발간논문

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Small punch creep testing has received attention due to the convenience of using smaller specimens than those of conventional uniaxial creep tests, which enables creep testing on developing or currently operational components. However, precedent studies have shown that it is necessary to consider friction between the punch and specimen when computing uniaxial equivalent stress from a finite element model. In this study, small punch creep behaviors of AISI 316L stainless steel, which is widely used in high temperature-high pressure machineries, have been compared for the two different ceramic balls such as Si3N4 and Al2O3. The optimal range of the friction coefficient is 0.4~0.5 at 650℃ for the best fit between experimental and simulation data of AISI 316 L stainless steel. The higher the friction coefficient, the longer the creep rupture time is. Therefore, the type of ceramic ball used must be specified for standardization of small punch creep testing.

keyword : small punch creep, multi-axial stress state, creep, AISI 316L stainless steel, friction coefficient, finite element analysis

Carbon fiber reinforced plastics (CFRP), which have been developed for their high mechanical properties, are insufficient to secure machinery. This paper investigates the use of magnetic abrasive finishing methods and the characteristics of surface roughness for mirror machining of CFRP. The cylindrical surface of CFRP was ground using a diamond paste with sizes of 0.1, 0.5, 1 and 6㎛. Consequently, an effective surface roughness of 0.03㎛(Ra) could be obtained via a paste size of 0.5㎛. The surface roughness was not improved due to epoxy abrasion between the carbon fiber and the epoxy.

keyword : carbon fiber reinforced plastics, CFRP, composites, machining, wear, SEM

To analyze the effect of the pore size of graphite in a pore-forming agent, graphite was added to porous ceramics of Al2O3-SiO2-ZrO2 systems. The graphite had 45~75, 100~125, 150~180, and 75~180㎛ dimensions. The properties of the ceramics, such as apparent porosity, density, dynamic elastic modulus, mechanical strength, and permeability, were investigated. The average pore size increased from 15.35㎛ to 22.32㎛ with the increase of the graphite size. The sample with the largest average pore size showed the highest mechanical strength and gas permeability. This was due to the sample with the largest pore size at the same porosity having fewer pores and larger distance between the pores than the sample with the smallest pore size, making cracks less likely to propagate. In addition, the large pore size reduced the repulsive power originating from the drag force between the gas and internal pore walls.

keyword : porous materials, powder processing, mechanical properties, image analysis, gas permeability

The tensile and creep behaviors of Alloy 263, which is a wrought Ni-base superalloy used for gas turbine combustion systems, was studied. Anomalous increase of yield strength and abrupt decrease of elongation with increasing temperature were observed after tensile testing at an intermediate temperature. Elongation of the superalloy decreased as the temperature increased to and above 540℃, and it reached a minimum value at 760℃. It was found that creep strain was also very low at the same temperature. Inhomogeneous deformation with intensive slip bands was observed in the specimens tested at low temperature. A thermally-assisted dislocation climb process was regularly conducted at high temperature. Twinning was found to be an important mechanism of both tensile and creep deformations of the superalloy at an intermediate temperature where ductility minimum was observed.

keyword : Ni-base superalloy, tensile, creep, deformation twin

Thermomechanical fatigue (TMF) behavior of heat resistant austenitic stainless steel was evaluated in the temperature range from 100℃ to peak temperatures of 600 to 800℃; The fatigue lives under TMF conditions were plotted against the plastic strain range and the dissipated energy per cycle. In the expression of the inelastic strain range versus fatigue life, the TMF data obtained at different temperature ranges were located close to a single line with a small deviation; however, when the dissipated energy per cycle, calculated from the area of the stress-strain hysteresis loops at the half of the fatigue life, was plotted against the fatigue life, the data showed greater scattering than the TMF life against the inelastic strain range. A noticeable stress relaxation in the stress-strain hysteresis curve took place at the peak temperatures higher than 700℃, but all specimens in this study exhibited cyclic hardening behavior with TMF cycles. Re-crystallization occurred during the TMF cycle concurrent with the formation of fine subgrains in the re-crystallized region, which is considered to cause the cyclic hardening of the steel.

keyword : alloys, deformation, fatigue, transmission electron microscopy, TEM, thermo-mechanical fatigue

This study aimed to evaluate the performance of diffusion barriers in order to prevent fuel-cladding chemical interaction (FCCI) between the metallic fuels and the cladding materials, a potential hazard for nuclear fuel in sodium-cooled fast reactors. In order to prevent FCCI, Zr or V metal is deposited on the ferriticmartensitic stainless steel surface by physical vapor deposition with a thickness up to 5㎛. The diffusion couple tests using uranium alloy (U-10Zr) and a rare earth metal such as Ce-La alloy and Nd were performed at temperatures between 660~800℃. Microstructural analysis using SEM was carried out over the coupled specimen. The results show that significant interdiffusion and an associated eutectic reaction ocurred in the specimen without a diffusion barrier. However, with the exception of the local dissolution of the Zr layer in the Ce-La alloy, the specimens deposited with Zr and V exhibited superior eutectic resistance to the uranium alloy and rare earth metal.

keyword : metals, vapor deposition, diffusion, scamning electron microscopy, eutectic

This paper reports the relationship between the surface roughness and thermal emissivity of graphite (IG-11) in nuclear reactors. The roughness was controlled by changing the oxidization time, resulting in 0, 6, and 11% losses of mass. The levels of roughness were 0.40, 0.72 and 1.09㎛ for the weight loss of 0, 6 and 11%, respectively. The binders and graphite fillers were found to have sequentially oxidized with a higher thermal emission for the highly oxidized sample, but with a lower emission when measured at a higher temperature. Our study suggests a method for predicting the thermal emission rate of graphite in a nuclear reactor based on roughness measurement.

keyword : carbon and graphite, scanning electron microscopy, SEM, thermal emissivity, surface roughness, isotropic nuclear graphite

Nanopowders of MgO, Al2O3 and SiO2 were made by high-energy ball milling. The fast sintering of nanostructured Al2O3-MgSiO3-SiO2 composites was investigated from mechanically activated powders of MgO, Al2O3 and SiO2 by a pulsed-current activated sintering process. Nanocrystalline materials have received much attention as advanced engineering materials with improved physical and mechanical properties; in particular greater strength, hardness, excellent ductility and toughness. Highly dense nanostructured Al2O3-MgSiO3-SiO2 composites were produced with simultaneous application of 80 MPa and pulsed output current of 2800A within 2 minutes. The sintering behavior, grain size and mechanical properties of Al2O3-MgSiO3-SiO2 composites were investigated.

keyword : compound, nanostructure, sintering, mechanical properties, pulsed current activated combustion

The shape of a dendrite tip has long been approximated by a paraboloid of revolution, but many attempts have been made as well to more accurately match the dendrite tip profile using other mathematical functions: power function, 4th order polynomial, and hyperbolic function. In the present work, dendrite tip shapes were matched by parabolic function. The differences between the dendrite tip shapes of pivalic acid(PVA)-ethanol(Eth) and succinonitrile(SCN)-salol systems, characterized by anisotropic and isotropic solid-liquid interfacial properties, respectively, were quantitatively treated using shape parameters. The PVA-Eth system showed a slightly higher Z/R value than the SCN-salol system, their Z/R values lying in the range 2-4. (Z is the distance from the tip beyond which the parabolic fit starts to deviate from the profile, and R the tip radius.) λP is the distance from the tip beyond which side branching starts to appear, and is larger in the PVA-Eth system than the SCNsalol system. λP is different for both sides of the 2-dimensional dendrite profile. The difference of λP between both sides of the dendrite is larger for PVA-Eth system than for SCN-salol, implying that the dendrite of PVA-Eth is less symmetric than that of SCN-salol.

keyword : crystal growth, solidification, interfaces, anisotropy, dendrite tip morphology

The newly developed α-case controlled mold material for Ti investment castings was suggested in this research. The Al2O3 mold containing interstitial TiO2 and substitutional Ti3Al was manufactured by the reaction between Al2O3 and Ti. It is obvious that as the TiO2 and Ti3Al content in the mold surface were increased, the depth of the interfacial reaction was significantly reduced. In addition, substitutional Ti5Si3 in the mold surface owing to the reaction between Ti and SiO2 from the binder was effective for α-case reduction. Therefore, the α-case reduction was accomplished by the diffusion barrier effect of interstitial TiO2, substitutional Ti3Al and Ti5Si3.

keyword : titanium, Liquid, solidification, microstructure, X-ray diffraction