발간논문

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Precipitation reaction during aging treatment has been investigated for Ni base superalloy, Nimonic 80A. The Rietveld analysis of X-ray diffraction profiles obtained from the powder samples, which had been electrochemically extracted and collected on the filter paper, has been employed for the quantification of the constituent phases such as γ` and carbides. The average particle size of γ` phase could be calculated from the Rietveld analysis. As the aging time increases, the fraction of γ` phase increases and its average particle size varies proportionally to (aging time)^½, which implies that the diffusion of solute atom is the controlling step for the γ` particle growth. The fractional increase of γ` phase is mostly caused by the particle growth controlled by the diffusion of solute elements like Ti and Al. The fractional change of γ` phase has been expressed against aging time in the form of the well known Avrami equation. Both MC-type carbide and M_(23)C_6-type carbide has been observed in as-solution treated alloy as well as the aged one. While MC carbide has been dissolved into γ matrix with aging treatment progressing. M_(23)C_6 carbide has been precipitated during aging treatment. This type of carbide reaction has been explained by calculating the thermodynamic equilibrium of Nimonic 80A.

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Nickel-based single crystal superalloy shows directional coarsening which has been explained by the interaction of interface dislocation and composition in matrix. After compression test at 700, 800℃ and 900℃, γ` precipitates were observed on (001) and (010) planes through scanning and transmission electron microscopes. Strains of γmatrix and γ` precipitate were measured using XRD. (010)plane exhibited the increasing tendency of directional coarsening of γ precipitate with increasing temperature. Due to the interaction of dislocation and Cr, Co diffused into matrix channel perpendicular to stress axis. γ` precipitates were sheared by dislocations along the direction parallel to stress axis and showed faster rate of strain with increasing temperature compared to γmatrix, which was never seen at low stress state such as in creep test. So it was concluded that stress intensity and temperature affected microstructure of γ` precipitates.

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In the present study, tensile fracture behavior of Cu-(9-x)Ni-xMn-6Sn alloys was examined as functions of aging time and an amount of Mn addition, based on the detailed SEM micrographic and fractographic observations. Discontinuous precipitation colonies, consisted of equilibrium α and γ phases, were observed around the grain boundary areas, and the amount of these colonies increased with increasing Mn addition and aging time. For those alloys where the Mn addition was restricted below 1%, typical fracture mode was intergranular dimpled rupture. In Cu-6Ni-3Mn-6Sn alloys, on the other hand, first cracks occur at discontinuous precipitation colonies and, the cracks appear to coalesce through void sheeting mechanism with the presence of strong shear stress between cracks.

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The effect of Cu on the hot workability of austenitic stainless steel has been investigated by performing tensile tests and examination by scanning electron microscopy and Auger electron spectroscopy. Reduction of area decreased linearly with increasing Cu contents at deformation temperature above 1100℃. But, at the deformation temperature below 1100℃, the effect of Cu on reduction of area changed drastically at about 3.2% Cu. At lower Cu contents, reduction of area decreased in the same manner with that observed above 1100℃. However, as Cu contents exceeded about 3.2%, reduction of area rapidly decreased with increasing Cu contents. Fracture mode changed from ductile rupture to intergranular decohesion as Cu contents increased in the ductility trough. The occurrence of embrittlement would be attributed to the segregation of Cu and S at austenite grain boundary.

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Dynamic torsional behavior of tungsten heavy alloys fabricated by mechanical alloying was investigated in order to evaluate the possibility of the adiabatic shear band formation. Dynamic torsional tests were conducted for tungsten alloy specimens fabricated with different sintering temperatures after mechanical alloying, and then the test data were compared with those of a conventionally processed specimen. The refinement of tungsten particles could be obtained after mechanical alloying, although contiguity, i.e., volume fraction of the interfacial area between tungsten particles, was seriously increased, thereby leading to very low ductility and impact energy. The dynamic test results indicated that interfacial fracture between tungsten particles occurred over broad deformed areas in these specimens, expecting the low possibility of the shear band formation. However, application of partial mechanical alloying was effective in the shear band formation since a wide matrix pool acted as a site of localized deformation. These findings suggested that the idea of making matrix pools by partial mechanical alloying would be useful for improving the penetration performance of tungsten heavy alloys.

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Quality of casting is dependent on the degree of casting defecets such as shrinkage, porosity, segregation, and etc. In this study, the sensitivity analysis was carried out to investigate the effect of casting parameters on the thermal history of the casting. Prediction of microshrinkage formation within the investment cast diesel valve seal ring was also made by computer simulation using ProCAST^(TM). The calculated thermal history was utilized to get the thermal gradient, the cooling rate, and the solidification velocity of the whole casting during solidification. In sensitivity analysis, the solidification time of the casting was largely dependent upon the metal pouring temperature and the thermal conductivity of the mould. The microshrinkage formation was closely related to the lower thermal gradient region in the Stellite alloy casting. The G/√R value, called Niyama criterion, successfully predicted the distribution of microshrinkage in the actual investment casting and the critical G/√R value for microshrinkage formation in Stellite alloy casting was found as 2℃ min.^(½)/㎝ for the diesel valve seal ring.

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The glass forming ranges of the ternary Al-Mg-Y and Al-Ni-Y systems have been predicted and compared with experimental measurements. A thermodynamic method (CALPHAD method) has been used to obtain T_o curves of five bounding binary systems, Al-Ni, Ni-Y, Y-Al, Al-Ni and Ni-Y, and ternary Al-Mg-Y and Al-Ni-Y systems. Glass transition temperature T_g in these systems has been approximated by crystallization temperature T_x, which was derived from the relationship with hole formation enthalpy. Based on the magnitude between T_o and T_g, the glass forming range has been determined. The range has been calculated to be rather wide because of the upper limit concept for T_g. This approach can be used as a guide to predict the glass forming range of binary, ternary and even multicomponent systems.

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Formation behavior and characteristics of the interface which were in-situ formed between TiB₂ reactant powder and α-Ti matrix were investigated. Furthermore, the effect of a TiB growth on densification of Ti/TiB₂ mixture was also examined. Mixed powders of TiB₂ or B₄C powder with CP Ti powders were compacted and pre-sintered at 700℃ for 1 hr followed by sintering at 900, 1000. 1100, 1200. 1300℃ for 3hrs, respectively. In addition, another specimens were sintered at 1000℃ for various times in order to study the formation behavior of TiB reinforcement in-situ formed in the α-Ti matrix. TiB was formed through the different mechanisms - formation of fine TiB and formation of coarse TiB by Ostwald ripening or coalescence of fine TiB. There was no crystallographic relationship between TiB and matrix. There were voids at the interface between TiB reinforcement and α-Ti matrix because of the preferential growth of TiB without any crystallographic relationship with α-Ti matrix and the surface energy between Ti matrix and TiB reinforcements, and so densification of Ti/TiB₂ compacts was hindered by the coarse TiB growth.

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The powder metallurgical fabrication of Bi₂Te₃ thermoelectric materials using mechanical grinding process has been studied with specific interest to control the microstructure and increase the figure of merit. The figure of merit of sintered bodies of milled Bi₂Te₃ powder decreased with increasing milling time, mainly due to the increase of the electrical resistivity with milling time. It has been known that microstructure refinement by the simple milling process was not effective to improve the thermoelectric properties of Bi₂Te₃ thermoelectric materials. Bi₂Te₃ sintered bodies with addition of nano size ZrO₂ particles from 1vol.% to 4vol.% showed higher figure of merit than that without addition of nano size ZrO₂ particles. Especially, figure of merit of Bi₂Te₃ sintered bodies with addition of 1vol.% ZrO₂ increased 1.6 times compared to the value of the specimen without addition of ZrO₂. Addition of nano size second particles to thermoelectric materials by mechanical grinding process seemed to be useful method to increase figure of merit.

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The microstructure of galvanized steel in Ni containing galvanizing bath have been investigated. An Al-killed steel plate containing low Si was used as substrate for galvanizing. The size of spangle and alloyed layer were observed under an optical microscope, SEM and EDS. The spangles of galvanized steel in Zn bath appeared to be "regular", while those of the galvanized in Zn-0.1Ni bath appeared to be "minimized" with higher brightness. The alloyed layer of galvanized steel in Zn-Ni bath were thinner than the one galvanized in Zn bath. The reduction of alloyed layer by addition of Ni in Zn bath was due to the formation of Zn-Fe-Ni compounds which act as a diffusion barrier of Fe-Zn at the ζ-(η-Zn) boundary.

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