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The deformation maps are constructed based on a dynamic material model for hot working of extruded Al 2024 composites reinforced with 15 vol. % SiC_p at the temperatures ranging from 320 to 520℃ and the strain rates ranging from 0.1 to 3.0/sec by torsion test. The values of strain rate sensitivity(m) of the composites were used for establishing the deformation maps. The efficiency of power dissipation, expressed as η= 2m/(2m＋1), is plotted as a function of temperature(T), pass strain(ε_i), and strain rate(ε`) to provide the optimum hot working condition for dynamic recrystallization (DRX). Also, the DRX domain of the composites has been analyzed from the relationship between the efficiency and deformed microstructure. With the increase in the deformation temperature and the decrease in the strain rate, the efficiency of the composites increased. The characteristics of these results have been investigated with the help of hot ductility measurement and Zener-Hollomon parameter(Z=ε` exp(Q/RT). It was found that the optimum hot working condition for DRX is 430∼450℃ and 0.5/sec.

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An inter-relationship between electrical conductivity and microstructure of mechanically mixed dual phase materials was investigated to clarify the conductive mechanism using insoluble systems such as Si SiO₂ and Pb elements. The microstructure of Si/Pb system showed a typical granular structure that Pb particles with a diameter of 0.2㎛ to 5㎛ were dispersed in Si matrix. On the other hand, SiO₂/pb system exhibited a percolation structure that Pb particles with non-uniform shape of sub-㎛ sizes were embedded in amorphous SiO₂ matrix. The morphologies of two mechanically mixed systems were quite different each other. This is believed to be due to the difference of plastic deformability of Si and SiO₂ elements. We also found that no considerable dissolution of Si and SiO₂ into Pb was detected. The two insoluble systems showed a positive TCR(Temperature Coefficient of Resistance) property from 4.2K to 300K and had a same conductor-insulator (semiconductor) transition composition at 15 vol.% of Pb. This can be concluded that the mechanism of electrical conductivity in these systems can be interpreted as a percolation theory. A very high electrical resistivity of these systems is attributed to the percolation structure (clusters) having many links and deadends, and introduced many defects (dislocation, void, vacancy, stacking fault) and internal stress during mechanical mixing process. These factors will interfere with the movement of conductive electrons significantly. The mechanism of electrical conductivity of these systems can be explained by Cayley-tree networks.

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New manufacturing processes, such as thermochemical and mechanochemical process, have been developed to obtain nanostructure W base composite powders using metal organic precursor as the starting materials. Nanostructure WC/Co and W/Cu powders which have homogeneous mixing state of component particles with 60㎚∼100㎚ particle size can be synthesized by the thermochemical and mechanochemical processing method. Nanostructure WC/Co powder has better sinterability than conventional submicron WC/Co powder. The combination of a thermochemical process and mechanical process is a useful method to obtain full density W-20 wt.%Cu composite alloy without addition of sintering activator by liquid phase sintering. The present study focuses on investigating the technical issues in manufacturing nanostructure W base composite powders by chemical processes.

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Nanocrystalline Al-Ti alloy powder was produced by reactive ball milling (RBM) in a hydrogen atmosphere. The microstructure of as-RBMed powder was nanocomposite type consisted of nano-sized Al and nano-sized TiH₂. Thermal analysis of as-milled powder showed that the decomposition of TiH₂ and the subsequent formation of Al₃Ti occurred at 370∼480℃. The powder was consolidated by hot extrusion at 500℃. The grain size of as-extruded specimens decreased with decreasing consolidation temperature and shortening consolidation time before hot extrusion. The grain size was about 50∼100㎚ for Al-5at.%Ti and about 100∼200㎚ for Al-10at.%Ti. The hardness of Al-5at.%Ti specimens synthesized by RBM and subsequent hot extrusion wag 25∼75% higher than that of Al-8wt.%Ti alloys produced by mechanical alloying (MA) in Ar atmosphere and hot extrusion. Room temperature and high temperature (300, 400, 500℃) tensile strengths of RBM Al-5at.%Ti alloys were superior to those of MA Al-8wt.%Ti alleys. But the ductility of RBM alloys decreases. It is considered that the deterioration in ductility of nanocomposite Al-Ti alloys was attributed to the increase of the interface area between Al and Al_3Ti and its high energy. SEM fractograph showed that fracture progressed intergranularly.

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Ti-45at%Al-1.6at%Mn intermetallic compounds were fabricated from elemental powder mixtures by the reactive sintering method using an Electro-Pressure Sintering(EPS) equipment. Effects of heating rate, holding temperature/time and cooling rate on microstructure and densification were studied. When EPS process was carried out by a high heating rate of 100℃/min, exudation of molten Al from an elemental powder mixture was observed. Such an exudation was found to be preventable by the pre-diffusion treatment below Al melting temperature or EPS process with a low heating rate of 5℃/min. When mold temperature twas held to 1100℃, porosity decreased with increasing in holding time, and a dense titanium aluminide having the porosity below 1% could be obtained at and above the holding time of 30min. Cooling rate during EPS process affected microstructure of sintered material, and a fully homogenized titanium aluminide having a fine lamellar structure was achieved when the cooling rate was lower than 50℃/min after holding at 1250℃ for 120min.

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Sintered Fe-4 wt.%Ni-1.5 wt.%Cu-0.5 wt.%Mo steels were purse plasma nitrided and the influence of nitriding treatment conditions(temperature, gas mixture, pulse ratio) on the surface layer microstructure the micropore formation behavior and the compound layer properties of the steel has been studied. The pulse ratio was varied from 0.1 to 1, and the temperature and gas ratio(N₂:H₂) were varied from 500℃ to 570℃ and 4:1, 1:1, 1:4, respectively. The number and size of micropore in the compound layer were decreased in a pulsed DC glow discharge by reducing the plasma power and the partial pressure of nitrogen gas. The cross-sectional WDX analysis showed that the penetration depth of nitrogen in sintered steed was 3 times thicker than that of bulk material.

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The fracture toughness of the thermal cycle simulates weld HAZ (heat-affected zone) of SA 508 Cl.3 RPV (reactor pressure vessel) steel was evaluated in the ductile-brittle transition region. Reference temperature(T_0), and master curve for each region in the weld HAZ were determined from the three point bending tests at low temperatures, by using the Weibull`s statistical method as described in ASTM E1921. Most specimens were conformed to validate at the test temperatures. It was shown that the new test method, which evaluates the fracture toughness in the transition region, was effectively applicable to the weld HAZ. The fracture toughness test results indicated that the coarse grained HAZ region near the weld fusion line possesses relatively good fracture toughness. In contrast, the minimum toughness value was noted at the subcritically reheated HAZ region adjacent to the base metal. The volume fraction of tempered martensite, mean sizes of grains and precipitates were quantitatively analysed as microstructural factors, a discussion on the effects of these factors on fracture toughness of the weld HAZ is presented.

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The Fe ion distribution ratio in the CaO-MgO-Fe₂O₃-FeO-SiO₂ slag was measured under the condition of equilibrium with the atmospheric air at 1873k. The effects of slag composition on the Fe ion distribution ratio were discussed and quantified. As the basicity and the MnO content in slaty increase, the Fe ion distribution ratio increases. However, The effect of iron oxide content shows to be dependant on the basicity of slag. As the iron oxide content increases, the Fe ion distribution ratio increases far the basic slag (basicity≥1.5) but decreases for acidic slag. It is revealed that the redox reaction of iron oxide in steelmaking slag is controlled by the complex anion formation reaction of iron oxide. The contents of Al₂O₃ and P₂O_5 show same effect on the Fe ion distribution ratio as SiO₂ content. The equations obtained by analyzing the effect of slag compositions influencing the Fe ion distribution ratio are as follows; ·log(Fe^(2+)/Fe^(3+)) ＝ -0.0275 F^2_1 ＋ 0.403 F₁ － 2.249 [F₁≡20N_(CaO) ＋ 7N_(MgO) ＋ 66N_(MnO)] ·log(Fe^(2+)/Fe^(3+)) ＝ -0.0258 F^2_2 ＋ 0.374 F₂ － 2.140 [F₂≡20N_(CaO) ＋ 5N_(MgO) ＋ 3N_(Fe_2O_3) ＋ 68N_(MnO)] ·log(Fe^(2+)/Fe^(3+)) ＝ -0.000649 F^2_3 ＋ 0.0212 F₃ － 0.916 [F₃≡(%CaO) ＋ 4(%MnO) ·log(Fe^(2+)/Fe^(3+)) ＝ -0.00107 F^2_4 ＋ 0.721 F₄ － 1.982 [F₄≡(%CaO) ＋ 0.38(%Fe₂O₃ ＋ %FeO) ＋ 3.2(%MnO)]

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A study has made on the effects of morphology on mechanical properties of dual phase(ferrite ＋ martensite) steels produced by combination of controlled rolling and step quenching (CRSQ). The structures obtained by step quenching are generally coarser and have inferior mechanical properties compared to those by intermediate quenching or intermediate air cooling. However, step quenching process has an advantage in fuel efficiency by omitting extra heat treatment. Austenite grain can be refined by controlled rolling prior to step quenching. Therefore, appyling controlled rolling to step quenching would improve at strength and ductility by refining microstructure and save energy by reducing process. In this new process, the strength was increased by 7%, and the ductility was improved by 16% in comparison with specimens made by step quenching alone. The optimum combination of strength and ductility was obtained by holding at 760℃ for 10min.

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