발간논문

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Effects of cold rolling process on the changes of crystal orientations were studied by X-ray pole figure and four circle X-ray goniometer in single crystals of Fe-1.7% Si. The crystals initially oriented for (hkl) [110] and (111) [112] maintained their initial orientation almost unchanged up to the finale cold rolling. Thus these were thought to be the stable end orientation. While the (001) [100], (110) [001], (110) [111], (110) [112] and (112) [111] orientations were rotated around [001]//ND or [110]//TD or [111]//RD axis and changed their orientation to the stable end orientations, such as (001) [110], (111) [112], (211) [111] and (121) [101].

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A wrought nickel-base superalloy aimed to be used at above 1000℃ was designed by means of a statistical method. The alloy designed and investigated was based on the Ni-Cr-W-Ti-Al system, where the strangthening was achieved by a solid solution hardening superimposed with the gamma prime precipitation hardening. The new alloy exibited both excellent creep rupture strength and better corrosion resistance, and the rupture life of the alloy had reached up to 550 hr at 1000℃ under the stress of 4㎏/㎟. The main precipitates contributing to the elevation of high temperature strength were found to be the randomly distributed coarse alpha tungsten particles together with the uniformly distributed fine gamma prime particles. In this alloy. the carbide of M_(23)C_6 type was appeared to be stable precipitate after a long time heat treatment at 900℃ whereas that of M_6C type was stable at 1000℃.

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The kinetics of gold cementation was studied by the use of rotating disk method using zinc powder. The rate constant was proportional to the square root of agitation speed as suggested by the Levich equation, Κ = 0.62 D＾(2/3) v＾(-l/6) ω＾(1/2). In the absence of initial zinc ions and above 40℃, it is considered that the overall cementation rate would be controlled by mass transfer rate through the aqueous diffusion boundary, however, below 40℃, the reaction mechanism would be changed. When zinc powder is added as a precipitant, the rate constant increases with increasing agitation until a condition of full particle suspension is roached. Thereafter, the specific rate constant is essentially independent of any further increase in agitation speed.

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A study has been made on the effects of additional elements on solidification structure, decomposition of supersaturated solid solution, and changes of microhardness in Al-Cr and Al-Cr-(Fe, Co, Ni) alloys made in flake form by the atomization-splat quenching method. Also, the variations in vickers hardness and high temperature tensile strength have been investigated in order to estimate the mechanical properties of the extruded bars. In Al-Cr-(Fe, Co, Ni) alloys, the progress of solidification was affected not only by the heat flow, but also by the intermetallic compounds acting as nucleation sites. As a result, a more homogeneous solidification structure was obtained. The crystallographic structure of intermetallic compounds in Al-Cr-Fe alloy was observed to have icosahedral symmetry. In the case of isochronal aging (1 hr) of splats, maximum microhardness was observed at 420℃ in Al-Cr-Fe, Al-Cr-Ni alloys, and at 400℃ in Al-Cr-Co alloy. The isothermal aging showed no changes of microhardness at 300℃, but a decrease was observed after 10 hr at 400℃ The isothermal aging of the extruded bars showed that Al-Cr-(Fe, Co, Ni) alloys had good thermal stability up to 350℃ and Al-Cr-Fe alloys had the most excellent tensive strength at the high temperature.

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The formation of austenite from tempered martensite during the intercritical annealing in an Fe/1.6Mn/0.7Si/0.1C dual phase steel has been investigated by optical and transmission electron microscopy. It was found that the rate of austenite formation from tempered martensite was controlled by nucleation and growth mechanism. The morphological change of matrix structure and the carbide precipitations during tempering strongly influenced the kinetics and morphogoloies of the austenite formation. It was also observed that austenites formed initially at Grain boundaries and they coalesed into gobular austenites in later stage. Growth of acicular austenites along the laths boundaries were found to be confined within prior austenite grains.

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Variations in the solidification structures and mechanical properties of Fe-Al-C cast iron were investigated using thermal analysis and melt quenching methods. The liquidus and stabel eutectic reaction temperatures were increased with the increases in aluminum contents. At the constant carbon content, the tensile strengthes were increased with increasing Al contents but the changes were small at the carbon contents over 3.5%. The maximum tensile strength of 490MPa was obtained from the specimen containing 2.64%C and 4.44%Al. Al the same tensile strength level, hardnesses were decreased with increasing Al contents. The factor that controlling the tensile strength was appeared to be the amount of primary austenite while that controlling the hardness was the amount of ferrite in the matrix.

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A Study has been made on the effects of addition of TiC, ZrC, VC, TaC, Mo2C, WC, TiN, Cr and P on the mechanical and chemical properties of Cr3C2-15% Ni cemented cardbides. The results were as follows. The morphology and grain size of chromium carbides in Cr3C2-15% Ni alloys were changed with carbon contents of the system. Addition of small amount of ZrC and TiN was effective in inhibiting carbide grain growth. Small amount of phosphorous added to the Nickel binder had effects on lowering the sintering temperature and on modifying grain size of the carbides, which in turn improves the mechanical properties of Cr3C2-15% Ni system. Oxidation and corrosion resistance of the Cr3C2-15% Ni alloy were much better than those of Wc-15% Co alloy and 18-8 stainless steel.

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