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Metastable liquid phase separation(MLPS) in Cu-Co-Fe ternary system was investigated using an electromagnetic levitation technique. The temperature of the metastable liquid phase separation(Tsep) was measured directly from the time-temperature profile during the cooling in levitated state. The metastable liquid miscibility boundary of the Cu-Co-Fe ternary was determined using the measured Tsep and the compositional analysis of the separated phases. Metastable liquid phase separated structures of Cu-Co-Fe system were found to be consistent with (Co, Fe)-Cu quasi binary phase diagram. In Cu-rich alloys, phase separation generally appeared as dispersed L1(Co, Fe rich phase) droplets in a L2(Cu rich phase) matrix, whereas for alloys containing Cu less than 45-47 wt%(depending on the alloy composition), L2 droplets formed in a L1 matrix. Secondary metastable liquid phase separation were observed in specimens which were cooled rapidly against a copper substrate.

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The characterization of boride such as morphology, distribution, crystal structure, and chemical composition was investigated using Ti-47Al-2Cr-4Nb alloy containing boron. The predominant boride morphology varied from a curved lacy type to a long fibrous needle (or plate) type and a blocky type consisting of bunches of hexagonal needles as the composition increases from 0.1 at.%B to 3.0 at.%B. The borides were different in distribution, chemical composition and crystal structure depending on their morphologies. The lacy boride at low B content appeared as the layered structure consisting of ordered β₂(B2), Ti₃B₄(D7_b), and TiB (B_f) Phase. The fibrous needle (or plate) type boride was determined to be TiB(B27) phase, whereas blocky boride was revealed to be TiB₂(C32) Phase well known in binary TiAl alloys containing B.

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Phase transformation, microstructrual changes and interfacial reactions of Pt/Ti thin films were investigated using X-ray diffraction, Auger electron spectroscopy, scanning electron microscopy and transmission electron microscopy. Pt/Ti thin films were deposited on SiO₂/Si or SiN_x/Si substrates by a DC magnetron sputtering system. Specimens were annealed at various temperatures in oxygen ambient or vacuum. The as-deposited Pt/Ti thin films reveal well-defined interfaces, columnar structures of Pt, and a relatively smooth surface. Extensive interdiffusion was found during annealing in oxygen ambient. In the SiO₂ substrate, oxygen was diffused into the Pt/Ti layer from the free surface as well as from the SiO₂ side. Annealing ambient, annealing temperature and substrate strongly influenced the microstructual change in Pt/Ti thin films.

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Multilayer coatings of TiN/ZrN and Ta/TaN were deposited on a high-speed tool-steel substrate by a reactive sputtering method, and their mechanical properties were evaluated in terms of processing parameters, such as the nitrogen flux, the compositional modulation wavelength, and the annealing temperature. The hardness and the adhesion strength of the TiN/ZrN multilayer coatings increased with the nitrogen flux, but excessive nitrogen flux caused a deterioration of the properties due to the formation of microcracks. A flux ratio, N₂/Ar, of 0.2 was found to be the optimum condition for the mechanical properties. The two properties of the coatings increased with the annealing temperature also, although temperatures higher than 400℃ were detrimental for the mechanical properties because of oxidation. A smaller compositional modulation wavelength resulted in both higher hardness and better adhesion, which agreed with previous observation on the effect of the coherency strains of the TiN/ZrN interface on the hardness and the adhesion. The TaN film deposited with the F_(N2)/F_(Ar) ratio of 0.4 in the reactive sputtering process exhibits the highest crystallinity, and the highest hardness and the adhesion strength of the Ta/TaN multilayers. The hardness and adhesion strength of the Ta/TaN multilayers becomes deteriorated with increasing the annealing temperature in the heat treatment right after depositing the layers. Therefore, post-annealing treatments are not desirable for the Ta/TaN multilayers from the standpoint of mechanical properties. Also the hardness of Ta/TaN multilayers increases with decreasing the compositional modulation wavelength, but the adhesion property of the layers is nearly independent of the wavelength.

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Mg alloys(AZ91) were manufactured by sand-casting and gravity-casting method, and Al alloys(ADC12) by die-casting for the application of automotive engine parts. Internal friction of each alloy was measured using the sonic resonance system by free decay method as a function of frequency and temperature. To compare with the measured internal friction value of Mg(AZ91), Al(ADC12) alloys produced in this study, we obtained die cast Mg alloys(AZ91) manufactured by Toyota, Daihatsu and GM, and Al alloys(ADC10) by Toyota. For Mg alloys, specimens produced by die casting showed higher value in internal friction than ones by sand-casting and gravity-casting, mainly due to the difference in grain size. Mg alloys produced by die-casting showed higher internal friction than Al alloys produced by the same method. Internal friction values measured at 393K for Mg alloys(AZ91) increased with increasing frequency.

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Fe-Cr-Al alloy has been used in automobiles to meet the social needs for clean exhaust gases and reduced weight for better fuel economy. Various other types of high performance Fe-Cr-Al alloy are now being used in the automobile exhaust system. This paper described the changes in the microstructures, mechanical properties and fracture behaviors for automobile exhaust system parts application. As the amount of 0.05 wt.%B or 0.5 wt.%Zr addition was added, the grain size became smaller. The results of XRD were observed α-ferrite(α-Fe) for Fe-(5∼20)wt.%Cr-(5, 6, 8)wt.%Al alloys. However, Fe-(5∼20)wt.%Cr-10 wt.% Al alloys were observed α-ferrite(α-Fe) and α₁(Fe₃Al). Rockwell hardness was increased by increasing Cr and Al amount. Yield strength was decreased by adding 0.5wt.%Zr. Regardless of Cr amount, fracture behaviors in 5 wt.%Al added alloys revealed quasi-cleavage and ductile fracture mode, and 10 wt.%Al added alloys revealed brittle fracture mode with cleavage and river pattern. However the fracture mode in 0.05 wt.%B and 0.5 wt.%Zr added alloy revealed quasi-cleavage and dimple fracture mode due to strengthening of grain boundaries, respectively.

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A series of cylindrical shaped monolithic AA6061 aluminum alloy ingots, haying 50 ㎜ diameter and 85 ㎜ length, were cast either by gravity die casting or squeeze casting under 100 ㎫ pressure. Also, AA6061/Al₂O₃composite was manufactured by melt infiltration under 100 MPa using Al₂O₃ short-fiber preform. The resulting microstructures and mechanical properties of these castings were examined in the as-cast and heat-treated conditions. The specimens were solution heat treated at 540℃ for 1hr, quenched in cold water, and finally aged in salt bath at 180℃ fur various hours. Squeeze casting produced sound casting of refined microstructure and also decreased microsegregation in the interdendritic regions due to rapid heat extration and increase in solubility of the solutes. Melt infiltration of AA6061 aluminum alloy into Al₂O₃ short-fiber preforms was successful under 100 MPa, and the matrix produced even finer microstructure, compared to monolithic squeeze cast AA6061. Maximun microvickers hardness and tensile strength values were high in the order of melt infiltrated composite, squeeze cast monolithic and gravity die cast monolithic in the as-cast condition. Also, subsequent heat treatment improved hardness and tensile strength and the maximum values were reached with shorter ageing time in the same order.

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This study is concerned with the effects of tempering temperature on wear properties and surface roughness of a high speed steel (HSS) roll manufactured by centrifugal casting method. Hot-rolling simulation test was carried out using a high-temperature wear tester capable of controlling speed, load, and temperature. The test results revealed that the peak-tampered roll specimen showed the best wear resistance because it contained a hard matrix. However, its surface roughness deteriorated as the scratching wear proceeded, thereby leading to increase in friction coefficient. In the over-tempered specimens containing a number of fine spherical carbides in the matrix, the abrasive wear occurred predominantly as fine carbides were fallen off from the worn surface, and thus the surface roughness was enhanced by more homogeneous wear of matrix and carbides. These findings suggested that over-tempering would be more desirable in order to improve wear resistance with consideration of the surface roughness of the HSS roll.

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Turbine casings used in fossil power plants are typical thick-section components serviced under high-temperature, high-pressure conditions. Hence, there components experiences fatigue damage due to thermal stress during operational transients as well as creep damage during steady operation. Cracks of turbine casing usually propagate by creep and fatigue. In this study, creep-fatigue crack growth tests were carried out using 0.5Cr-0.5Mo-0.25V casing steel taken from a retired turbine casing. The crack growth rate due to creep was characterized by the C_t parameter and that due to fatigue was characterized by ΔK according to well-known Paris Law. From the experimental results, metals for creep-fatigue crack growth behavior of 0.5Cr-0.5Mo-0.25V casting steel were investigated based on various material constitutive laws. The proposed relation between crack growth rate and fracture parameters such as (C_t)_(avg), ΔK can be used in predicting residual life of turbine casings.

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The effects of retained austenite on the formability of a 0.1C-1.5Si-1.5Mn-0.5Cu TRIP-aided cold rolled steel sheet were investigated after various heat treatments. Tensile tests and limiting dome height (LDH) tests were conducted, and the changes of retained austenite volume fractions as a function of tensile strain were measured using X-ray diffraction. The results showed plausible relationships between the formability and retained austenite parameters such as stability and initial volume fraction of retained austenite. The formability was improved with the increase of volume fraction of retained austenite. However, when the volume fraction of retained austenite was same, better formability was obtained in the specimens with high stability of retained austenite, meaning that the strain induced transformation of retained austenite to martensite could be stably progressed and resulted in the improvement of formability. Thus, the conditions of intercritical annealing and isothermal treatment in TRIP-aided cold rolled steel sheets should be established in consideration of volume fraction and stability of retained austenite. It was concluded that the effects of retained austenite on the formability of TRIP-aided cold rolled steel sheets could be clearly understood in terms of the stability and the initial volume fraction of retained austenite.

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