발간논문

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Variation of damping capacity with aging time in an Fe-17%Mn alloy with different carbon contents has been studied from the kinetic viewpoint. The average of decreasing velocity of damping capacity at each aging temperature increases with increasing carbon content and reaches maximum around 0.12wt%C, and then decreases with further carbon content. This indicates that the decreasing velocity of damping capacity can be represented by an equation V_ｄ= K·A_d·C_s, where K is constant, .A_d the total interfacial area of damping sources per unit volume, and C_s the amount of carbon segregated at damping sources. The activation energy for the damping decrease process is 6 ㎉/㏖ regardless of the carbon content. Such a small activation energy suggests that carbon segregation at damping sources is caused by interaction of carbon atoms with damping sources without long range diffusion of carbon atoms through lattices.

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The present work has studied the behavior of precipitation during aging process of a Weldalite 049 alloy using high resolution electron microscopy and in-situ hot-stage microscopy. The results show that the quenched specimen is observed the site of few nm G.P.I and Ll₂type of β phase, T₁ and θ` phases are precipitated from subgrain boundary in the in-situ experimental work and the S` phase is nucleated from the Cu and Mg clusters. It has been suggested that it is necessary to characterize the microstructures of alloys and particularly the local structures such as the interface, lattice defects and lattice mismatch to control the precipitation behavior. The high resolution electron microscopy technique has a great advantage in characterizing the local structure on the atomic-scale.

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The development of microstructures in micro alloyed forging steels was controlled by applying different cooling rate after hat rolling. The water quenching led to the formation of lath and auto-tempered martensite while the air cooling resulted in bainite and M-A constituents. As the cooling rate increased, the size of M-A constituents decreases and the constituents elongated to the specific direction. Since high carbon twinned martensites in the air cooled specimen caused the brittle fracture, the air cooled specimen showed a low toughness.

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Crystalline phases and mechanical properties of sintered hydroxyapatite/Ti (HAp/Ti), processed either by mechanical milling or mechanical alloying, have been investigated. Crystalline phases of sintered HAp/Ti were changed with Ti vol%, powder processing method and sintering temperature. When sintered at 1100℃ or 1300℃, formation of calcium titanate compounds were facilitated with HAp/Ti powders fabricated by mechanical alloying, compared to powders processed by mechanical milling. HAp/Ti with 20 vol% Ti, fabricated by mechanical alloying and sintered at 1300℃ for 4 hours in vacuum, showed flexural strength of 52 MPa and K_(10) value of 1.5 MP_(a m)^½, compared to 33MPa and 0.84 Mp_(a m)^½ of the pure HAp sintered sample.

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Single crystal specimens were solidified by modified Bridgman method. The surface recrystallized single crystal specimens were prepared by shot peening followed by heat treatment. Recrystallization of single crystal superalloys was supposed to decrease its creep rupture lives, however, creep tests showed that the creep life of the recrystallized specimen was similar(or slightly longer) to that of the single crystal specimen at 982℃ 240MPa and 1050℃ 140MPa in air. The surface oxide layer of the recrystallized specimens was thicker than that of the single crystal specimens after creep rupture at 982℃, 240MPa and 1050℃, 140MPa in air. Surface oxidation was enhanced by the existence of recrystallized grain boundaries. The creep ruptured microstructures showed that the cracks were initiated at the recrystallized grain boundaries, which were perpendicular to the stress direction. The initiated cracks propagated along the recrystallized grain boundaries but the crack growth was obstructed by the surface oxide layer. In vacuum, the single crystal specimens showed longer creep rupture lives than the surface recrystallized specimens at 982℃, 240MPa. Creep at 871℃ 500MPa in air, where oxidation is less severe, was similar to the result at 982℃, 240MPa in vacuum.

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A study was made of the effects of alloying elements on the microstructure and microfracture mechanism of squeeze cast AZ91-X(X=Y, Sr, Nd) magnesium alloys. Particular emphasis was placed on the role of Y and Nd, which are important alloying elements in magnesium alloys fabricated by rapid solidified process. Microstructural observation, in situ fracture test, and fractographic observation were conducted on the alloys to clarify the microfracture process. Microstructural analyses indicated that the grain refinement could be achieved by small addition of alloying elements, although Mg_(17)Al_(12) phases were still present along grain boundary. From in situ observation of the AZ-Sr alloy, coarse needle-shaped particles and Mg_17)Al_(12) phases located along grain boundary provided easily intergranular fracture sites under low stress intensity factor levels, resulting in the drop in toughness. In the AZ-Y and the AZ-Nd alloys, on the other hand, a number of planar slip bands and twinnings were well developed inside grains, thereby leading to the enhancement of overall mechanical properties. These findings indicated that small addition of Y or Nd was very effective in improving mechanical properties on the basis of the well-developed twinnings and slip bands.

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Effect of δ-ferrite and sulphur contents on the hot ductility of Type 304 stainless steel has been studied. The hot ductility of 304 steel was improved by the presence of about 8%δ-ferrite in as mast structure. It is due to the retardation of grain growth during continuous casting or reheating and the acceleration of recrystallization at γ/δ interfaces under the high temperature deformation conditions. The hot ductility of steel containing high sulphur was decreased drastically at temperatures below 1200℃ due to the reprecipitation of (Fe, Cr, Mn)_xS_y at grain boundaries by continuous cooling during hot rolling process.

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Solderability and interfacial reaction between 43Sn-57Bi-X solders and Cu-substrate were studied. The solderability of 43Sn-57Bi-X solders on Cu substrate was examined as a function of soldering temperature, soldering time and types of flux. Cu/solder joints were subjected to aging treatments for up to 60days to see interfacial reaction at 100℃ and then examined changes of microstructure and interfacial compound by optical microscopy and EDS. Addition of small amount of Cu, Ag and Zn, respectively, to solder result in decreased spread area on Cu substrate with RMA-flux treatment. but addtion of Ag and Zn showed increased spread area with R-flux treatment. According to the result of EDS, it was analyzed that the interfacial zone between solder and Cu-substrate was composed of Cu₃Sn and Cu_6Sn_5. But addition of Zn resulted in suppression of Cu_xSn_y, compound formation because of superior tendency to Cu_xZn_y compound formation.

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Ni was bonded by liquid phase diffusion using pure boron (B) as an insert material, and bonding mechanism and its application to other metals were studied. B powder, under 300 mesh, was coated on the polished bonding surface. Bonding specimen was packed in a quartz tube under 1.6×10^(-1) Pa vacuum atmosphere, and heated to 1433-1473K. As results, at the bonding temperature of 1433K, 1453K and 1473K, initial width of liquid layer was 87 ㎛, 81 ㎛, 80 ㎛, and maximum width of liquid layer was 138 ㎛, 146 ㎛ and 158 ㎛ respectively with the thickness of insert material of 25 ㎛. Bonding mechanism was similar to conventional TLP bonding except the formation of liquid metal on the joint and initial width of liquid layer. Other metals such as Cu, AISI304 steel, Alloy713C and Rene80 superalloy were also liquid phase diffusion bonded by using pure B.

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The dissolution behavior of nickel in iodine/iodide solutions was investigated using a rotating disc. Experimental variables studied here included pH of the solution, temperature, concentration of lixiviants, and rotating speed of the nickel disc. Nickel easily dissolved when pH of the solution was about 2. However, the dissolution rate dramatically decreased when the pH of the solution became greater than 3. The dissolution rate increased at faster rotating speed, higher initial concentration of iodine, and temperature of the solution. The overall dissolution reaction rate was found to be influenced partially by surface chemical reaction and largely by mass transfer of triiodide through diffusion boundary layer at the liquid/solid interface with an activation energy value of 17.2 kJ/㏖. The possibility of reutilization of used iodine/iodide solution by electrowinning in an electrochemical cell was investigated and the results were promising.

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