechanical and fracture properties of Mn-Mo-Ni low alloy steels and their heat affected zones (HAZ`s). Three kinds of steels whose composition were varied from the composition specification of SA 508 steel (Grade 3) were fabricated by vacuum-induction melting and heat-treatment, and their mechanical and fracture properties were examined. ASTM E1921 standard test method was employed to measure fracture toughness in the transition temperature region of the steels. As C content decreased and Mo and Ni contents increased, the amount of fine M2C carbides was greatly increased, while that of coarse cementites was decreased, thereby leading to the improvement of tensile and fracture properties. Brittle martensite-austenite constituents were also formed in these steels during cooling, but did not deteriorate fracture toughness because they were decomposed to ferrite and fine carbides after tempering. Their simulated heat affected zones also had sufficient impact toughness after post-weld heat-treatment. These findings suggested that the low-alloy steels with high strength and toughness could be designed by decreasing C and Mn contents and by increasing Mo and Ni contents.

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Effect of nitrogen contents on the microstructure, tensile and corrosion behaviors of as-solutionized CD4MCU (Fe-25Cr-5Ni-2.8Cu-2Mo) cast duplex stainless steel was examined in the present study. The slow strain rate test was conducted at a nominal strain rate of 1×10^(-6)/sec in air and 3.5% NaCl+5% H2SO4 solution for studying the stress corrosion cracking behavior of the present alloy. It was observed that the volume fraction of austenitic phase in CD4MCU alloy increased from 38% to 58% with increasing nitrogen content from 0 to 0.27 wt.%. The tensile behavior of CD4MCU alloy showed a complex trend depending on the volume fraction and shape of austentic phases as associated with nitrogen content. The resistance to general corrosion of CD4MCU alloy was significantly improved with the addition of nitrogen. The difference in the magnitude of the improvement with different nitrogen contents was, however, not significant for the present alloy. The % change in tensile elongation in 3.5% NaCl+5% H2SO4 solution with respect to air for slow strain rate test was gradually reduced with increasing nitrogen content up to 0.27%. The stress corrosion cracks observed in high-nitrogen CD4MCU alloy was confined within the ferritic phases and not fully developed to the sharp cracks due to the globular shaped austenitic phases.

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This study aimed at improving hardness by tempering of VC/carbon steel surface-alloyed material fabricated by high-energy electron-beam irradiation. The mixture of VC powders and flux (50%MgO-50%CaO) was placed on a plain carbon steel substrate, and then electron beam was irradiated on this mixture using an electron beam accelerator. The surface-alloyed layer of 1.8 mm in thickness was homogeneously formed without defects. The microstructural analysis indicated that VC particles were coarsely formed along solidification cell boundaries, and the matrix inside cells was mostly composed of lath-type martensite and fine cuboidal VC particles. A large amount (10.1 vol.%) of these VC particles in the lath-type martensitic matrix improved hardness four times greater than that of the substrate. When the VC/steel surface alloyed material was tempered, a number of fine VC particles were precipitated in, the tempered martensitic matrix, thereby leading to the additional hardness improvement. Therefore, the hardness of the surface-alloyed layer was further enhanced by tempering, while the residual stress in the layer could be removed and the fracture toughness could be increased.

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Devitrified amorphous alloy nanocomposites with nanoscale precipitates are regarded as new prospective structural materials with superior mechanical properties such as high strength and high shear resistance. Several different models for the strengthening mechanism in partially devitrified amorphous nanocomposite materials have been proposed; shear resistance model, matrix solute enrichment model and phase mixture model. However, the strengthening mechanism of the devitrified amorphous nanocomposites has not been clearly elucidated. In order to analyse the hardening behaviour of the partially devitrified amorphous alloys, the change in solute concentration of Zr-based alloys during the devitrification process by heat treatment was investigated. It was found that the solute concentration of the remaining amorphous matrix increases during the devitrification when the solute concentration of the initial amorphous alloy is higher than that of the precipitate. On the other hand, the solute is diluted in the remaining amorphous matrix during the devitrification when the solute concentration of the initial amorphous alloy is lower than that of the precipitate. Investigating this solute concentration change and the associated mechanical properties shows that it is the phase mixtures model, and not the solute concentration model, that provides an adequate description of the devitrified amorphous nanocomposites.

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The present study was concerned with the effects of reinforcements on damping capacity in cast Al-Si composites. Bending vibration tests were conducted on A356 alloy, Al-SiCp composites, and Al-SiCp-C composites using impulse-frequency response techniques, and their damping capacity and elastic modulus were evaluated. The test results indicated that the damping capacity was increased in the order of the A356 alloy, Al-SiCp composites, and Al-SiCp-C composites. Particularly in the Al-SiCp-C composites, the addition of 4 vol.% graphites was very effective on the enhancement of damping capacity, while SiC particulates contributed to the improvement of hardness and strength. The elastic moduli calculated from the Euler-Bernoulli beam theory for the A356 alloy and the composites were matched well with the measured moduli within the error range of 3~7%.

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Titanium diboride and zirconium diboride powders were synthesized with TiO2(ZrO2)-B2O3-Mg by employing a magnesium thermite technique. In place of B2O3, H3BO3 was also tried as a source of boron with limited success. Removing the by-product MgO by acid leaching, metal borides of submicron size with basically spherical shape could be obtained. In the case of TiB2, a small amount of TiN was formed besides MgO during the synthesis reaction due to high temperature exposure of the reactants to air. Though ZrB2 could be similarly synthesized as TiB2 using the same technique, it was difficult to remove residual ZrO2 even after acid leaching. The presence of ZrN was not as apparent as that of TiN. The spark plasma sintering method was used to sinter the synthesized TiB2 without using any additives. At temperatures of 1600℃ and higher, relative density greater than 98% was obtained. The maximum Vickers hardness of the sintered body was 3360(32.9 GPa). Two sets of commercial powders with particle size of 3 and 16 μm respectively were tested at 2000℃ for comparison but they showed relative density less than 80%. The surface conditions of the sintered commercial powders were unfit for the hardness test.

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Microstructure and mechanical properties of TiAlN thin films, deposited on quenched and tempered STD61 tool steel by Arc Ion Plating, were studied using XRD, XPS, hardness, adhesion and wear tester. From the results of XPS analysis, chemical binding energy of Ti, Al and N was proportional to Al contents. The TiAlN film was grown with the (200) orientation. As the Al content in the thin film increased, the hardness increased and the grain refined, but the adhesion of thin film decreased because of the increase of compression stress due to excess of limit Al content. The wear resistance of TiAlN film increased with increasing of Al contents. But, when the Al contents exceed, the thin film did not tolerate the continuous wear load and then the fracture occured because of the high internal stress of the thin film.

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