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Two Al-Fe-V-Si alloys, Al-3Fe-1V-2Si and Al-6Fe-1V-2Si (in wt.%) alloys, have been produced via melt-overflow process (a single roll strip casting process). This process offers relatively fast solidification rate and can produce near net shape products such as thin gauge strips with the fine microstructure free from segregation. This study is aimed at understanding evolution of microstructure during this process. It has been shown that the microstructure of the alloys consists of various phases: bcc phase, icosahedral phase, and hexagonal phase. All the phases are structurally related: i. e. constructed by several icosahedra and octahedra and show similar chemical compositions. The volume fraction and distribution of these phases depend on the solidification rate and the degree of undercooling. The degree of recalescence occurring during solidification also affects the final microstructure, in that there is a transformation of thermally unstable phases to the stable phases during solidification. The solidification sequences in Al-Fe-V-Si alloys are described in terms of the formation of microquasicrystalline phase and the accumulation and transformation of that phase to the metastable and stable intermetallic phases.

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Second phases in the melt-overflowed Al-Fe-V-Si alloy have been investigated, and found to be classified into three different types : e.g. icosahedral phase, hexagonal phase, and bcc α-AlFeSi. Crystallographic orientation relationships among the phases were determined by electron diffraction. All the phases have similar atomic arrangements, which are constructed by several (Fe+V) and (Al+Si) icosahedra connected through chains of (Al+Si) octahedra, and thereby providing close orientation relationships among them. Phase decomposition behavior of the second phases has been also investigated by means of hot-stage in-situ TEM technique. During heating icosahedral phase transforms to thermally more stable bcc phase, and such transformation is controlled by Si diffusion. On the other hand, transformation of the hexagonal phase to bcc phase is not controlled by diffusion.

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Tensile and fracture behaviors of double-forged and aged Ti-6Al-4V alloys, with and without solution heat treatment and/or rapid cooling, were examined in the present study. Depending on the forging temperature and heat treatment condition, the microstructures of Ti-6Al-4V alloys varied greatly, including equiaxed α+transformed β. elongated α+equiaxed α, α` or acicular α in the prior β grain, and significantly affected the tensile and fracture properties at 25 and 316℃. Tensile and fracture failure modes of Ti-6Al-4V alloys with different microstructures were discussed based on the detailed fractographic examinations.

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Fatigue crack propagation behaviors of Ti-6Al-4V alloys, produced through different thermomechanical processing routes, were examined at 25 and 316℃. The microstructure containing acicular α+β phases in the coarse prior β grain demonstrated better resistance to fatigue crack propagation (FCP) than equiaxed α+transformed β or α` structures. The crack tortuosity and crack closure in the acicular α structure were higher than those of the other microstructures, which was believed to be responsible for the improved FCP resistance. At 316℃, both equiaxed α+ transformed β and α` structures did not show any degradation in FCP resistance. For acicular α structure which showed the highest FCP resistance at room temperature, on the other hand, the crack tortuosity was greatly reduced and, resultantly, FCP resistance was reduced at 316℃.

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Low temperature tensile properties and SCC resistances of super duplex stainless steels and their weldments are investigated. Tensile strengths increase remarkably with decreasing test temperature, while elongations decrease steeply at -196℃ after showing peak or constant value down to -100℃. Owing to the low tensile deformation of weld region, elongations of welded specimen decrease in comparison to those of unwelded specimen. The welded tensile specimen is fractured through weld region at -196℃ due to the fact that the finely dispersed ferrite phase in the austenite matrix increases an opportunity to supply the crack propagation path through the brittle ferrite phase at low temperature. The stress corrosion cracking initiates preferentially at the surface ferrite phase of base metal region and propagates through ferrite phase. When the corrosion crack meets with the fibrously aligned austenite phase to the tensile direction, the ferrite phase around austenite continues to corrode. Eventually, fracture of the austenite phase begins without enduring the tensile load. The addition of Cu+W to the super duplex stainless steel deteriorates the SCC resistance in boilling MgCl₂ solution, possibly due to the increment of pits in the ferrite phase and reduction of N content in the austenite phase.

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Effects of cathodic hydrogen charging on tensile properties of Al 8090 alloy were examined as functions of charging time, strain rate, specimen orientation and aging condition. Round bar specimens were cathodically charged in 0.1N HCl solution under an applied potential of -1800mV vs. SCE for 12 and 24 hours, respectively. It was found that Al 8090 alloy showed a substantial decrease in tensile elongation with hydrogen charging, while the decrease in tensile elongation was linearly proportional to (charging time)^(1/2), suggesting that Al 8090 alloy is susceptible to hydrogen embrittlement. SEM fractographic examinations demonstrated that fracture mode was intergranular dimpled rupture, regardless of cathodic hydrogen charging. Hydrogen charging, however, appeared to reduce the plasticity around the dimples on the intergranularly fractured surface of Al 8090 alloy. T-oriented Al 8090 alloy specimens showed higher sensitivity to hydrogen embrittlement, indicating that grain boundary played an important role in hydrogen-assisted deformation (and fracture) for Al 8090 alloy. Overaged Al 8090 alloy specimen showed slightly higher sensitivity to hydrogen embrittlement than peakaged counterpart, suggesting that hydrogen charging further reduced the plasticity in overaged condition.

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Fe-20Cr-5Al(-0.06La) alloy thin strip was spontaneously strained by growth stress which was developed during isothermal high temperature oxidation. The activation energy, Q, for high temperature strain(creep) was determined from the relation between oxidation temperature and stain rate. In La free alloy, the activation energy was calculated to about 332.6 kJ/㏖. The compressive stress of about 651.9 ㎫ and tensile stress of about 10.1 ㎫ was developed in α-alumina oxide film and metal substrate respectively. The high growth stress was relaxed through the not only creep of metal substrate but also local detachment of α-alumina oxide film from the metal substrate. In La added alloy, the activation energy was calculated to about 263.6 kJ/㏖. which was seriously lower than the activation energy for La free alloy. In this case, α-alumina oxide film was not spalled or fractured. The addition of La contributed to not only the decrease of growth stress but also the stress relaxation through creep. It was concluded that easy stress relaxation process was crucial to enhance the adherence between α-alumina film and metal substrate.

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Nanocrystalline materials have been modeled as a mixture of crystallite and intercrystallite phases in order to investigate the effect of grain size on hardness of the nanocrystalline materials. The hardness has been calculated using the rule of mixtures based on the volume fractions of crystallite and intercrystallite phases. The critical grain size, d_c, concept suggested by Nieh and Wadsworth was applied for the calculation of the hardness as a function of grain size. The theoretical hardness as a function of grain size were compared with the published experimental data. The calculated hardness showed a change in gradient and a negative Hall-Petch plots which can be observed in some experimental results. The inverse H-P relationship doesn`t occur and just the gradient changes at dc when d_c>d_(peak), (d_(peak) is the grain size at the peak hardness point, eq.(9)). On the other hand, the inverse H-P relationship appears when d_c
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Oxidation behavior of 321 stainless steel was studied. After solution treatment, specimens were polished up to 1㎛ Al₂O₃ grade and then subjected to oxidation treatment in dry air of 500∼1200℃. TEM, ESCA, SEM and EDS were used to study oxidation behavior. According to the results of TEM analysis after 500℃ oxidation treatment, it was found that thin amorphous Fe oxide was formed on the surface and polycrystalline (Cr, Fe)₂O₃ was formed below the amorphous Fe oxide layer. For the specimen oxidized at 1200℃, Ti was detected at oxide island on large oxide scale of 321 stainless steel, and it was concluded that Ti has negligible effect on resistance to oxidation. Most of Ni was detected at the interface between metal and (Cr, Fe)₂O₃, and between Fe₂O₃ and (Cr, Fe)₂O₃. (Received July 21, 1998)

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The present study is concerned with γ-(Ti_(52)Al_(48))_(100-x)B_x(x=0, 0.5, 2, 5) alloys produced by mechanical milling/vacuum hot pressing using melt-extracted powders. Microstructure of as-pressed alloys exhibit dual phase equiaxed microstructure of α₂ and γ with a mean grain size of 200 ㎚. Besides α₂ and γ phase, binary and 0.5%B alloys contains Ti₂AlN and Al₂O₃ phases located along the grain boundaries and show appreciable coarsening in grain and dispersoid sizes during annealing treatment at 1300℃ for 5 h. On the other hand, 2%B and 5%B alloys contains fine boride particles within the γ grains and show minimal coarsening during annealing. Room temperature compression tests of as-pressed alloys reveal low ductility, but very high yield strength >2100 ㎫. After annealing treatment, mechanically milled alloys show much higher yield strength than conventional PM and IM processed alloys, with equivalent ductility to IM processed alloys. 5%B alloy with smaller grain size shows higher yield strength than binary alloy at room temperature. When test temperature is increased to 850℃ 5%B alloy shows much lower strength than binary alloy, indicating the deformation of fine 5%B alloy is dominated by the grain boundary sliding mechanism.

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