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An investigation has been made on the effect of nitrogen content on the hot ductility of austenitic stainless steels containing 18 wt.%Cr-8 wt.%Ni(type 304 steel) and 16.6 wt.%Cr-10.5 wt.%Ni-2 wt.%Mo (type 316 steel) in the temperature range from 900℃ to 1200℃. It was found that increasing nitrogen content in the type 316 steel decreased the hot ductility below the temperature of 1200℃ due to the decrease of stacking fault energy. In the type 304 steel containing δ-ferrite phase, hot ductility was improved with an increase in nitrogen content up to 320 ppm, as a result of decreased amount of δ-ferrite. The addition of nitrogen above 320 ppm, however, reduced the hot ductility due to the decrease in stacking fault energy.

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The creep properties of the mechanically alloyed Al-10Ti-xSi(x = 0, 2, 4) alloy was investigated in a wide range of stress (50∼310 MPa) and temperature (300∼450℃). The transition of creep mechanism has been observed depending on stress and temperature regimes, i.e. Coble creep at low stresses and temperatures and dislocation creep at high stresses and temperatures. Each creep mechanism could be well explained by using the theoretical Coble creep equation and modified semi-empirical power law dislocation creep equation taking into account of the threshold stress. The transition stress from diffusional creep to dislocation creep becomes higher with an addition of Si. This is due to the comparatively larger threshold stress with an increase of Si content, which was arisen from the increase of elastic modulus as a result of Si incorporation in the Al₃Ti particles. Thus the addition of Si enhances the creep resistance of mechanically alloyed Al-10Ti alloy.

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Two different formation mechanisms of the icosahedral quasicrystalline phase in Al-Cu-Fe alloys were suggested according to the different cooling rate regimes. During conventional easting, where the cooling rate is as moderate as 1∼10²℃/s, the icosahedral phase is formed by a peritectic reaction between the λ-Al_(13)Fe⁴ primary phase and liquid. For melt spinning, where the cooling rate is as high as 10³∼10^6℃ /s, the icosahedral phase is formed by direct nucleation and grain growth from the undercooled melt. The relative amount of the icosahedral phase increased as the cooling rate decreased in conventional casting. During melt spinning, however, the relative amount of the icosahedral phase increased with an increase of the cooling rate. For an Al_(62.5)Cu_(25.5)Fe_(12) alloy, an almost single icosahedral quasicrystalline phase was obtained by conventional casting followed by a subsequent heat treatment at 750℃ for 3 hours. As the elemental Si was added up to 5 at.%, the relative volume fraction of the icosahedral phase gradually decreased in both as-cast and heat treated (Al, Si)-Cu-Fe alloys. For the as-cast Al_(50)Cu_(20)Fe_(15)Si_(15) alloy, however, the major phase was identified as an icosahedral phase related 1/1 cubic approximant.

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A methodology to control the interfacial microstructures, while suppressing the formation of Al₄C₃ in wrought Al alloy composites reinforced with SiC, was demonstrated. Thermodynamic calculations were carried out to elucidate how one can select process parameters in terms of alloy composition and fabrication temperature to obtain intended interfaces. Experimental verifications were conducted using SEM and TEM to validate calculated results. The reaction mechanisms for forming various interfaces were identified both theoretically and experimentally. The evaluations of the interfacial bonding strengths and interfacial stability, at elevated temperatures were carried for various interface type.

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In order to improve wear resistance and oxidation resistance simultaneously, the duplex-treatment of pack cementation coating and plasma nitriding was performed on AISI H13 steel and STS 403 steel. Cyclic oxidation test at 900℃ was performed and the oxidation behaviors of the duplex-treated specimens was compared with those of plasma nitrided specimens in terms of microstructure through SEM/EDS and weight change as a function of oxidation cycles. Special attention was paid to the effect of the single element (Al only) or multi-element (Al + Cr) diffusion coating on the oxidation behavior of the duplex treated specimens layer. Based upon the results from cyclic oxidation test performed at 900℃, the specimens treated by plasma nitriding only showed a destruction of nitrided layer after 20 cycles and rapid oxidation of the substrate. While both types of duplex-treated specimens showed a much better oxidation resistance than the plasma nitrided specimen, still better oxidation resistance could be observed from the specimen treated by multi-element (Al + Cr) diffusion coating and plasma nitriding than the specimen treated by single-element (Al) diffusion coating and plasma nitriding. This could be attributed to the fact that improved oxidation resistance was provided by forming a Cr₂O₃ layer as an additional protective layer on the surface of the multi-element duplex-treated specimen.

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It was investigated the influence of process variables, such as time, temperature, pressure and current density on the hardness, microstructure and surface carbon content during plasma carburizing of a low carbon Cr-Mo steel(0.176C, 0.119Si, 1.014Cr, 0.387Mo). As the process variables increased, the effective case depth in plasma carburized steel increased up to 50% in comparison to that of gas carburizing. With increasing time, cementite formed along the austenite grain boundaries at carburized surface and thickened. Also the surface carbon content increased initially, then its rate slowed down. Owing to the enhanced carbon adsorption rate on the surface during plasma carburizing, the supersaturation of austenite with respect to carbon occurred faster than in conventional gas carburizing process. The effective case depth increased with the surface carbon content. It is considered that the formation of carburized layer of low carbon Cr-Mo steel during plasma carburizing depends on the surface carbon content so that the use of plasma would not influence the diffusion rate of carbon in steel.

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High-rate unbalanced magnetron sputtering process was employed to deposit a STS304 stainless steel film on S45C steel. The deposition of stainless steel film was carried out at a pressure of 2×10^(-3) torr, various target power densities, substrate bias voltages and distances of target-to-substrate. Deposition rate of the STS304 stainless steel films up to 1.3㎛/min could be obtained, but it decreased more rapidly by increasing target-to-substrate distance than by decreasing ion current. The results from XRD and EDS analysis showed that unbalanced magnetron sputtering of stainless steel target could successfully transfer the multi-element composition of the bulk material to the coating without any dramatic stoichiometric variation. SEM observation of STS304 film indicated that the structure of the film was a columnar structure(zone T structure) with a fine grain size. The Knoop hardness of coated STS304 film was measured to be approximately H_k800∼850 and this high value could be attributed to the fine grain size of the film and the residual compress stress on the film. The corrosion property of the film was evaluated using salt spray test and the results indicated that the corrosion property of stainless steel coated S45C steel was similar to that of the electroplated Cr coated S45C steel.

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New types of two-dimensional thermodynamic stability diagrams were presented to predict high temperature corrosion behavior of multi-component alloys in mixed oxidant environments. The new stability diagrams were constructed by computer-assisted phase equilibria calculations of the multi-component reacting system. The usefulness of the new stability diagrams is discussed with the Fe-Cr alloys in oxidizing-chloridizing environments. The new stability diagrams can provide valuable information on the internal oxidation behavior of alloys and the stability of ternary phases such as FeCr₂O₄, which cannot be obtained from the conventional stability diagrams. Using reaction paths on the new stability diagrams, more mechanistic predictions can be made for the mixed corrosion behavior of alloys.

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A phase mixture model is considered in which a mechanically alloyed, particle reinforced metal matrix composite with ultrafine microstructure is treated as a mixture of a matrix phase, a reinforcing particle phase and a boundary phase. The finite element method is employed in conjunction with a unit cell of the composite to investigate the compressive and tensile behaviour of the system. The reinforcing ceramic particles are taken to be elastic. A unified constitutive model based on dislocation density evolution is used to describe the plastic flow behaviour of the matrix; grain size effects are included. A yield criterion for porous materials, including the evolution of density, is applied to the boundary phase. The boundary phase is assumed to have the mechanical properties of a quasi-amorphous material. The effects of the volume fraction of the reinforcing particles, overall effective density, grain size of the matrix, and the density of the boundary phase on the overall mechanical properties are discussed. The calculated stress-strain curves based on the unit cell model are used to simulate an indentation test, and compared with experimental measurements.

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The problem of furnace annealing method for Silicon direct bonding is intrinsically or extrinsically generated gases like H₂O, H₂ and hydrocarbon etc., which made the unbonded area at the bonding interface. The purpose of this research is to develop a new annealing method to remove the unbonded area. Linear annealing method used a halogen lamp as heat source with hemi-ellipse reflecting mirror and induced temperature gradient over the surface of weakly bonded silicon wafers. The interface pores, frequently appeared in the furnace annealing method at all temperature ranges, were rarely found with linear annealing method. The bonding strength of linear-annealed wafers increased continuously with increasing annealing temperature.

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