발간논문

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The precipitation and growth behaviors of γ′ phase in Mar-M247 type Ni-base Superalloy have been investigated mainly by micro-Vickers hardness test, scanning electron microscopy, transmission electron microscopy and electrical resistivity measurements. The γ′ precipitates were observed to be spherical at the early stage of aging time at all aging temperature and to be cuboidal at the later stage at 920, 970℃. The hardness of specimens aged in various conditions closely related to the size of γ′ precipitates. The growth kinetics of γ′ precipitates followed the prediction of Lifshitz-Slyozov-Wagner(L-S-W) theory of diffusion controlled growth at all aging temperatures(820, 870, 920, 970℃). The activation energy for the growth of γ′ precipitates was estimated 233kJ/㏖ which was nearly similar to those of diffusion for solute atoms(Co,Cr,Ti) in Ni₃Al and Ni-base Superalloys.

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Effects of the aging treatment on the mechanical properties of 0.5-2.0% Cu-added hot rolled steel sheets were investigated by Vickers hardness measurement, tensile test and scanning and transmission electron microscopy. The hardness of the specimen coiled at 450℃ was greater than that of 650℃. As increasing Cu contents, the hardness increased and the time to reach peak hardness decreased. It was also found that the aging at 550℃ led to the maximum hardness for all composition. The maximum tensile strength of 1.0, 1.5 and 2.0% Cu-bearing steels aged at 550℃ was 553. 679 and 766㎫ respectively. It was confirmed by TEM observation that Cu-clusters were finely dispersed throughout the grain inside. The size of Cu-clusters was about 10-20㎚ at peak-aged condition.

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The effects of alloying elements Cu and Cu+W addition, distribution and content of austenite phase on low temperature tensile and impact properites of 25%Cr-7%Ni-4%Mo-0.28%N super duplex stainless steel were investigated. The dispersed structure shows lower tensile strength and shorter elongation than fibrous structure due to the neck initiation at the austenite film precipitated at prior ferrite grain boundaries. Elongation value of fibrous structure decreased with the addition of Cu and Cu+W. With decreasing tensile deformation temperature, the maximum elongation peak was resulted in the dispersed structure due to the increase of the α′ martensite content. The level of α′ martensite content was greater in dispersed structure than in fibrous one. Low temperature impact values of the dispersed structure were greater when compared to the fibrous one due to the mutual interference of austenite and ferrite phases. Impact value decreased with the addition of Cu and Cu+W for both fibrous and dispersed structures.

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The objective of this study is to demonstrate that the white tin exhibits the behavior of mechanical equation of state for plastic deformation at room temperature, as was first suggested by Hart. The existence of the mechanical equation of state can be verified by a scaling relation of stress-strain rate curves obtained at various plastic strain levels for a particular material. Variety of metals and alloys with cubic structure have been reported to show the behavior of mechanical equation of state. In this study, an explicit form of mechanical equation of state for plastic deformation has been provided based on the internal variable theory of inelastic deformation, recently proposed and verified by the scaling of stress vs. strain rate curves obtained from a series of load relaxation tests. The principal advantage of load relaxation test is to provide a broad range of strain rates with a minimal plastic straining, so that the flow behavior can be obtained without changing the internal structure appreciably. The effect of temperature on the flow behavior of pure tin has also been investigated by conducting load relaxation tests at various temperatures. The activation energy for plastic deformation in pure tin at the temperature range from room temperature to 140℃ was obtained as 46.4kJ/mole, which is very similar to that for grain boundary self diffusion.

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In order to achieve good deep drawability it is imperative to obtain well developed {111}//ND texture which gives rise to the higher plastic strain ratios. However, it has been found that no {111} texture developed after a normal rolling process and subseguent annealing processe. Therefore an unconventional rolling process which causes shear deformation has been introduced for 99.99 pct aluminum and Al-1.2Mn-0.4Mg alloy. The effect of initial texture and microstructure on the shear texture has been studied. It is found that the rolling texture using asymmetric rolls could be characterized by α fibre and γ fibre. This experimental results were able to be explained by the Taylor-Bishop-Hill theory. The evolution of recrystallization texture from the specimen having the shear texture components was well predicted using the maximum energy release model suggested by Lee.

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The cold forgeability and carburization properties of new grade steel(0.22C-0.10Si-0.41Mn-1.0Cr-0.0022B) was compared with those of commercial boron steel and alloy steel. The reduction of Mn, Si content in new grade steel resulted in the improvement of the critical compression limit(82.3%, ε=1.73). The depth of internal oxidation layer in new grade steel(15㎛) was smaller than that of commercial boron steel(20㎛) and the surface hardness by case hardening was higher(735 Hv) compared to commercia) boron steel(633 Hv) with the same case depth. Hardenability of a new grade steel was almost the same as that of commercial boron steel while cold forgeability and carburization properties was superior to those of the commercial boron steel and alloy steel.

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The aim of the present investigation was to develop a new titanium alloy with improved superplastic performance (higher elongation and lower forming temperature) through alloying with Fe, Ni and Al. A total of nine alloy compositions were used in which Fe content was fixed while Ni and A1 contents were varied. The alloy Ti-5Fe-4Al-2Ni was proved to be promising in that it gives a large elongation over 1500% at a low deformation temperature of 650℃, which was made possible mainly due to the existence of Fe and Ni. Alloying with Ni within suitable amount was found to increase the strain rate sensitivity and to decrease the flow stress, while the effect of Al addition was found to be insignificant to the superplastic behavior but its content should be limited. Possible mechanisms and optimum volume fractions for the superplasticity in the present alloy system were also discussed.

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A numerical model has been suggested to predict and analyze the shape of a growing billet produced by spray casting process using the scanning gas atomizer. It is important to understand the mechanism of billet growth because a billet with a desired final shape can be obtained by optimum combination of several process parameters. The shape of a growing billet has been determined by the flow rate of alloy melt, spray mass distribution, rotation and withdrawal speed of the substrate, and scanning motion of the gas atomizer. Scanning motion has been controlled by the profile of cam which determines scanning angle and scanning speed of the gas atomizer. The effects of the most dominant process parameters, such a s withdrawal speed of the substrate and the cam profile, on the shape of the growing billet have been discussed. This numerical model can also serve as a basis for heat transfer analysis of the growing billet.

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Effects of the addition of Cr, Mo, Nb, and B to Fe-25at%Al intermetallic compound on the intergranular corrosion behavior were investigated using potentiostat. The specimens were casted by the vacuum arc melting. The subsequent homogenization and the stabilization led to the homogeneous DO₃ structure of the specimen. After the corrosion tests, the surface of the tested specimens were observed by the optical microscopy and scanning electron microscopy(SEM). For Fe-25at%Al intemetallic compound, the addition of Cr and Mo proved beneficial to decrease the grain boundary attack by decreasing activation current density. Addition of B and Nb resulted in a higher activation current density and also a higher passive current density. These results indicated the role of Cr and Mo in improving the intergranular corrosion resistance of Fe-25at%Al intermetallic compound. Boron and Nb addition to Fe-25at%Al accelerated the granular and also the intergranular corrosion. Fe-25at%Al containing Cr and Mo showed remarkably improved intergranular corrosion resistance in comparison with Born and Nb addition to Fe-25at%Al.

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This paper was focused on the extrusion process of SiC_p/2024 Al composites. Aluminum powders mixed with SiC particles of 10, 20 Vol.% were degassed at 450℃ during 90min. Powder-extruded composites was extruded at 500℃ under the extrusion ratio of 25. The billet-extruded composites was extruded at 500℃ after hot pressing as a reference material. It was found that the relative density of powder-extruded composites was slightly lower than that of the billet-extruded composites. However, microhardness of the powder-extruded composites was superior to billet-extruded composites. This is attributable to the maintenance of aluminum powder property caused by short thermal-exposure time during powder extrusion process. The powder-extruded composites exhibits the lower tensile strength and elongation, comparing with billet-extruded composites. This may be due to smaller extrusion pressure, which could not be sufficiently eliminated interfacial defects such as pores.

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