발간논문

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A study was made of the effects of microstructural factors on microfracture mechanism of five commercial work roll materials. Particular emphasis was placed on the role of hard carbides and graphites formed during roll casting process and the matrix phases. Microstructural observation, in situ fracture test, and fractographic observation were conducted on the rolls to clarify the microfracture process. From in situ observation of the high speed steel roll, hard carbides located along cell boundary provided easily intercellular fracture sites under low stress intensity factor levels. The similar fracture behavior was shown in the high chromium iron roll and the Ni-grain roll, yielding in low fracture toughnesses. In the Adamite roll and the ductile cast iron roll, on the other hand, fracture occurred along a tortuous transcellular path since carbides or graphites were well spaced and of a spherical shape, thereby leading to the enhancement of fracture properties. These findings indicated that the crack propagation path through the tough matrix was effective in improving fracture properties, according to the fact that the crack propagated along the intercellular boundaries only when it met directly microcracks formed at intercellular carbides.

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The effect of α phase precipitation on the tensile property and fatigue limit of -C type Ti-3Al-8V-6Cr-4Mo-4Zr alloy has been investigated. For the control of α phase precipitates morphology, it has been carried out aging treatments in the temperature range of 430℃ to 580℃ to the annealed or annealed and cold drawn specimens. The shape, size and distribution of α phase precipitates were changed with the annealing or aging temperature. In case of aged at 430℃ to the annealed and cold drawn specimen, a lot of fine α phase precipitates have been distributed homogeniously all over the matrix, and it has shown the highest tensile strength of 1,543㎫. However, the highest fatigue limit of 705㎫ and the highest endurance ratio of 0.552 have been obtained from the specimens that has been aged at 530℃. In this case, the fine α phases have been precipitated by a separation reaction of β→β+α. On the other hand, in the specimens annealed at 800℃ or 900℃, namely, near or above the β transus, the grain growth has been notable. The α phases were gradually precipitated along the grain boundaries as the aging temperature increased. In this study, the orientation relationship between α phase precipitates and matrix was identified as (110)_β//(11￣01)_α and [001]_β//[011￣1]_α Moreover, the most of α phase precipitates have shown the type 1 α(obeying the Burgers orientation relation).

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The effect of multi-pass hot rolling and subsequent heat treatments of Zr-2.5wt%Nb alloy on microstructures and mechanical properties was investigated in a view point of texture, impurity atoms and phase transformation using DSC, XRD and TEM The multi-pass hot rolled specimen showed high yield strength above 1000㎫ and low elongation below 3% due to strong interaction between dislocation substructures and impurity atoms of Fe and Cr during hot rolling. The stress relieve annealing induced some precipitation of Nb atoms resulting in still high strength and low ductility due to strain aging effect. However, the remarkable increase of elongation up to 20% with yield strength of 750㎫ after recrystallization heat treatment was caused by fine recrystallized grains with Zr(Cr,Fe)₂ precipitates. (Received January 30, 1997)

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ASP30 high speed steel billets showing different carbide microstructures of M₂C carbide cell and M_6C carbide cell have been prepared by the spray forming process. The variations of carbide distribution and bend fracture properties after hot forging have been investigated. Direct hot forging of spray formed materials has resulted in the formation of carbide clusters along hot working direction and bend fracture properties have been deteriorated. On the other hand, the metastable M₂C carbide has been completely decomposed to MC and M_6C carbides after high temperature exposure. The carbides are more uniformly distributed after forging and the bend properties increase significantly with decomposition treatment. The formation of M₂C carbide during solidification is preferable for obtaining a fine homogeneous carbide distribution after decomposition treatment and hot forging. The bend fracture properties of the spray formed ASP30 alloy have been found to be comparable to those of ASP30 alloy fabricated by powder metallurgy.

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To investigate the behavior of hydrogen in dual phase steels, stress relaxation test and hydrogen permeation test have been performed. The dual phase steels were heat-treated in two different ways, an intermediate air cooling and an intermediate quenching, to change martensite volume fraction and morphology. The electrochemical permeation results showed that ferrite-martensite interfaces and martensite lattices were major hydrogen trap sites. The results of stress relaxation tests were as follows: at the low initial strain, hardening was observed as the results of hydrogen-precipitate(martensite)-dislocation interactions but at the high initial strain, softening was occurred because of the growth of microvoid. Hydrogen embrittlement was completely reversible ; after removal of the hydrogen by anodic desorption at +200㎷ for 20 hours, the total elongation was the same before the hydrogen charging.

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The superior oxidation-resistance, wear-resistance and creep-resistance make Ni-aluminide intermetallic compounds have merits for high-temperature applications. Simultaneous synthesis and densification of NiAl and Ni₃Al from elemental Ni and Al powders was investigated. Combustion synthseis was carried out under the uniaxial compressive pressure and preheating of metal mould die. It was shown that both the compressive pressure and preheating of metal mould die strongly affect the density, the grain size, the hardness and the wear loss of NiAl and Ni₃Al product. A relative density of NiAl and Ni₃Al products as much as 99.8% was achived under compressive pressure of 275㎫ and preheating temperature of 600℃.

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It is well known that a newly developed Al-Zn-Mg-Mn alloy has high strength and ductility with excellent weldability. However, comparing the mechanical properties between the extruded and the rolled alloys containing 0.8wt.% Mn, the extruded alloy with fine fibrous grain structure is shown to have higher strength than that of the rolled one consisting of coarse equiaxed grains In this study, the effect of themomechanical treatment on the tensile min the rolled Al-Zn-Mg-Mn. alloy containing 0.8wt.% Mn has been studied. Even though the grain size of 30% cold rolled and recrystallized alloys(ITMT-30 alloys) was coarser than that of 60% cold rolled and recrystallized alloys(ITMT-60 alloys), it was found that the tensile properties of ITMT-30 alloys was higher. It can be explained by the hardening effect by substructure as well as grain size refinement. The final thermomechanical treatment(FTMT) of the rolled alloy improves remarkably yield strength and tensile strength with good ductility as compared with only T6 treated rolled alloy. The strength of FTMT treated alloy is compatible to the extruded Al-Zn-Mg-Mn alloy. Increase of the strength is related to the precipitation hardening and the work hardening effect by FTMT.

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Present investigation has been made to study the superplastic deformation behavior of Ti-6Al-4V alloy with respect to the variation of α/β volume fraction. Load relaxation and superplastic deformation tests have been carried out at 600℃ and 800℃, to obtain stress-strain rate curves and tensile elongations for the microstructures of 3 and 11㎛ grain sizes. The experimental results have been analyzed by the theory of inelastic deformation which consists of two mechanisms, i.e., the grain matrix deformation and the phase/grain boundary sliding. Grain matrix deformation observed at 600℃ is consistent with an iso-stress model for the deformation of fine grain microstructures (3㎛) in which the β volume fractions are varied. However, the coarse grain microstructures (11㎛) reveal the deformation behavior of mixed type between. iso-stress mode and iso-strain rate mode. Phase boundary sliding occurring at 800℃ is the most effective in 48% β structure, resulting in the largest elongation, The sliding resistance in the different types of grain boundary is increased in the order of α/β≫α/α≒ β/β.

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The anneal-induced enthalpy relaxation behavior was examined calorimetrically for the amorphous (Al_(1-x)Y_x)_(80)Ni_(20)(0.25≤X≤0.75) alloys with a ribbon shape. When the samples annealed at temperatures below T_g or T_x are heated, an excess endothermic reaction (enthalpy relaxation) occurred above the annealing temperature (T_a) followed by a broad exothermic reaction. The onset peak temperature (T_m) of the endothermic reaction increased in a continuous manner with the annealing time (t_a). The exothermic reaction is thought to be due to the annihilation of various kinds of frozen-in defects and the endothermic one to the development of topological and compositional short-range ordering among the constituent atoms. The enthalpy relaxation was easily occurred with Y content. The easiness of the structural relaxation with Y content can be well explained by the interaction between Al and Y atoms as predicted from the difference in the melting temperature of compounds corresponding their compositions in the equilibrium phase diagrams. A bulky amorphous alloy with a diameter of 2㎜ was obtained by water quenching for Al_(20)La_(50)Ni_(20) composition. The amorphous nature of bulky alloy had a relaxed structure more than that of a ribbon shape, while a n amorphous nature was nearly the same.

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The effect of the intermediate annealing and the secondary cold rolling on the evolution of the cube-texture in the high purity aluminium condenser foils was studied by X-ray texture measurements and TEM observations. The intermediate annealing increased the orientation density near {112}<111>. The variation of the orientation density during the intermediate annealing was explained by the orientation dependence of the recovery rate. The cube-orientation formed during the intermediate annealing disappeared after the secondary cold rolling. The second cold rolling increased the dislocation density. The intermediate annealing and the secondary cold rolling was effective to obtain a higher level of the cube-orientation in the annealing texture. This was interpreted by the selective growth mechanism acting on the recrystallization.

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