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The effects of Mn addition and homogenization heat treatment on the tensile properties in as-extruded Al-Mg-Si-Mn alloys have been studied to develop a new Mn bearing alloy without heat treatment after the extrusion process. When the ingots were hot extruded at 450℃ with constant extrusion ratio(9:1) and speed (10 ㎜/sec), the optimum condition for extrusion press effect, which results in improvement of strength due to the formation of texture, was found to be the largest. Three kinds of homogenization heat treatment were conducted to determine the optimum condition for the extrusion process. Adding manganese to Al-Mg-Si based alloy in the range of 0.2wt.%∼1.0wt.%, Mn-dispersoids composed of Al, Mn and Si was formed. Though the size and the density of Mn-dispersoid were somewhat reduced after extrusion process, as the content of Mn was increased, the density of Mn-dispersoid was increased. Also homogenization heat treatment affected the size and the distribution of Mn-dispersoid before and after extrusion process. The effect of Mn addition on the tensile properties was small in peak aged condition of 1.0wt.%Mn bearing alloy but the properties were improved by the combination of the addition of Mn and the homogenization heat treatment in as-extruded state.

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Shape recovery stress of thermo-mechanically treated Ti-Ni and Ti-Ni-Cu shape memory alloys has been investigated by means of thermal cycling test under constant load and also and effect of thermo-mechanical treatment on transformation behaviour and transformation temperatures has been examined by means of electrical resistivity measurements, differential scanning calorimetry and X-ray diffraction. Transformation behaviour was not changed by thermo-mechanical treatments in Ti-Ni-Cu alloys, while it changed from B2(cubic)-B19`(monoclinic) to B2-R(rhombohedral)-B19` in the Ti-Ni alloy. Martensitic transformation start temperature increased silghtly by thermo-mechanical treatments in Ti-Ni-Cu alloys, while it decreased largely in the Ti-Ni alloy. Shape recovery stress of Ti-Ni-Cu alloys was larger than that of the Ti-Ni alloy. This was attributed to the fact that the maximum recoverable elongation of Ti-Ni-Cu alloys was larger than that of the Ti-Ni alloy. And also shape recovery stress depended of Cu-content. It increased from 290MPa to 400MPa with increasing Cu-content up to 5at%, and then decreased from 400MPa to 375MPa with increasing Cu-content from 5at% to 10at%.

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Formation of retained austenite and tensile properties of austempered Fe-Mn-Si-P steel after intercritically annealing the initially normalized specimen have been investigated. Two type of retained austenite, film and granular, were observed. The retained austenite of film type existing between the bainitic ferrite lath increased with increase of intercritical annealing temperature. The isolated granular type increased with increase of austempering temperature. With decrease of intercritical annealing temperature and increase of time, retained austenite and carbon contents increased, contents of retained austenite and carbon were shown with increasing austempering time. Due to the removal of martensite and increment of retained austenite contents, tensile strength decreased and elongation increased with increasing austempering time up to 5 min. The TS×El value increased with increase of austempering time and retained austenite concents. The maximum TS×El value of TS×El 3000 above obtained at the retained austenite of 12% and the austempering temperature of 400℃

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The changes in the microstructure, mechanical properties and DO₃↔ B2 transition temperature were investigated for Fe-25at.%Al alloys when Cr, Mo. Nb and B were added. As the amount of B addition increased, the grain size became smaller, the grain boundaries appeares clearer, and the amount of second phase precipitates increased. The DO₃↔ B2 transition temperature increased by about 70℃ with the addition of 2at.%oMo. Mechanical properties such as hardness(Rockwell & Microvickers) and yield strength were degraded by adding 6at.%Cr. The strength was increased by precipitate in the grains and along the grain boundaries with the addition of Nb. The Fe-25at.%Al alloy showed brittle cleavage fracture surface in the charpy impact test specimen. The addition of 6at.%Cr changed fracture mode from brittle cleavage fracture to transgranular cleavage fracture. When B was added, fracture occurred in transgranular dimple fracture mode. However, as the amount of Nb addition increased, crack propagated along the grain boundaries where brittle precipitates formed, showing intergranular fracture.

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Isothermal forgings of mill-annealed, unidirectionally-rolled, and cross-rolled Ti-6Al-4V alloys were performed at temperatures of 830, 880 and 925℃ with the true strain rates of 1×10^(-3)/sec and 5×10^(-3)/sec. Flow stresses and microstructural evolution of the specimens during the forging were characterized. The flow behavior and microstructural changes during forging have been found to depend on initial microstructure of the specimens and forging conditions. Dynamic recrystallization and strain softening occurred in unidirectionally-rolled specimens, which had elongated α grains. On the other hand, mill-annealed and cross-rolled specimens, of which initial microstructure do not have elongated α grains, did not show distinct dynamic recrystallization under the same forging conditions. It was also shown that the Zener-Hollomon parameter is a good factor to characterize the steady-state stresses and the final grain size of the unidirectionally-rolled specimens during/after the forging under the given conditions.

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Austenitic stainless steel was melted by air induction melting(AIM) method. Cast structure and amount of nonmetallic inclusion were observed. Ductility at room temperature and hot workability at high temperature were investigated. Air induction melted stainless steel ingot was remelted by electroslag remelting(ESR) process. Slag was composed of CaF₂, CaO and Al₂O₃. ESR ingot exhibited a defect free and unidirectionally grown columnar structure with a good surface finish. The amount of sulphide type inclusion was significantly reduced during ESR. Because of directional solidification structure and reduction of inclusion amount, mechanical properties such as ductility and hot workability of ESR treated ingot were much improved from those of AIM ingot.

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In order to investigate the effect of particle size on gaseous reduction rate of iron ore, two kinds of iron oxide pellets of different diameters were reduced with CO-CO₂ gas mixture. Results were analyzed using two models, the intermediate model and the unreacted-core model. From the analysis of the former model, the effective diffusivity through product layer, D_s, the effective diffusivity through reactant layer, D_(so), and volume-based chemical reaction rate constant k_v, were not changed with particle diameter while Thiele`s modulus φ and effectiveness factor, E₁ were changed. From that of the latter model, the effective diffusivity through product layer, D_s was not varied with particle diameter, while the interface chemical reaction rate constant, k_c was varied. Investigation revealed that particle size affected a gas-solid reaction modules and that this affected gaseous reduction rate.

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The limiting current type oxygen sensor is known for excellent characteristics at the low temperatures and throughout the wide oxygen concentration range. In this study, 3 ㏖% Y₂O₃ doped poly-crystal and 15 ㏖% Y₂O₃ doped single-crystal zirconia were used as the solid electrolyte of limiting current type oxygen sensors. The limiting current of the sensor using polycrystalline YSZ was observed over the oxygen concentration range of up to 62.5% and at a temperature range of 560℃∼720℃ in O₂-N₂ mixtures. The sensor using single crystal-YSZ had lower operating voltage than the sensor with polycrystalline YSZ probably due to its low electrolytic resistance.

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Effects of shot peening(SP) on the intergranular and pitting corrosion behavior of AISI 316 type stainless steels(SS) have been studied by using the electrochemical techniques. Samples were solutionized for an hour at 1050℃ and sensitized for five hours at 650℃ in Ar gas atmosphere. Subsequently shot-peening treatments were carried out on the heat-treated surface of AISI 316SS for 1, 2, 4, and 10 minutes using the steel balls of Φ 0.5 ㎜. The structure and corrosion morphology of samples were investigated using TEM, SEM and XRD. The Cr-carbide(Cr_(23)C_6) peaks were not observed from X-ray diffraction pattern studies as SP treatment time increased. Electrochemical measurements showed that, in the case of solution treated samples, the corrosion and pitting potential decreased for SP treated SS, whereas reactivation, passivation and active current density increased as the SP time increased. For the samples with sensitization treatment pitting potential increased as the SP time increased from 1 min to 4 min, but reactivation current density decreased with SP time due to the SP induced removal of Cr-carbide(Cr_(23)C_6) from the grain boundaries. The corrosion morphology of shot-peened SS after sensitization showed the appearance of reduced intergranular corrosion, due to the removal of chromium carbide(Cr_(23)C_6) as SP time increased.

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The corrosion rate of amorphous Fe-8Cr-13P-7B alloy is significantly low in comparison with that of Fe-8Cr-20B alloy, showing that the addition of phosphorus to the amorphous Fe-8Cr alloy is quite effective in increasing the corrosion resistance. Elemental phosphorus was accumulated on the alloy surface during the initial active dissolution. The formation of elemental phosphorus layer accelerates the cathodic reaction of hydrogen evolution and oxygen reduction and also suppresses the active current density. This leads to sharp ennoblement in the corrosion potential of Fe-8Cr-13P-7B alloy from the active state to the passive state with immersion time, while the corrosion potential of Fe-8Cr-20B alloy is unchanged.

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