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Hot compression test was performed to examine the effect of degree of undercooling on dynamic ferrite transformation behavior. Austenite undercooling was controlled by cooling rate from austenitization temperature to deformation temperature. The degree of undercooling was determined as ΔT (=Ae_3 - Ar_3). Ae_3 temperature was calculated by using Thermo-Calc and Ar_3 temperature was measured by dilatometry. The fine grained ferrite was formed during 70% deformation just above Ar_3 temperature. It was observed that the average ferrite grain size was 2 ㎛ regardless of temperature. At this temperature the flow stress was decreased by ferrite formation during deformation. The amount of ferrite transformed dynamically was increased as ΔT was increased. While, the ferrite grain size was mainly influenced by the amount of deformation. It was also confirmed that the critical strain of dynamic ferrite at high cooling rate to deformation temperature was shifted to lower strain compared to that at low cooling rate with the aid of increasing undercooling.

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The main emphasis of the present study has been placed on understanding of the effects of applied stress and strain modes on the behavior of strain induced martensite transformation in relation to formablity and mechanical properties of TRIP-aided cold-rolled steel sheets. Two kinds of cold-rolled steel sheets were prepared, and uniaxial tensile test and plane strain tensile test were conducted. Under the plane strain condition, strain induced martensite transformation was retarded than that under uniaxial tensile condition, and the rate of martensite transformation was decreased at the same major strain. This may be inferred from the fact that the applied shear stress and shear strain on test specimen under the plane strain condition was lower than under the uniaxial tensile condition. The difference of stress or strain mode plays an important role against the change of strain induced martensite transformation behavior.

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The structure of Cu precipitates in a Cu-bearing High Strength Low Alloy steel has been characterized by using high resolution electron microscopy and with the combined use of a computer-aided image processing system. The twinned 9R and distorted intermediate structures were found in the precipitates smaller than about 10 nm in diameter. In the precipitates of about 15 nm in diameter, two kinds of structures, which are the 3R transformed from 9R and the multitwinned f.c.c. formed directly from coherent b.c.c. particle by the lattice shear deformation were observed. The f.c.c. precipitates have a Kurdjumov-Sachs orientation relationship to the matrix, and the diffuse characterization of atoms was observed within twin band with 2-3 atomic layers is observed. The distorted f.c.c. structure was found in rod shaped precipitates of about 30 nm in length.

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This paper concerns the kinetics of the self-annealing mechanism in electroplated Cu thin films. The annealing kinetics of electroplated Cu thin films is characterized by monitoring the change in the sheet resistance and X-ray diffraction intensity with the variation of film thickness and temperature. The kinetic parameters of annealing, the time exponent and the activation energy, are determined by fitting the results to the KJMA (Kolmogorov, Johnson, Mehl and Avrami) model. It is found that the film thickness has little influence on the activation energy but does change the time exponent. From subsequent analysis of the final grain structure using transmission electron microscopy, it is then concluded that the self-annealing occurs mainly by the process of the recrystallization and that annealing kinetics dependence on film thickness stems from the hindering of grain boundary migration at the Cu/substrate interface. In case the film thickness is smaller than the nuclei size or when there is not enough stored energy in the film to drive nucleation and grain growth, the self-annealing may occur by recovery.

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A theoretical model for the influence of residual stress on the out-of-plane deflection in a free-standing thick diamond films (the bowing phenomenon) is presented. The variation in residual stress with film thickness is believed to be a cause of the bowing. In this study, the stress variation is assumed to be produced by a gradual increase of substrate deformation resulting from the layer-by-layer deposition of the film. The model was developed using infinitesimal plate-bending theory, considering the two deformation modes of contraction or expansion and bending. To verify the suggested model, diamond films were fabricated on Si, Mo and W substrates of varying thicknesses using microwave plasma assisted chemical vapor deposition. The model`s predictions on bowing, based on intrinsic stress value measured by the curvature method, were in good agreement with the bowing curvature of the as-released films measured by a profilometer. This confirms that the bowing of thick films depends on the intrinsic stress variation of the film associated with gradual increase in substrate deformation. Finally, a method of eliminating bowing by depositing multi-layers with different stresses was discussed.

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Microstructural evolution of a low alloy steel tempered at various temperature in the range of 300-700℃ using induction heating method was examined. A series of microstructural evolution such as precipitation of acicular cementite with particular orientation and formation of spheroidized cementite was observed with increasing tempering temperature. Comparing to the tempered steel with salt-bath, similar tempered microstructure could be obtained at higher temperature by induction heating. Tensile stress-strain curves of the tempered steel revealed that the excellent formability of the low alloy steel would be achieved by tempering in the range of 500-700℃ with induction heating method which is commercially applicable heat-treatment process.

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Creep properties of directionally solidified Ti-46A1-1.5Mo-0.2C alloys with well aligned lamellar microstructure have been dramatically improved. The ingots were manufactured by seeding technique directional solidification. Directionally solidified Ti-46A1-1.5Mo-0.2C alloys showed excellent creep properties. However, the creep resistance decreased as the angle between lamellar orientation and loading axis increased. It is found that the control of the lamellar orientation is most important to improve the creep resistance of TiAI alloys. In the results of TEM analysis of creep specimen before and after creep test, it was found that no carbides were formed before creep test. But, after creep test, carbides were formed in γ lath, γ / γ interfaces, γ / α_2 interfaces and dislocations. In the case of creep test at 750℃, the quantity of carbides found in dislocations decreased as the load level increased. It was thought that superior thermal stability of Ti-46A1-1.5Mo-0.2C alloy suppressed the dynamic recrystallization and the coarsening of the lamellar microstructure, and the excellent creep resistance was maintained for long time due to this thermal stability.

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Valve seat inserts should have a good wear resistance because they are used in the valve train area of internal combustion engines. Currently, the most of the valve seats are being manufactured by P/M method having the three steps of sintering, Cu-infiltrating and heat treatment. In this study, the effect of Co contents on microstructures and mechanical properties have been investigated at each steps. On sintering specimens, increase of Co content results in decrease of the hardness because of decrease of the ratio of martensite by decrease of diffusion of Cr-hard particles. On Cu-infiltrating specimens, the decrease of radial crushing strength with the increase of Co is resulted from the residual pores by Kirkendall`s effect. And after heat treatment, the increase of Co results in decrease the ratio of the residual austenite of the matrix because of over-diffusion of the Cr. According to the result of the engine simulation test, matrix can not be hardened without the addition of Co-hard particles, the excess addition of Co-hard particles reduce the wear resistance by the increase of residual pores. Therefore, the appropriate amount of Co-hard particles is 6 wt%(Co 3.6 wt%).

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Plasma sprayed zirconia-base coatings are of general interest as thermal barrier coatings in diesel and gas turbine engines. However, residual pores and cracks in these coatings decrease the mechanical properties. Therefore the improvements to ZrO_2-25CeO_2-2.5Y_2O_3(CYSZ) coatings by SiC impregnation were studied. The effect of the nano-SiC sealing on the wear properties was also studied. The coatings were produced by an atmospheric plasma spraying process. The CYSZ coatings were sealed by repeated polycarbosilane (PCS) solution infiltration and pyrolysis. Through thermogravimetry analysis, the PCS precursor solution started to decompose at 100℃ and finished at 800℃. After 800℃ pyrolysis process, the SiC phase was amorphous. DSC analysis revealed that the crystallization of the amorphous silicon carbide started at about 1030℃ and finished at 1100℃. The mean grain size determined using the Sherrer and Wilson formula from the breath of the SiC(220) line was not larger than 5 nm. Under dry condition sliding wear against sintered ZrO_2-8Y_2O_3 (YSZ), weight losses were markedly decreased by SiC sealing. The sealing resulted in the wear transition, which corresponded to the change in the mechanism from a brittle fracture mode causing severe wear to a plastic deformation mode causing mild wear. The friction coefficient was also decreased by both the surface smoothing effect of the SiC sealing and the possible excess carbon of the nonstoichiometric SiC. During sliding wear of unsealed and sealed coatings, a stress induced tetragonal to monoclinic martensitic phase transformation, which deteriorates wear resistance, was not observed.

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The isothermal oxidation behaviors of plasma sprayed NiCrAlY bond coatings (Ni22Cr10Al1Y and Ni31Cr11Al 0.6Y) were evaluated. Two unique microstructures such as oxide stringers and improperly flattened zones can be observed in the atmospheric plasma sprayed bond coatings. Structure and chemical composition of oxide stringer were examined by TEM and EDS. Improperly flattened zones had a high density of open porosities. These features affected the oxidation behaviors at both transient and steady state stages. At transient`oxidation stage, NiO, Cr_2O_3 and α-Al_2O_3 were observed on the surface simultaneously. Oxide stringers and improperly flattened zones had an especially high density of NiO and Cr_2O_3, which implied that these regions had inherently lower contents of Al. During steady-state oxidation, continuous α-Al_2O_3 reduced the diffusion rate of oxygen and thus rate of isothermal oxidation became slow. As the oxidation times increased, depletion of Al, the formation of NiAl_2O_4 layer and the formation of Cr_2O_3 occurred subsequently. And the hemispherical protrusions were also detected on the surface at room temperature. These protrusions were examined carefully.

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