발간논문

Home > KJMM 논문 > 발간논문

In the past few years, oxide dispersion strengthened(ODS) alloys produced by mechanical alloying(MA) techniques have become increasingly interesting for structure applications in nuclear fission and fusion power plant. The mechanical properties of ferritic 17% Cr ODS alloys with and without the addition of Ti and Mp and 17％ Cr ODS were investigated and comparison with those MA-956 alloys in the temperature range of 973K to 1573K. Nano-sized oxide dispersoids in the 17Cr-3Mo-1Ti-0.25Y_(2)O_3) alloys suppressed the grain growth during annealing at high temperature and resulted in a remarkable improvement of the high temperature strength. The oxides containing Ti was the finest and showed the most uniform dispersion.

keyword : ODS, Ferritic alloys, Mechanical alloying, Nano-sized oxides, Microstructure, Tensile properties, MA- 956, Heat-resistant steel

Effects of martensite morphology and tempering on quasi-static and dynamic deformation behavior of dual phase steels were investigated in this study. Dynamic torsional tests were conducted on six steel specimens, which had different martensite morphology and tempering effect, using a torsional Kolsky bar, and then the test data were compared via microstructures, tensile properties, and fracture mode. In the intermediate annealed (IA) specimens, small martensites were well distributed in the ferrite matrix, but bulky martensites were mixed with ferrites in the step quenched (SQ) specimens. Under a dynamic loading condition, the fracture mode of SQ-specimens was changed from cleavage to ductile fracture as the tempering temperature increased, whereas IA-specimens had a ductile fracture mode, irrespective of tempering. These phenomena were analyzed using a rule of mixtures, phase continuity, and interfacial strength between martensite and ferrite.

keyword : Dual phase steel, Tempering, Dynamic deformation, Martensite, Ferrite, Step quenching, Intermediate annealing

Recently, it is proved that delayed hydride cracking(DHC) is accompanied by deformation twinning through the texture analysis of a fracture surface. Thus, in order to understand the operation of deformation twinning with regard to deformation, texture variations with strain by cross rolling were investigated using Zr-2.5％Nb alloy with (1120)<1010> texture. Deformation twinning occurred predominantly in the range of 5-15% strain by rolling. It is appeared that the amount of deformation twinning was dependent on the direction of the rolling plane. The basal poles were rotated to the direction normal to rolling plane with strain. A (0002) texture was fully reversed after 15% of strain. The reason for the change texture was identified to be the operation of (1012) and (1121) twinning systems through the analysis of inverse pole figure, mainly. It appeared that the easiness of twinning operation was affected by the relationship between the direction of major strain and the twinning shear and by the orientation and/or distribution of neighboring grains. The contribution of the deformation twinning to the amount of strain by rolling was quantitatively calculated using change in basal pole components.

keyword : Texture, Inverse pole figure, Deformation twinning, Deformation mechanism, Zr-2.5%Nb alloy

The mechanical property improvement of a strip-cast Zr-base bulk amorphous alloy was explained by clarifying fracture mechanisms. Effects of crystalline phase particles formed in the amorphous matrix during strip casting on strength, ductility, and fracture toughness were explored in detail by directly observing microfracture processes using an in situ loading stage installed inside a scanning electron microscope (SEM) chamber. The compressive and fracture toughness test results indicated that ductility, strength, and fracture toughness of the atrip-cast amorphous alloy were higher than those of the as-cast monolithic amorphous alloy, although the strip-cast alloy contained a considerable amount (4.5 vol. %) of hard, brittle crystalline particles. This improvement of mechanical properties in the strip-cast alloy could be plausibly explained by mechanisms of (1) blocking of crack propagation, (2) formation of multiple shear bands, and (3) crack deflection by crystalline particles. Such property improvement presents new applicability of the strip-cast amorphous alloy containing crystalline particles to structures and components requiring excellent fracture toughness.

keyword : Bulk amorphous alloy, Strip casting, Crystalline particle, Multiple shear band, Crack deflection

Titanium-base composites with TiC particulates were synthesized by reacting mixed elemental titanium alloy powder compacts with hydrocarbon gas such as propane gas, and sintered in a vacuum. Titanium carbide was formed as a thin layer around the surface of each titanium particle in the compact during the reaction. Various transient carbides and hydride were also formed during the reaction but completely disappeared during the high temperature vacuum sintering, leaving only titanium carbide globules. The density and structure of the sintered composites were significantly affected by the initial titanium powder size and the reaction parameters. The volume percent of the titanium carbide could be precisely controlled by adjusting the reaction temperature and time. The rate of titanium carbide formation increases with the reaction temperature but shows parabolic behavior with time. Based on the results, a model for the titanium carbide formation inside a titanium powder compact was proposed.

keyword : Titanium base composite, Titanium carbide, Propane gas, Reaction synthesis, Sintering

In this study, optimum conditions of aluminium dispersion plating on steel mesh plate for obtaining porous layer has been presented. The process parameters of those conditions have been investigated extensively and the method of making microporous structure of steel electrode has been developed. The optimum process conditions are the particle size : 50-75 μm, the current density : 15 A/dm^(2), pH : 3.5, temperature : 60℃, the agitation rate : 500-600 rpm, the particle concentration : 40-50 g/l, and the distance rate between anode and cathode : 3
keyword : Porous electrode, Powder suspension, Composite plating, Nickel sulfamate bath

Since the intermetallic compound Ni_(3)Al has attractive high-temperature properties, Ni_(3)Al thin foils could be used as lightweight, high-temperature structural materials. The floating zone (FZ) method makes it possible to fabricate the thin foils with smooth and crack-free surface by cold rolling up to 96% without any intermediate annealing steps. For the practical application of Ni_(3)Al foils, the other properties such as oxidation resistance, wear resistance and friction resistance should be considered. The 3-NiAl, which is one of the important intermetallic compound in Ni-Al systems together with γ-Ni_(3)Al, have been used as coating materials of Ni-base superalloys because of its excellent oxidation resistance. In this study, the NiAl-based coating was intended to deposit on the Ni_(3)Al foils through RF magnetron sputtering. The binary Ni-50Al coating, however, was not able to form the stable α-Al_(2)O_(3) oxide. Moreover, the rapid Al diffusion between coating layer and substrate was occurred. On the contrary, it was found that the Ni-47Al-3Ti-0.1Y coating showed superior oxidation resistance due to stable α-Al_(2)O_(3) formation and good retardation of Al diffusion from coating layer.

keyword : Ni3Al thin foil, NiA1 coating, Oxidation resistance, RF magnetron sputtering, Floating zone method

The corrosion behaviors of Zr-based alloys were investigated with a series of Zr-1.5Nb-xSn ternary alloys which had the variations of β_(Nb) content and final annealing condition. As a result of microstructural study, the _(enriched) was observed in the samples annealed at 640℃, regardless of Sn content. The volume fraction of the precipitates and the Nb concentration in the precipitates did tend to be increased with increasing the Sn content. The samples annealed at 470℃ and 570℃, where β_(Nb) precipitated, exhibited better corrosion resistance than those annealed at 640℃ where β_(Zr) precipitated. The corrosion resistance of the Zr-1.5Nb-xSn alloy system did tend to increase with increasing Sn content. This would be resulted from the reduction of Nb solubility in matrix with increasing Sn content. Thus it eventually facilitate the formation of β_(Nb) phase from the solid solution state of Nb in Zr matrix. The Nb concentration in β_(Nb) phase increased with increasing the Sn content. This would contribute to the formation of β_(Nb) from soluble Nb that existed in the solid solution of Zr matrix.

keyword : Zr, Sn content, Annealing, Zr-Nb alloy, Corrosion, Microstructure

The effect of Cu addition to the Zr-based alloy that developed for nuclear fuel cladding was investigated. The alloys were finally heat-treated at 470, 520, 580℃ for 2.5 hours. The microstructures were examined by using optical microscope. Transmission electron microscopy (TEM) equipped with energy dispersive X-ray spectroscopy (EDS) also has been used to study the composition and structure of intermetallic precipitates. The corrosion tests of the alloys having different Cu content were tested in both 360°C water and 400℃ steam environment. In addition, the oxide film characteristics were analyzed by using low-angle X-ray diffraction (XRD) and Electrochemical Impedance Spectroscopy (EIS). With the increase of Cu content, the Cu concentration in precipitates was increased. The corrosion test showed that the weight gain increased consistently with the increase of Cu content. In the effect of final annealing, the corrosion resistance decreased gradually with the increase of annealing temperature. As a result of oxide film analysis, the oxide films of the sample having good corrosion resistance had a high fraction of tetragonal ZrO₂ and dense oxide layer.

keyword : Zr, Fuel cladding, Corrosion, Final annealing temperature, Low-angle XRD, EIS, Oxide, Tetragonal ZrO2

The corrosion fatigue properties of thermally insulated pipeline were investigated in synthetic groundwater by electrochemical test, corrosion fatigue test and SEM analysis. Since the potential difference between the weldment and the base metal was small, the pipeline steel was not susceptible to galvanic corrosion. No fatigue limit was apparent for corrosion fatigue testing with remarkable reduction of fatigue lifetime. The effects of insulation and stress on the corrosion fatigue resistance of pipeline steel were studied through linear polarization resistance (LPR) and electrochemical impedance spectroscopy (EIS) during corrosion fatigue testing. The result of LPR measurement indicated that the corrosion rate was determined not by the water content of PUR foam but by the magnitude of applied stress. The better corrosion fatigue property of the insulated steel was attributable to the formation of a protective rust layer under the insulation, which reduced the acceleration of corrosion process and the propagation of fatigue crack. This was confirmed by the results of EIS measurements and SEM observation.

keyword : Corrosion fatigue, Thermally insulated pipeline, Synthetic groundwater, Insulation, EIS, Lifetime