• Journal of Powder Materials
• /
• v.22 no.1
• /
• pp.6-9
• /
• 2015

In this study, the behavior of densification of copper powders during high-pressure torsion (HPT) at room temperature is investigated using the finite element method. The simulation results show that the center of the workpiece is the first to reach the true density of copper during the compressive stage because the pressure is higher at the center than the periphery. Subsequently, whole workpiece reaches true density after compression due to the high pressure. In addition, the effective strain is increased along the radius during torsional stage. After one rotation, the periphery shows that the effective strain is increased up to 25, which is extensive deformation. These high pressure and severe strain do not only play a key role in consolidation of copper powders but also make the matrix harder by grain refinement.

• Journal of Korea Foundry Society
• /
• v.33 no.3
• /
• pp.127-133
• /
• 2013

The influences of a small amount of CaO addition on the microstructure and corrosion behavior of AZ81 casting alloy have been investigated by means of optical microscopy, scanning electron microscopy, X-ray photoelectron spectroscopy, immersion and electrochemical tests. The added CaO led to the refinement of ${\alpha}$-Mg grains and the decrease in ${\beta}$ precipitate content by the formation of an $Al_2Ca$ phase. The AZ81-CaO alloy had a better corrosion resistance than the AZ81 alloy. The microstructural characterization on the corroded surface revealed that the enhanced corrosion resistance of the CaO-containing alloy may well be ascribed to the increased barrier effect of precipitates formed more continuously along the grain boundaries and the incorporation of Al and Ca elements into the corrosion film, by which it became more protective.

• Journal of Korea Foundry Society
• /
• v.19 no.1
• /
• pp.38-46
• /
• 1999

Microstructural characteristics and strengthening behavior in Mg-5wt%Zn-0.6wtZr alloys have been investigated by a combination of optical, secondary electron and transmission electron microscopy, differential thermal analysis, and hardness and tensile, creep property measurements. The result have been compared with those of Mg-5wt%Zn alloys. The as-squeeze cast microstructure consisted of dendrite ${\alpha}-Mg$, interdendrite or intergranular $Mg_7Zn_3$ and fine dispersoids of $ZnZr_2$. The size of secondary solidification phases in Mg-5wt%Zn-0.6wtZr alloys was significantly smaller than that of the Mg-5wt%Zn alloys due to the existence of fine dispersoid of $ZnZr_2$ which also effected the refinement of grain size. TEM study showed that the main cause of age hardening is formation of fine rodlike ${\beta}_1\;'$ precipitates as well as fine $ZnZr_2$ dispersoids. Due to the observed microstructural characteristics mechanical propeties of Mg-5wt%Zn-0.6wtZr alloys was found to be superior to those of Mg-5wt%Zn alloys.

• Journal of the Korean Society of Safety
• /
• v.27 no.3
• /
• pp.15-19
• /
• 2012

The fatigue behavior of AM60 magnesium alloy produced by equal channel angular pressing(ECAP) process was investigated through fatigue lifetime and fatigue crack propagation rate tests. The grain structure of the material was refined from 19.2 ${\mu}m$ to 2.3 ${\mu}m$ after 6 passes of ECAP at 493 K. The yield strength(YS) and ultimate tensile strength (UTS) increase after two passes but decrease with further pressing, although the grain size becomes finer with increasing pass number. The softening effect due to texture anisotropy overwhelmed the strengthening effect due to grain refinement after 2 passes. A large enhancement in fatigue strength was achieved after two ECAP passes. The current finding suggests that two passed material is better than the multi-passed material in view of the static strength and fatigue performance.

• Journal of Powder Materials
• /
• v.17 no.1
• /
• pp.52-58
• /
• 2010

Bulk nanostructured metallic materials are generally synthesized by bottom-up processing which starts from powders for assembling bulk materials. In this study, the bottom-up powder metallurgy and High Pressure Torsion (HPT) approaches were combined to achieve both full density and grain refinement at the same time. After the HPT process at 473K, the disk samples reached a steady state condition when the microstructure and properties no longer evolve, and equilibrium boundaries with high angle grain boundaries (HAGBs) were dominant. The well dispersed alumina particles played important role of obstacles to dislocation glide and to grain growth, and thus, reduced the grain size at elevated temperature. The small grain size with HAGBs resulted in high strength and good ductility.

• Transactions of Materials Processing
• /
• v.14 no.6 s.78
• /
• pp.502-508
• /
• 2005

Hypereutectic Al-Si alloys have been regarded attractive for automotive and aerospace application, due to high specific strength, good wear resistance, high thermal stability, low thermal expansion coefficient and good creep resistance. Spray casting of hypereutectic Al-Si alloy has been reported to provide distinct advantages over ingot metallurgy (IM) or rapid solidification/powder metallurgy (RS/PM) process in terms of microstructure refinement. In this study, hypereutectic Al-25Si-2.0Cu-1.0Mg alloy was prepared by OSPREY spray casting process. The change of strain rate sensitivity and Creep transition were analyzed by using the load relaxation test and constant creep test. High temperature deformation behavior of the hypereutectic Al-Si alloy has been investigated by applying the internal variable theory proposed by Chang et al. Especially, the creep resistance of spray casted hypereutectic Al-Si alloy can be enhanced considerably by the accumulation of prestrain.

• Korean Journal of Metals and Materials
• /
• v.48 no.12
• /
• pp.1078-1083
• /
• 2010

In this study, high temperature oxidation behavior of Alloy 617 was investigated to evaluate the effect of grain size for Alloy 617. The grain size of grain-refined Alloy 617 (GR617) was reduced to $5{\mu}m$ from $71{\mu}m$ for as-received Alloy 617 (AR617) by recrystallization after cold rolling. After high temperature aging, the oxide layers of AR617 and GR617 consisted of $Cr_2O_3$ external oxide scale and $Al_2O_3$ internal oxide. The external oxide scale resulted in a Cr-depleted zone and a carbide free zone below the scale. The depth of the carbide free zone was deeply formed in GR617. On the other hand, the depth of the internal oxide layer in GR617 was shorter than that in AR617. After a 3-point bending test, crack propagation of GR617 was more restricted than that of AR617 because of the different microstructure of the internal oxide.

• Journal of Powder Materials
• /
• v.26 no.2
• /
• pp.101-106
• /
• 2019

Ni-based oxide dispersion strengthened (ODS) alloys have a higher usable temperature and better high-temperature mechanical properties than conventional superalloys. They are therefore being explored for applications in various fields such as those of aerospace and gas turbines. In general, ODS alloys are manufactured from alloy powders by mechanical alloying of element powders. However, our research team produces alloy powders in which the $Ni_5Y$ intermetallic phase is formed by an atomizing process. In this study, mechanical alloying was performed using a planetary mill to analyze the milling behavior of Ni-based oxide dispersions strengthened alloy powder in which the $Ni_5Y$ is the intermetallic phase. As the milling time increased, the $Ni_5Y$ intermetallic phase was refined. These results are confirmed by SEM and EPMA analysis on microstructure. In addition, it is confirmed that as the milling increased, the mechanical properties of Ni-based ODS alloy powder improve due to grain refinement by plastic deformation.

• Advances in aircraft and spacecraft science
• /
• v.10 no.2
• /
• pp.127-140
• /
• 2023

The present article focuses on the thermoelastic deformation behavior of inhomogeneous functionally graded metal/ceramic cylindrical shell structure with multiple perforations using 2D finite element approximation. Here, cylindrical shell structure is considered with single (1×1) and multiple (2×2, 3×3 and 4×4) perforations. The temperature-dependent elastic and thermal properties of functionally graded material are evaluated using Voigt's micromechanical material scheme via power-law function. The kinematics of the proposed model is based on the equivalent single-layer first-order shear deformation mid-plane theory with five degrees-of-freedom. Here, 2D isoparametric finite element solutions are obtained using eight-node quadrilateral elements. The mesh refinement of present finite element model is performed to confirm the appropriate number of elements and nodes for the analysis purpose. Subsequently, a comparison test is conducted to demonstrate the accuracy of present results. In later section, numerous numerical illustrations are demonstrated at different set of conditions by varying structural, material and loading parameters and that confirms the significance of various parameters such as power-law index, aspect ratio, thickness ratio, curvature ratio, number of perforations and temperature on the deformation characteristics of functionally graded cylindrical shell structure.

• Child Health Nursing Research
• /
• v.29 no.3
• /
• pp.182-194
• /
• 2023

Purpose: This study analyzed research trends in infant and toddler rearing behavior among family caregivers over a 10-year period (2010-2021). Methods: Text network analysis and topic modeling were employed on data collected from relevant papers, following the extraction and refinement of semantic morphemes. A semantic-centered network was constructed by extracting words from 2,613 English-language abstracts. Data analysis was performed using NetMiner 4.5.0. Results: Frequency analysis, degree centrality, and eigenvector centrality all revealed the terms ''scale," ''program," and ''education" among the top 10 keywords associated with infant and toddler rearing behaviors among family caregivers. The keywords extracted from the analysis were divided into two clusters through cohesion analysis. Additionally, they were classified into two topic groups using topic modeling: "program and evaluation" (64.37%) and "caregivers' role and competency in child development" (35.63%). Conclusion: The roles and competencies of family caregivers are essential for the development of infants and toddlers. Intervention programs and evaluations are necessary to improve rearing behaviors. Future research should determine the role of nurses in supporting family caregivers. Additionally, it should facilitate the development of nursing strategies and intervention programs to promote positive rearing practices.