발간논문

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Thermodynamic phase calculation and its experimental confirmation were made on a series of wrought Ni base superalloys. The Gibbs free energy of the constituent phases such as γ, γ`, σ, etc can be expressed by applying the Sublattice model. Minimum Gibbs free energy condition was selected as the criteria for determining the equilibrium state. The calculated phase fraction and the transition temperature were experimentally validated by comparing the calculated value with the measured one. Isothermal aging treatment followed by electrochemical extraction and quantitative X-ray diffraction was employed for experimental validation of the calculated phase fraction, and the differential thermal analysis for the phase transition temperature. The calculated values for both phase fraction and transition temperature are well matched with the measured values. The calculated phase diagrams offer lots of useful information such as solution treatment temperature, TCP(Topologically Close Packed) phase formation, γ` phase fraction and driving force for precipitation, etc.

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An investigation was performed to establish the optimum cooling and rolling conditions for producing high carbon steel wire rods for cold drawing and good mechnical properties. Especially, an attention was paid on suppressing the occurrence of proeutectoid cementite film on the austenite grain boundaries in high carbon steels during cooling since its presence causes the wire breakage during cold drawing processes or applications. Continuous cooling simulation, hot compression test, etc. were conducted on steels with different carbon content, 0.8-1.42%C. The precipitation of the proeutectoid cementite film in high carbon steels was suppressed if the deformed steels were held for over 0.5-10 sec after hot rolling and slow-cooled to the precipitation temperature of the proeutectoid cementite.

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Tensile behavior of mechanically alloyed Al-8.5Fe-1.3V-1.7Si alloy extrusions, with various grain sizes ranging from 0.1 to 0.6 ㎛, were examined within the temperature range between 25℃ and 250℃. It was found that both tensile strength and ductility gradually decreased with increasing temperature and/or decreasing strain rate. The SEM fractographic examination showed that MA/PM Al-Fe-V-Si alloy extrusions failed by dimpled rupture, regardless of test conditions. The size and depth of dimples, however, differed considerably with different testing temperatures and strain rates. TEM micrographs of deformed specimens showed that the formation of dislocation structures within the grain was extremely limited at each test temperatures, and was further restricted at elevated temperatures. The present observations suggest that the enhanced dynamic recovery is responsible for the nil work hardening and plastic instabilities between growing voids at elevated temperatures, leading to a reduced tensile ductility.

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The effect of Mg content on fatigue behavior in Al-Mg alloys for auto body applications was investigated in this study. The increase of Mg content in Al-5.5 wt% Mg and Al-6.6 wt% Mg alloys enhanced the tensile strength and elongation from 284 ㎫ and 26.4% to 321 ㎫ and 31.2%. But the Al-6.6 wt% Mg alloy showed lower fatigue strength compared to that of Al-5.5 wt% Mg alloy due to the small precipitates along grain boundaries. These small precipitates were identified as Al_8Mg_5 phase which plays an important role as fatigue crack initiation site. The fatigue limits of Al-5.5 wt% Mg and Al-6.6 wt% Mg alloys showed 0.56 and 0.44, respectively. The fatigue crack growth rate of Al-6.6 wt% Mg alloy sheets is faster than that of Al-5.5 wt% Mg alloy sheets.

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Experiments were conducted on 0.15%C steel with an initial grain size of ∼40 ㎛ for obtaining an ultra-fine grain structure and enhanced tensile properties. ECA pressing was successfully conducted at 200℃ by designing the channel angle and the arc of curvature at the outer point of intersection of two channels properly. The Vickers hardness increased significantly by a factor of 1.6 even with a single pass. Subsequently, the hardness increased slightly after second pass. The microstructure was refined after a single pass and consisted of parallel bands of elongated subgrains. These subgrains were developed to fine grains by repeating the pass. At least for 0.15%C steel, it was possible to achieve equiaxed fine grains of ∼0.5 ㎛ by applying 2 passes such that the sample was rotated by 180℃ in the second pass.

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The effects of martensite morphology on the quasi-static and dynamic deformation behavior of dual phase steels were investigated in this study. Quasi-static and dynamic torsional tests were conducted using a torsional Kolsky bar for four steel specimens, which had different martensite morphologies and volume fractions, and then the test data were compared via microstructures, tensile properties, and fracture mode. In the intermediate quenched (IQ) steel specimens, very fine fibrous martensites were well distributed in the ferrite matrix, whereas bulk martensites were mixed with ferrite grains in the step quenched (SQ) specimens. The quasi-static torsional properties were similar to tensile properties, and in IQ-specimens, fracture occurred in a ductile mode whereas cleavage fracture were predominated in SQ-specimens. Under a dynamic loading condition, the fracture mode of SQ-specimens was changed from cleavage to ductile fracture, whereas IQ-specimens had a ductile fracture mode, irrespective of loading rate. This phenomena were analyzed using a shear lag model, phase continuity, and thermal softening effect of the martensite.

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Numerical and experimental studies of equal-channel angular pressing was carried out to verify theoretical deformation models. Finite element method was used to calculate the grid deformation and the effective plastic strain field with various die shapes. When the curvature angle is zero, a homogeneous shear deformation occurs on the cross section of the specimen, which is well predicted by the theoretical models. As the curvature angle increases, the shear deformation is found to be comparatively uniform at the upper part of the specimen, and the effective plastic strain is in a good agreement with the theoretical value. However, the shear deformation at the lower part of the specimen becomes smaller, and the effective plastic strain is considerably different from the theoretical result. Model experiments using plexiglass molds and plasticine billets supported well with the results of the finite element analysis.

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To shorten the processes of MMC fabrication, a number of techniques are being studied. In this study, a new process for MMC`s fabrication was developed utilizing vacuum hot pressing and subsequent extrusion. MMC`s with pure Al and 2024Al matrices reinforced with various volume fractions of SiC particulates were produced at 400℃ by vacuum hot pressing(VHP), followed by direct extrusion with extrusion ratios of 10:1 at 480℃ at constant ram speed of 2㎜/sec. The effects of extrusion pressure and relative density according to the volume fractions of the reinforcement were compared with the resultant metallurgical microstructures. Young`s moduli of the manufactured composites were measured by the sonic resonance method, damping capacity by free decay method. Measured Young`s moduli of the composites at room temperature were compared with the predictions based on Rule of Mixtures, Halpin-Tsai and Mean-field models. Measured Young`s moduli of the composites are in good agreement with the predictions based on Mean-field model. The composites reinforced with finer particles showed higher damping capacity under the same amount of SiC reinforcement.

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P/M high speed steels containing 0∼12 wt%Co were fabricated by gas atomizing, hipping and hot working. The microstructure, carbides and mechanical properties were evaluated before and after heat treatment. The primary carbides after the heat treatment were MC and M_6C type. The volume of the carbide increases from 16 to 20% with V addition. But Co does not affect the carbide volume. The hardness and softening resistance at high temperature were increased with the Co content. The maximum bend strength and wear resistance were obtained between 5∼9 wt%Co.

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The microstructure and mechanical properties of Ti/TiN film coated on AISI 304 stainless steels have been studied. AISI 304 stainless steels containing 0.1∼1.0 wt% Ti were fabricated by using vacuum furnace and followed by solutionization treatment at 1050℃ for 1hr. The specimens were coated by Ti and TiN with 1㎛ and 2㎛ thickness by electron-beam PVD method. The microstructure and phase analysis were carried out by using XRD, WDS and SEM. Mechanical properties such as hardness (Micro-Vickers) and wear resistance were examined. Coated films showed fine columnar structure and some defects. Surface roughness increased in all specimens after TiN coating. XRD patterns showed that the TiN(111) peak was major in TiN single-layer and the other peaks were very weak, but TiN(220) and TiN(200) peaks were developed in Ti/TiN double-layer. The hardness of the coating film was higher in Ti/TiN double-layer than in TiN single-layer and not affected by the Ti content of substrate. Ti/TiN double-layer showed better wear resistance than TiN single-layer. The observed wear traces were sheared type in all coated specimens.

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