발간논문

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A new Cu-Zr-Al-Be monolithic bulk amorphous alloy was developed utilizing minimal use of toxic and expensive Be. The developed alloy not only exhibits high plasticity (~7%) but also possesses a large glass forming ability (GFA) (Φ12 mm). The possible mechanisms underlying the enhancement of the glass forming ability and the plasticity exhibited by this alloy are discussed based on the dimensionless parameter γ and the structural changes. In addition, alloy design strategy for a simultaneous improvement of GFA and plasticity are proposed from the perspective of atomic packing state.

keyword : Bulk amorphous alloy, Glass forming ability, Plasticity

Strain hardening behavior of ultrafine grained (UFG) dual-phase (DP) steels via equal-channel angular pressing and subsequent intercritical annealing followed by water quenching was investigated at ambient temperature. The formation of UFG martensite islands of ~1㎛ was not confined to the prior pearlite colonies, but uniformly distributed throughout UFG ferrite matrix. Their room temperature tensile properties were examined and compared to those of coarse grained DP steels. Although uniform and total elongations were not degraded, the strengths of UFG-DP steels were much higher than that of coarse grained DP steels. In contrast to conventional UFG steels fabricated by equal-channel angular pressings, UFG-DP steels exhibited significant strain hardening rate, which was attributed to uniform distribution of island-typed martensite as well as grain refinement of each constituent phase throughout the microstructure. Such strain hardening behavior was discussed in relation to modified C-J analysis based on Swift relationship.

keyword : Equal-channel angular pressing, Ultrafine grain, Dual-phase steel, Strain hardening behavior

The fracture property improvement of Zr-based bulk amorphous alloy containing ductile crystalline particles was explained by direct observation of microfracture processes using an in situ loading stage installed inside a scanning electron microscope (SEM) chamber. Strength of the amorphous alloy containing crystalline particles were lower than that of the monolithic amorphous alloy, while the ductility was higher. According to the in situ microfracture observation, crystalline particles initiated shear bands under low stress levels, acted as blocking sites of shear band or crack propagation, and provided the stable crack growth which could be confirmed by the R-curve analysis, although they negatively affected apparent fracture toughness. This increase in fracture resistance with increasing crack length improved overall fracture properties of the alloy containing crystalline particles. The improvement could be explained by mechanisms of blocking of crack or shear band propagation, formation of multiple shear bands, crack blunting, and shear band branching.

keyword : Bulk amorphous alloy, Ductile crystalline particle, Fracture toughness, Shear band, R-curve analysis

In the present study, effects of specimen thickness and notch shape on drop weight tear test (DWTT) properties of API X70 and X80 line-pipe steels fabricated by varying hot-rolling conditions were investigated. The DWTT was conducted on the rolled steels, and the results were discussed in comparison with the Charpy V-notch (CVN) impact test and crack tip opening angle (CTOA) test data. The DWTT results indicated that the steels rolled in the single phase region had the higher upper shelf energy (USE) than the steel rolled in the two phase region because their microstructures were composed of acicular ferrites, which was similar to the CVN test results. The DWTT energy transition temperature (ETT) increased with increasing specimen thickness due to the increase in constraint state, and the ETT of the Chevron notch DWTT was slightly higher than that of pressed notch DWTT. The measured CTOA showed better correlation with the DWTT propagation energy than with the CVN or DWTT total energy as it was related with the crack propagation speed. Since the value of sin(2CTOA) was linearly proportional to the DWTT energy, it could be used as a useful parameter to estimate the CTOA.

keyword : Line-pipe steel, Drop-weight tear test, Charpy V-notch (CVN) impact test, Crack tip opening angle

The application of spray forming process to the fabrication of highly alloyed Cu-Cr materials has been considered in this study, in order to achieve a fine scale microstructure. Solidification and microstructural evolution of Cu-10Cr alloy during spray forming have been rationalized by quantitative analyses of average enthalpy and liquid fraction in the preform surface layer. The results gave an insight to establish quantitative guidelines for process optimization and to understand microstructural evolution during spray forming. The microstructure of spray formed Cu-10Cr alloy was characterized by an equiaxed cell structure in which smaller sized Cr particles were formed at Cu cell boundaries. In contrast, the ingot-cast billet had a dendritic microstructure comprising coarse primary Cr dendrites and fine Cu-Cr eutectic. The Cr particles had a good thermal stability in the Cu matrix because no significant growth occurred after exposure at elevated temperatures up to 900℃. After hot extrusion, the spray formed Cu-10Cr alloy had a good combination of strength and ductility.

keyword : Cu-Cr, Spray forming, Solidification, Enthalpy, Liquid fraction, Equiaxed microstructure, Strength

The effect of Al content on the fluidity, tensile and creep properties of Mg-xAl-Zn alloy was investigated. The molten Mg-(3~11)Al-Zn alloys of 700℃ were poured into the mould designed to estimate fluidity. Tensile and creep tests were performed for the alloys and microstructure and fractured region were observed to investigate the mechanism of creep deformation behavior. Results showed the fluidity increased with higher AI content. Creep resistance decreased while tensile strength improved with increasing AI amount. Microstructure revealed that the morphology of Mg(17)AI(12) is the main factor determining tensile and creep properties of as cast Mg-xAl-Zn alloys. Final purpose of this study is to develop the heat resistant Mg alloys to apply transportation systems such as automobile parts. Further study on the effect of heat treatment and additional alloying elements on the microstructure and properties of Mg-xAl alloys is going to be performed.

keyword : Magnesium, Fluidity, Tensile properties, Creep properties

The effects of the oxygen concentration on the hydrogen sorption properties of an NEG(non-evaporable getter) alloy, Zr(57)V(36)Fe7, were investigated. The desired level of the oxygen concentration was obtained by letting an argon gas with 2.98 vol% O2 flow through a tube furnace containing the alloy powder at 423 K for a predetermined time period. The sorption speed of the alloy decreased almost linearly with the increase of the oxygen concentration. The reason for this can be explained as follows: the activated metal surface area decreases with the increase of the oxygen concentration and the activation of the metal surface covered with oxygen requires more energy to remove it.

keyword : Non-Evaporation Getter, NEG, Hydrogen sorption speed, Activation, Oxidation

The impact loads of cavitation bubbles generated by ultrasonic vibratory device and their effects on the surface damage have been quantitatively evaluated for the 8.8Al-bronze, flame-quenched 8.8Al-bronze and SUS316 alloys. The cavitation impact loads were determined by calibrating the cavitation pulses through a steel ball drop test, and a linear relation between the cavitation pulse height and the impact load was established for each materials. The measured impact loads and frequency under the cavitation bubble collapse were correlated to the size and number of the pit formed on the surface by assuming that an individual pit is generated by a single collapse of bubble. The critical impact load Le required to form the pits (more than 5 μm) on the flame-quenched 8.8Al-bronze was 13.16N that is lower compared to those of the 8.8Al-bronze (13.33N) and SUS316 (13.9N) alloys. The very small amounts of cavitation bubbles were involved in the formation of pits, which were only 0.89~4.32%, 0.45~1.84% and 4.07~6.87% for the 8.8Al-bronze, flame quenched 8.8Al-bronze and SUS316 alloys, respectively.

keyword : Cavitation, Impact Load, Erosion, 8.8Al-bronze, SUS316, Pit

This work has metallographically examined three forged iron artifacts from the Wolsanni tomb complex constructed by Silla, and identified a pattern in the engineering sequences applied in their manufacture. The specific distribution of such phases as martensite, pearlite and ferrite consistently observed in their microstructure shows that the artifacts were manufactured in a sequence of shape-forging followed by steel-making. This is unique in that shape-forging comes before steel-making as opposed to another sequence where shape-forging comes after steel-making. The choice between these two, determined by the nature of available steel-making and iron-smelting processes, apparently played a crucial role in characterizing a local iron industry in ancient Korea. This article will compare the Wolsanni results with those of the Hwangnam Great Tomb of Silla and the Kaya tomb complex at Daeseongdong, Kimhae, and will propose that Silla and Kaya had developed a similar iron industry based on the production of low C iron and the particular steel-making by carburization. The invention of special iron artifacts named Cheoljeong must have been a necessary requirement for such unique iron technology to be successfully established.

keyword : Kyongju wolsanni tomb complex, Iron artifacts, Microstructure, Silla iron technology