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The deformation behavior of DSS depends on phase composition controlled by heat treatments. With data from stress relaxation tests the destabilization of dislocation structure is studied. The effect of growth rate of the local instability and strain rate on the shape of flow curve is also discussed.

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In this study, the effect of annealing temperatures, especially without the additional stabilization treatment, on the intergranular corrosion in STS 321 stainless steel was investigated. Nitric acid immersion and Electrochemical Potentiokinetic Reactivation(EPR) tests were periormed to find out an optimum annealing temperature. Also the effect of annealing temperature in relation with the sensitization, i.e. precipitation behavior of the Ti-precipitates, was studied. According to the microscopic results, titanium nitrides formed by solidification and also sulfides were observed in the hot band. On the other hand, titanium carbides and carbosulfides were detected in the annealed hot strip. The optimum annealing temperature for improving intergranular corrosion resistance was found about 1020∼1040℃. This was related to the increase of titanium carbide precipitation during sensitization.

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Adhesion strength and microhardness of (Ti_(1-x)Al_x)N coatings made by plasma enhanced chemical vapor deposition technique were investigated and the effect of heat treatment was characterized. The critical load of the (Ti_(1-x)Al_x)N coating to substrate was higher than 50N for X<0.15 and it decreased below 10N for X>0.15. Using 0.5 ㎛ thick TiN interlayer beneath (Ti_(1-x)Al_x)N coating, the adhesion strength could be improved. The residual chlorine content in the coating degraded the Knoop microhardness from 6500 to 1500 ㎏/㎟. The heat treatment at 1000℃ increased the microhardness due to the removal of the residual chlorine and the AlN precipitation. Residual stress of the coating from the difference of thermal expansion coefficient caused crack in coated layer, which was beneficial to improve the adhesion strength. In TiN and (Ti_(1-x)Al_x)N/TiN coatings, however, crack could not be observed after heat treatment, and the adhesion strength of the films did not change either.

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HK-40 has been the predominant alloy for reformer or cracking tubes in petrochemical plants. The material degradation of HK-40 tubes in using high temperature is due to microstructural changes, especially about carbide behavior and crack formation. The structure of HK-40 is characterized by a skeletal eutectic carbide network and a huge amount of secondary carbides in an austenitic matrix. Carbides on grain boundary form continuous plate-like shape in the as-cast condition, and coalesce progressively to increase discontinuity of itself during creep. Cavities initiate at this grain boundary and cracking is caused by grain boundary sliding of carbides denuded area. The secondary carbides in matrix explicitly affect crack growth behavior and stress-rupture elongation. But the carbides precipitated in grain boundary at the first stage of heat treatment improve rupture time and strain by the grain boundary strengthening.

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The effect of Mo additions on the microstructure and the room temperature deformation behavior of directionally solidified (DS) and polycrystalline forms of two-phase TiAl alloys were systematically investigated. Tensile elongation as large as 6.5% was observed for a polycrystalline specimen with duplex structure, but it varied with the strain rate and the test environments. In case of the fully lamellar structure, however, tensile elongation lover than 1% was obtained both in polycrystalline and in DS specimens. It was found from directional solidification studies that the lamellar boundary orientation in DS ingot has been affected by the growth rate and Mo contents. In case of Ti-48.5at.%Al-0.6at.%Mo DS ingot grown at a growth rate of 300 ㎜/hr, the lamellar boundary was nearly perpendicular to the growth direction. However, the lamellar boundary orientation of Ti-47.5at.%Al-2.5at.%Mo DS ingot was nearly parallel to the growth direction when the directional solidification was performed at a growth rate of 90 ㎜/hr. These results have shown that solidification procedures of TiAl with a nearly stoichiometric composition has been affected by Mo addition and, therefore, the lamellar boundary orientation in DS ingot could be controlled by changing the growth rate and the content of Mo additions.

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In this study, dynamic deformation characteristics of three aluminum alloys, i.e., 2090 Al-Li, Weldalite 049, and 7039 Al, were investigated in order to analyze ballistic performance. The dynamic shear stress-shear strain curves were obtained by dynamic torsional tests using a torsional Kolsky bar, and then correlated with microstructure and the tendency of adiabatic shear banding. The results indicated that the dynamic shear stress-shear strain curves matched well with the microstructural observation, suggesting that the dynamic torsional test might be a powerful tool for evaluating ballistic performance of metallic materials. Measured dissipated energy during dynamic shear deformation, E_s was also found to be useful for explaining the dynamic deformation characteristics of the aluminum alloys.

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The effects of microstructure and fracture behavior on penetration performance, together with the dynamic torsional behavior using torsional Kolsky bar, of tungsten heavy alloys were investigated in the present study. Dynamic torsional tests were conducted for five alloys, one of which was fabricated by the double-cycled sintering process, and then the test data were compared via microstructures, mechanical properties, adiabatic shear banding, and fracture mode. The dynamic torsional test results indicated that in the double-sintered tungsten alloy whose tungsten particles were very coarse and irregular-shaped, cleavage fracture occurred in the center of the gage section with little shear deformation, whereas shear deformation was concentrated into the center of the gage section in the other conventional alloys. The deformation and fracture behavior of the double-sintered alloy correlated well with the observation of the impacted penetrator specimens and the in situ fracture test results, i.e., microcrack initiation at coarse tungsten particles and cleavage crack propagation through tungsten particles. These findings suggested that the cleavage fracture mode would be beneficial for the self-sharpening effect, and thus the improvement of the penetration performance of the double-sintered tungsten heavy alloy would be expected.

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Corrosion resistant Ni-Cr-Fe alloy(UNS NO6690) was prepared by vacuum induction rnelting(V.I.M). Hot workability was investigated at various temperatures and discontinuity was observed at around 1100℃. Vacuum induction melted Ni-Cr-Fe ingots were remelted by electroslag remelting(E.S.R) method. The surface of E.S.R ingots showed clean surface finish and was much influenced by flux composition. E.S.R ingots showed directionally grown grains. As melting rate decreased, the grains were elongated more. The amount and the size of nonmetallic inclusions were reduced after E.S.R. Hot workabilities of E.S.R treated ingots were much improved at various temperatures compared with those of V.I.M ingots(especially from 30 to 95% of reduction of area at 1100℃). The improvement of hot workability is attributed to the change of inclusion amount and morphology as well as cast grain size and directionality.

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The black iron oxide layers formed in the surface of two HSS rolls were investigated. The microstructural observation of the surface region of the HSS roll with more W and Mo additions indicated that microcracks were formed in the black iron oxide layers, and propagated along needle-shaped M₂C type carbides, thereby leading to the final separation of the black iron oxide layers from the roll surface. On the other hand, on the HSS roll with less W and Mo additions, the black iron oxide layers were hardly formed since they were fallen off easily from the roll surface. These results were explained by the molybdenum addition which affected the amount of M₂C carbides and the black iron oxide formation. In order to achieve the stable black iron oxide layer on the HSS roll surface, the molybdenum content should be controlled carefully.

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The influence of centerline segregation on the necking behaviour was examined. P added extra low carbon steel sheet contained centerline segregation zones, which were made in the continuous casting process and elongated in the rolling process. The centerline segregation zone was in high concentration of solution so that the deformation behaviour of centerline segregation was different from that of matrix. The difference of deformation made surface rough and accelerated necking. The necking occurred parallel to the rolling direction because of the centerline segregation zone when the P added extra low carbon steel sheets were stretched axisymmetrically.

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