발간논문

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The activity in the thermodynamic properties of the molten alloy systems make an offer very important information for metal refining. Activities of components in Ga-Sb molten alloy system were determined by the use of atomic absorption specetrometer (AAS) which was modified for our purpose. The heat of vaporization of an element was determined as the slope of the log(A˚T) vs 1/T plot, where A˚ is the absorbance of the vapor, and T is the temperature of the sample. Thus values obtained for Ga and Sb were close to, but some lower than those in the literature. The activities of Ga and Sb in molten Ga-Sb alloys at 1100K showed negative deviations from Raoult`s law over the whole composition range. The activity coeffients of composition elements at infinte dilution solution is γ˚ = 0.22, γ˚ = 0.18 at 1100K In addition other, thermodynamic functions was calculated on the basis of the result obtained. As a rusult of this study, atomic absorption spectrometry(AAS) was effective as activity mesurement method about alloy systems that involves not only monomeric species but also polymetric species in equilibrium vapor phase.

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This paper was focused to investigate the effect of extrusion and heating rate on the reactive synthesis and the microstructure of products formed by reactive synthesis. The mixing of elemental powders was carried out by V-blender. The mixed powders were canned, sealed under vacuum, and were hot extruded at 450℃ under extrusion ratio 20 : 1. For powder extruded and cold compacted specimens, thermal analysis have been performed. In the effect of extrusion, the ignition temperature(T₁) of extruded specimens was found to be lower than that of cold compacted specimens. This may be due to breakdown of aluminium oxide films and elongation of mixed powders by hot plastic-deformation. In the effect of heating rate, both ignition temperature(T₁) and combustion temperature(T_c) was found to be increased with increasing heating rate. In particular, the ignition temperature of extruded specimens was below the melting temperature of aluminium under the heating rate of 40℃/min. Also, phase of reaction products depended upon combustion temperature. Microstructure of reaction products consists of unreacted titanium, TiAl and Ti₃Al. However, unreacted titanium was not observed after heat treatment.

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The hot deformation in the range of 900℃∼1000℃, 0.05/sec∼1.15/sec were performed to study the effect of dynamic recrystallization in the controlled rolling of Nb-microalloyed structural was steel with decreasing temperature (T), increasing strain rate(ε), and pass strain (ε₁) of 0.2 and /or 0.5. The flow stress dependence(σ) of T, ε, and strain(ε) is also exaned. The critical strain of dynamic recrystallization(ε_o) was determined from the Lnσ/Lnε vs. ε plots. The Brain refinement can be obtained by progressing the process and controlling the degree of dynamic recrystallization. By increasing the pass stain from 0.2 to 0.5 at the penultimate deformation step in this process, dynamic recrystallization could be induced.

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The superplasticity of Fe-25Cr-7Ni-3Mo, Fe-25Cr-7Ni-1Mo-2W and Fe-22Cr-5Ni-1Mo-2W duplex stainless steels was investigated by tensile test at 850℃ with initial strain rate ranged from 10^(-3)s^(-1) to 10^(-1)s^(-1). The constituent phases in the duplex stainless were found to change following a sequence of α→α+γ→α+γ+σ→γ+σ through phase transformation with increasing time at 850℃. The substitution of W for Mo in the duplex stainless steels retarded the formation of σ phase due to the lower diffusivity of W compared to Mo. The tensile elongation of Fe-25Cr-7Ni-3Mo and Fe-25Cr-7Ni-2W alloys were larger than that of Fe-22Cr-5Ni-1Mo-ZW alloy due to the larger volume fraction of fine grained σ phase without remaining untransformed coarse α grains at 850℃. Significant strain-enhanced grain growth of γ and σ phase was observed maintaining equiaxed grain shape with low dislocation density during tens e deformation. These results indicate the grain boundary sliding is dominant mechanism of superplasticity in γ/σ duplex stainless steels at 850℃.

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Shape recovery force of Ti-50.2Ni and Ti-(50-X)Ni-XCu(at%)(X=5-20) shape memory alloys was measured by means of constant temperature tensile tests. Shape recovery force is almost independent of deformation temperature when deformation was made at temperatures below martensitic transformation start temperarure, Ms. However, it decreases significantly with rising deformation temperature when deformation was made at temperatures above Ms. In Ti-50.2Ni and Ti-45Ni-5Cu alloys whose transformation behaviour is B2(cubic)-B19`(monoclinic), shape recovery force increases with increasing prestrain from 2% to 4%, while keeps almost constant in Ti-Ni-Cu alloys whose Cu-content is above 10at% in which transformation occurs from B2 to B19(orthorhombic). The maximum shape recovery force decreases with increasing Cu-content.

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The mechanisms of the liquid metal formation at Ni/B/Ni (Ni: base metal, B: insert material) joint, which could be characterized as liquid phase diffusion bonding, has been investigated. Fine boron (B) powders (under 300 mesh) were coated on the elaborately polished bonding surfaces as an insert material. The specimen packed in quartz tube under 5×10^(-3) torr vacuum was heated in a induction furnace to 1283K-1473K. It is observed that inserted material B was diffused into base metal, and that liquid metal was formed by the reaction of B and Ni. Liquid metal was formed preferentially at the grain boundaries. The state of phases was analyzed to depend on B content because of the fact that content of B on the liquid layer was higher than the solid state base metal.

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Al/AlN FGMs were fabricated by the direct nitridation of Al/AlN powder compact. The density of a compact presintered at 600℃ in argon atmosphere followed by the reaction in nitrogen atmosphere at 900℃ for 4hours was improved to 97% of theoretical density. The reaction product was consisted of a dense ceramic layer on its surface and a ceramic-Al metal mixed layer beneath the layer. The reaction layer thickness was affected by the processing variables including AlN content. Al powder size and Mg content.

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Alumina ceramics and Inconel 600 metal were bonded using conventional brazing filler metal of Ag-Cu alloy after Ti and Zr coatings onto alumina surface. Then the effect of metal coatings on the interfacial structure and metallurgical behavior at interface between the filler metal and Inconel 600 were investigated. It was found that a prior sputter coating of Ti or Zr was effecive in wetting even to the alumina surface of difficult-to-wet with Ag-Cu brazing alloy. The bonding shear strength employing Ag-Cu brazing alloy was found to be increased from zero to 12 MPa with 3 ㎛ Ti coating. Although this bonding strength was lower than that of 25 MPa for using active brazing metal of Ag-Cu-Ti, the active metal coating method prior to brazing with Ag-Cu alloy might give a benificial effect associated with a lower melting temperature of eutectic Ag-Cu alloy compared to Ag-Cu-Ti. And also it was found that the Ti coating and/or Ti segregation during brazing enhanced the separation of silver- and copper-rich liquid phaes in molten filler metal causing a coarse microstructure.

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A relation between metallurgical microstructure changes and oxidation resistance of 18%Cr ferritic stainless steels at the temperatures between 800 and 1050℃ has been studied. It was found that the phase structure changes during isothermal oxidation above Ac₁ temperature influerced on the oxidation resistance. In case of Fe-18%Cr steel, a thin layer of austenitic phase was produced at the beneath scale/metal interface due to the depletion of Cr by forming chromia layer during oxidation. It may generate compressive stresses on the protective scale, and so micro cracks which allow abnormal oxidation by forming nodular oxides would be developed. It results in decrease in oxidation resistance. The addition of stabilizing elements such as Ti or Nb produces fully ferritic structure up to 1000℃. It results in improvement of oxidation resistance.

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The effect of tempering on the growth behavior of M₂C precipitates was examined. Applied tempering temperatures were 468, 482 and 510℃, and tempering time varied from 1 to 400 hours. The value of peak hardness was HRc 55 in all temperatures and the shift of peak hardness to earlier tempering time occurred with an increase in temperature. The peak hardness was obtained, when the average length and diameter of M₂C carbides were 4∼8 ㎚ and 1.5∼2.5 ㎚, respectively. The size of M₂C carbides increased with tempering time, and the size of carbides at 510℃ was larger than that at 482 or 468℃ in all tempering time. The aspect ratio of M₂C carbides was almost 3 in all tempering conditions. The amount of precipitated austenites increased with tempering time and temperatures. Experimental studies of grow th behavior of the carbides in this steel have been made by using TEM, SEM and X-ray diffraction.

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