• 연구논문집
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• s.33
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• pp.137-145
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• 2003

The effects of antimony addition on the microstructures and creep behavior of AZ31 magnesium alloy have been investigated. Constant load creep tests were carried out at temperatures ranging from $150^{\circ}C$ to $200^{\circ}C$, and an initial stress of 50MPa for AZ31 alloys containing antimony up to 0.84% by weight. Results show that small additions of antimony to AZ31 effectively decreased the creep extension and steady state creep rates. The steady state creep rate of AZ31 was reduced 2.5 times by the addition of 0.84% of antimony. The steady state creep rate of AZ31-0.84Sb alloy was controlled by dislocation climb in which the activation energy for creep was 128 kJ/mole. The microstructure of as-cast AZ31-0.84%Sb alloy showed the presence of $Mg_3Sb_2$ precipitates dispersed throughout the matrix. The main reason for the higher creep resistance in AZ31-Sb alloys is due to the presence $Mg_3Sb_2$, which effectively hindered the movement of dislocations during the elevated temperature creep.

• Journal of Welding and Joining
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• v.20 no.6
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• pp.30-30
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• 2002

It has been reported that the creep characteristics on weldment which is composed of weld metal(W.M), fusion line(F.L), heat-affected zone(HAZ), and base meta(B.M) could be unpredictably changed in severe service conditions such as high temperature and high pressure. However, the studies done on creep damage in power plant components have been mostly conducted on B.M and not the creep properties of the localized microstructures in weldment have been thoroughly investigated yet. In this paper, it is investigated the creep characteristics for three microstructures like coarse-grain HAZ(CGHAZ), W.M, and B.M in X20CrMoV121 steel weldment by the small punch-creep-(SP-Creep) test using miniaturized specimen(l0×10×0.5mm). The W.M microstructure possesses the higher creep resistance and shows lower creep strain rate than the B.M and CGHAZ. In the lower creep load the highest creep strain rate is exhibited in CGHAZ, whereas in the higher creep load the B.M represents the high creep strain rate. The power law correlation for all microstructures exists between creep rate and creep load at 600℃. The values of creep load index (n) based on creep strain rate for B.M, CGHAZ, and W.M are 7.54, 4.23, and 5.06, respectively and CGHAZ which shows coarse grains owing to high welding heat has the lowest creep loade index. In all creep loads, the creep life for W.M shows the highest value.

• Journal of Welding and Joining
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• v.20 no.6
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• pp.754-761
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• 2002

It has been reported that the creep characteristics on weldment which is composed of weld metal(W.M), fusion line(F.L), heat-affected zone(HAZ), and base meta(B.M) could be unpredictably changed in severe service conditions such as high temperature and high pressure. However, the studies done on creep damage in power plant components have been mostly conducted on B.M and not the creep properties of the localized microstructures in weldment have been thoroughly investigated yet. In this paper, it is investigated the creep characteristics for three microstructures like coarse-grain HAZ(CGHAZ), W.M, and B.M in X20CrMoV121 steel weldment by the small punch-creep-(SP-Creep) test using miniaturized specimen($10{\times}10{\times}0.5mm$). The W.M microstructure possesses the higher creep resistance and shows lower creep strain rate than the B.M and CGHAZ. In the lower creep load the highest creep strain rate is exhibited in CGHAZ, whereas in the higher creep load the B.M represents the high creep strain rate. The power law correlation for all microstructures exists between creep rate and creep load at $600^{\circ}C$. The values of creep load index (n) based on creep strain rate for B.M, CGHAZ, and W.M are 7.54, 4.23, and 5.06, respectively and CGHAZ which shows coarse grains owing to high welding heat has the lowest creep loade index. In all creep loads, the creep life for W.M shows the highest value.

• Journal of Korea Foundry Society
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• v.40 no.2
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• pp.7-15
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• 2020

The high-temperature stability of Mg-8.0Zn-1.6Y (wt.%) alloys upon the addition of Ca has been investigated by characterizing the ignition temperature, microstructure, tensile and creep properties. The ignition temperature increases with an increase in the Ca content, indicating that an addition of Ca enhances the ignition resistance of the Mg-Zn-Y alloy. The as-cast microstructures of all tested alloys mainly consisted of the dendritic α-Mg matrix and I-phase (Mg₃Zn₆Y) at the grain boundaries. In the Ca-added Mg-8.0Zn-1.6Y alloys, the Ca₂Mg₆Zn₃ phase forms, with this phase fraction increasing with an increase in the Ca contents. However, a high volume fraction of the Ca₂Mg₆Zn₃ phase rather deteriorates the mechanical properties. Therefore, a moderate amount of Ca element in Mg-8.0Zn-1.6Y alloys is effective for improving the tensile and creep properties of the Mg-Zn-Y alloy. The Mg-8.0Zn-1.6Y-0.3Ca alloy exhibits the highest tensile strength and the lowest creep strain among the alloys investigated in the present study. The creep resistance of Mg-Zn-Y-Ca alloys depends on the selection of the secondary solidification phase; i.e., when Ca₂Mg₆Zn₃ forms in an alloy containing a high level of Ca, the creep resistance deteriorates because Ca₂Mg₆Zn₃ is less stable than the I-phase at a high temperature.

• Journal of Korea Foundry Society
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• v.27 no.5
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• pp.198-202
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• 2007

In the present study, the high temperature mechanical properties and creep resistance of Mg-Zn-Y-Ca alloys has been investigated. The Mg-4Zn-0.8Y alloy consists of ${\alpha}$-Mg matrix and icosahedral quasicrystalline phase. Calcium addition into Mg-4n-0.8Y based alloy results in the formation of ${\tau}(Ca_{2}Mg_{6}Zn_{3})$ and $Mg_{2}Ca$ as the second solidification phases. Creep properties of the Mg-Zn-Y and Mg-Zn-Ca based alloys measured at applied stresses between 65 MPa and 85 MPa are significantly improved with adding calcium and yttrium, respectively. The improved creep resistance is due to the formation of thermally stable $Mg_{2}Ca$ phase.

• International Journal of Highway Engineering
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• v.6 no.3 s.21
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• pp.9-15
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• 2004

The cracking resistance of asphalt mixtures is generally evaluated by measuring a single parameter (i.e., Tensile strength, Stiffness). However, the use of a single parameter has been questioned in the evaluation of asphalt mixture cracking performance. The focus of this study was to clearly identify the key properties and characteristics associated with the cracking resistance of asphalt mixtures. Results of fracture, creep, and strength tests at multiple loading rates performed on the modified and unmodified mixtures showed that the mixture cracking resistance was primarily affected by the rate of micro-damage accumulation. This was reflected in the m-value, without affecting the fracture energy limit. It was also observed that the short loading time (elastic) stiffness alone could not differentiate the mixture cracking resistance of the mixtures. It was concluded that the key to characterize the cracking resistance of asphalt mixture is in the evaluation of the combined effects of creep and failure limits. It was also found that a residual dissipated energy parameter measured from Superpave IDT strength test gave the quick and useful way to distinguish the difference of cracking resistance of asphalt mixtures. Failure strain in the longer-term creep test appeared to be a useful parameter for evaluating the combined effects of creep and failure limits of asphalt mixtures.

• Journal of the Korean Society for Precision Engineering
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• v.29 no.10
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• pp.1137-1143
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• 2012

Creep characteristic is an important failure mechanism when evaluating engineering materials that are soft material as polymers or used as mechanical elements at high temperatures. One of the popular thermo-plastic polymers, Acrylonitrile Butadiene Styrene (ABS) which is used broadly for machine elements material, as it has excellent mechanical properties such as impact resistance, toughness and stiffness compared to other polymers, was studied for creep characteristic at different levels of stress and temperatures. From the experimental results, the creep limit of ABS at room temperature is 80 % of tensile strength which is higher than PE and lower than PC or PMMA. Also the creep limits decreased to linearly as the temperatures increased, up to $80^{\circ}C$ which is the softening temperature of Butadiene ($82^{\circ}C$). Also the secondary stage of creep among the three creep stages for different levels of stress and temperature was non-existent which occurred for many metals by strain hardening effect.

• Journal of Korea Foundry Society
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• v.26 no.1
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• pp.34-39
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• 2006

In the present study, the possibility of Mg-Zn-Y alloys as high temperature casting alloys has been investigated. The fluidity of alloys containing yttrium were better than that of commercial AZ91 alloy because the oxide layer on the surface reduced the reaction between melt, and air and mold, which would reduce the resistance during the process of filling the mold. However, this oxide film reduced the hot-tearing resistance. In the case of ZAW942, this alloy exhibited fluidity and hot-tearing resistance better than AZ91 alloy. Because of thermally stable quasicrystal and other phases obstructed the movement of grains, the creep resistance of alloys containing rare earth elements more than 2 wt% was better than that of AZ91 alloy.

• Journal of the Korean Society for Precision Engineering
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• v.20 no.6
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• pp.214-221
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• 2003

Tension mask assembly is positioned right behind the glass-made front panels of CRT type display devices. The frame-supported thin metal sheet contains numerous slits, through which electron beams are focused to enhance definition. Pretension is imposed on the masks, especially for enlarged flat screens, in order to avoid vibration due to acoustic or mechanical impact. High temperature assembly process subsequent to pretensioning, however, degenerates the creep resistance of common mask materials, and if tensile stress is high enough, tension on the mask may be loosened substantially due to creep deformation. In this study, the assembly is modeled as a combined structure of beams and wire array, and a numerical simulation is attempted for pretensioning followed by high temperature process. According to a model study, small amount of creep strain is likely to be generated, but its adverse influence is not negligible. Some structural modification measures to reduce the creep-induced tension loosening are proposed and evaluated. Also, optimal configuration of frame structure is sought for, which maintains high tension of masks and minimizes the possible creep of frame simultaneously.

• KEPCO Journal on Electric Power and Energy
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• v.6 no.1
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• pp.41-47
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• 2020

The contribution of damage mechanisms to failure of steam turbine casing made of Cr-Mo-V steel was investigated. Creep-fatigue interaction on the HP side corner of turbine casing was revealed as the root cause of the catastrophic failure performed by metallurgical analysis. The steady-state pressure and transient thermal stress were analyzed based on the actual operating condition of the thermal plant. Damage of creep-fatigue interaction to crack initiation was evaluated with multiaxial effects. The contribution ratio of creep and fatigue to the crack initiation was estimated to 3:1. Temporary geometrical correct action with repair weld was executed. For long-term operation, design improvement of casing equipment for creep resistance should be needed.