발간논문

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This study investigates the effect of heat treatment on the mechanical properties of a forged Nibased superalloy called Nimonic 80A. Nimonic 80A ingot samples were fabricated by vacuum spray casting to achieve a fine and homogenized microstructure. The ingot samples were subsequently hot-forged with the diameter of 220 mm at 1373 K. From the center to the surface of the forged Nimonic 80A, its average grain size decreased and its micro-Vickers hardness increased slightly. Solution treatment was carried out at 1353 K with 8 hours of air cooling followed by aging treatment, which was carried out in the range of 873- 1073 K with various times from 0.5 to 256 hours. To set the optimum aging conditions, micro-Vickers hardness tests were performed. The maximum hardness value of 388.0 Hv was obtained by aging at 973 K for 32 hours. Also, tensile tests were performed for optimum aging conditions at room temperature and 873 K. The results can be used effectively to perform reasonable heat treatment of Nimonic 80A superalloy.

keyword : nimonic 80A, aging, mechanical properties, hardness test, vacuum spray casting

9-12% Cr steels have been used in thermal power plants which repeat start and stop operations. Major factors of fatigue life are temperature, frequency, stress ratio, holding time, microstructure, and environment. Normally, fatigue life decreases at high temperature, low frequency, high stress ratio, and long holding time conditions. A Mod.9Cr-1Mo steel, called G91, was developed at ORNL (Oak Ridge National Laboratory, USA) and was adopted as a high-temperature structural material in the ASME Code in 2004. However, its low-cycle fatigue and fatigue crack growth characteristics have been rarely studied. In this work, we have investigated the low-cycle fatigue crack growth behaviors of G91 steel under various test conditions in terms of temperature and stress ratio. As temperature and stress ratio increase, the crack growth rate becomes faster and striation distance also increases. On the other hand, the number of branch cracks decreases.

keyword : fatigue crack growth, high temperature fatigue, G91, stress ratio, branch crack, striation

In this study, we prepared a sample of AISI 409L weld metals using automotive exhaust manifolds and evaluated their corrosion properties by conducting an anodic polarization test after 10 minute of heat treatment at 900℃. The specimens of AISI 409L transformed fully ferrite. Weld metal was refined more than base metal. Specimen of heat treatment at 900℃ and as weld specimen was formed precipitation. However heat treatment specimen was bulkly formed and coarser than the as weld specimen. The strength measured by 10 Hv highly at heat treatment specimens in comparison with as weld. The increase in strength is attributed to the precipitation of Ti. The result of heat treatment suggest that there was a decrease of current density and high corrosion potential. Following heat treatment process produced Ti precipitation and for this reason, it can restrain Cr-carbide so that steel will have more corrosion resistance.

keyword : metals, welding, precipitation, scanning electron microscopy (SEM), ferritic stainless steel

Hot ductility behavior of precipitation-hardened low-carbon iron alloys containing 0.02 wt% Ti and 0.05 wt% Nb was characterized by a hot tensile stress test. Carbon (0.05, 0.1, 0.25 wt%) and boron (0.002 wt%) contents were varied to study the effect of precipitates on the high-temperature embrittlement of the alloys in the temperature range of 600~800℃. Ductility loss was observed at 700℃ for the tested alloys. The cause of the ductility loss was mainly attributed to the carbides and ferrite films formed at the grain boundaries during deformation. Although the carbon content tended to raise the total fraction of Nb (C, N), the precipitates were formed mostly in the grain interior as the precipitation temperature was raised above the deformation temperature by the high carbon content. Hence, carbon in excess suppressed the hot ductility loss. Meanwhile, boron addition improved the hot ductility of the alloys. The improvement is likely due to the boron atoms capturing carbon atoms and thus retarding the carbide formation.

keyword : steel, hot ductility, precipitation, Nb, Ti alloy

Ultrafine-grained or nanostructured alloys usually lack the strain hardening capability needed to sustain uniform tensile deformation under high stresses. To circumvent this problem, we fabricated the Cubased composite reinforced with the 3-dimensionally interconnected Cu5Zr phase using the combined technique of rapid quenching and subsequent hot-rolling. The alloy exhibited a tensile ductility of ~2.5% together with a strength of 1.57 GPa, which exceeds the values of most commercially available Cu-Be alloys. In this study, we elucidated the structural origin of the high strength and tensile ductility of the developed alloy by examining the thermal stability of the Cu5Zr reinforcing phase and the energy (work) absorption capability of the Cu matrix.

keyword : eutectic structure, nanostructured alloy, Cu-Zr binary alloy, superlattice, hot-rolling

Low-carbon steels and a stainless steel were oxidized isothermally and cyclically between 1050℃ and 1200℃ for up to 100 min in air to find the effect of alloying elements of Si, Mn, Ni, and Cr on their oxidation. The most active alloying element of Si was scattered inside the oxide scale, at the scale-alloy interface and as internal oxide precipitates beneath the oxide scale. Manganese, which could not effectively improve the oxidation resistance, was rather uniformly distributed in the oxide scale. Nickel and chromium tended to present at the lower part of the oxide scale. Excessively thick porous scales formed on the low-carbon steels, whereas thin but non-adherent scales containing Cr2O3 formed on the stainless steel.

keyword : alloys, rolling, oxidation, scanning electron microscopy, steel

Nano-powders of 1.5TiAl and Al2O3 were synthesized from 1.5TiO2 and 3Al powders by high energy ball milling. Nanocrystalline Al2O3 reinforced composite was consolidated by pulsed current activated sintering within 2 minutes from mechanochemically synthesized powders of Al2O3 and 1.5TiAl. The relative density of the composite was 99.5%. The average hardness and fracture toughness values obtained were 1250 kg/mm2 and 10 MPa·m1/2, respectively.

keyword : compound, nanostructure, sintering, mechanical properties

The aim of this study is to investigate the effect of post heat treatment on bonding properties of roll cladded Ti/MS/Ti materials. First grade Ti sheets and SPCC mild steel sheets were prepared and then Ti/MS/Ti clad materials were fabricated by a cold rolling and post heat treatment process. Microstructure and point analysis of the Ti/MS interfaces were performed using the SEM and EDX Analyser. Diffusion bonding was observed at the interfaces of Ti/MS. The thickness of the diffusion layer increased with post heat treatment temperature and the diffusion layer was verified as having (ε+ζ)+(ζ+β-Ti) intermetallic compounds at 700℃ and an (ζ+β-Ti) intermetallic compound at 800℃, respectively. The micro Knoop hardness of mild steel decreased with post heat treatment temperature; however, those of Ti decreased at a range of 500~600 ℃ and showed a uniform value until 800℃ and then increased rapidly up to 900℃. The micro Knoop hardness value of the diffusion layer increased up to 700℃ and then saturated with post heat treatment. A T-type peel test was used to estimate the bonding forces of Ti/Mild steel interfaces. The bonding forces decreased up to 800℃ and then increased slightly with post heat treatment. The optimized temperature ranges for post heat treatment were 500~600℃ to obtain the proper formability for an additional plastic deformation process.

keyword : Ti/Mild steel/Ti clad material, post heat treatment, bonding interface property, intermetallic compound, diffusion Layer, bonding force

The microstructural changes of three pieces from an ancient iron pot were studied in order to identify present the material degradation due to repeated heating for one-thousand years. The microstructures of the pieces were divided into the areas of ferrite/graphite, ferrite/pearlite, and corroded oxidation. The area of ferrite/graphite was undergone by severe Galvanic corrosion, but that of ferrite/pearlite was not even during a thousand years` using . The shape of the graphites was coexisted with types of A, B, and C of as modern graphite classification. In the ferrite/pearlite area, abnormal acicula precipitates with a high aspect ratio of 0.2㎛thickness and several hundreds ㎛length were presented. They might be a kind of carbide in the ferrite matrix with its special precipitate plane.

keyword : ancient iron pot, microstructural change, ferrite/graphite/pearlite, high aspect ratio carbide

Electrical properties as a function of composition in silicon nitride (SiNx) films grown at low temperatures (<200℃) were studied for applications to photonic devices and thin film transistors. Both siliconrich and nitrogen-rich compositions were successfully produced in final films by controlling the source gas mixing ratio, R=[(N2 or NH3)/SiH4], and the RF plasma power. Depending on the film composition, the dielectric and optical properties of SiNx films varied substantially. Both the resistivity and breakdown field strength showed the maximum value at the stoichiometric composition (N/Si=1.33), and degraded as the composition deviated to either side. The electrical properties degraded more rapidly when the composition shifted toward the silicon-rich side than toward the nitrogen-rich side. The composition shift from the siliconrich side to the nitrogen-rich side accompanied the shift in the photoluminescence characteristic peak to a shorter wavelength, indicating an increase in the band gap. As long as the film composition is close to the stoichiometry, the breakdown field strength and the bulk resistivity showed adequate values for use as a gate dielectric layer down to 150℃ of the process temperature.

keyword : silicon nitride, dielectric, photonic device, PE-CVD, Low temperature