• Journal of the Korean Society for Heat Treatment
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• v.32 no.2
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• pp.68-73
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• 2019

${\alpha}$-titanium alloy has a relatively low heat treatment characteristic and it is mainly subjected to heat treatment for residual stress, recovery or dynamic recrystallization. In this study, commercially pure titanium hollow castings was fabricated by gravity casting. Heat treatments were carried out at $750^{\circ}C$, $850^{\circ}C$ and $950^{\circ}C$ to investigate the effect of post-heat treatment on microstructure and mechanical properties. Beta-transus temperature ($T_{\beta}$) was about $913^{\circ}C$, and equiaxed microstructure was shown at temperature below $T_{\beta}$ and lath-type microstructure at temperature above $T_{\beta}$. Microstructure and mechanical properties did not show any significant difference in the direction of solidification for titanium hollow billet, so it can be seen that it was a well-made material for extrusion process. The optimum heat treatment condition of hollow billet castings for the seamless tube production was $850^{\circ}C$, 4 hr, FC, indicating a combination of equiaxed microstructure and appropriate mechanical properties.

• Journal of Powder Materials
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• v.17 no.6
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• pp.470-476
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• 2010

In this study, Cu-10Sn and Cu-10Sn-2Ni-0.2Si alloys have been manufactured by spray casting in order to achieve a fine scale microstructure and high tensile strength, and investigated in terms of microstructural evolution, aging characteristics and tensile properties. Spray cast alloys had a much lower microhardness than continuous cast billet because of an improved homogenization and an extended Sn solid solubility. Spray cast Cu-Sn-Ni-Si alloy was characterized by an equiaxed grain microstructure with a small-sized (Ni, Si)-rich precipitates. Cold rolling of Cu-Sn-Ni-Si alloy increased a tensile strength to 1220 MPa, but subsequent ageing treatment reduced a ultimate tensile strength to 780 MPa with an elongation of 18%.

• Proceedings of the Korean Powder Metallurgy Institute Conference
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• 2006.09b
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• pp.1023-1024
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• 2006

Al-8Fe-2Mo-2V-1Zr alloys were prepared by the gas atomization/hot extrusion and the melt spinning/hot extrusion. For the gas atomized and extruded alloy, equiaxed grains with the average size of 400 nm and finely distributed dispersoids with their particle sizes ranging from 50nm to 200nm were observed. For the melt spun and hot extrusion processed alloy, refined microstructural feature consisting of equiaxed grains with the average size of 200nm and fine dispersoids with their particle sizes under 50nm appeared to exhibit a difference in microstructure. Strength of the latter alloy was higher than that for the former alloy up to elevated temperatures.

• Transactions of Materials Processing
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• v.29 no.1
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• pp.5-10
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• 2020

Heat treatment conditions of 88.5% asymmetrically cold rolled oxygen free copper (OFC) sheets have been studied to obtain an equiaxed fine microstructure with a grain size of less than 10 ㎛. The commercial OFC sheets with the thickness of 10 mm were asymmetrically cold rolled by using equal speed asymmetric rolling (ESAR) processes and total rolling reduction. The thickness of the rolled sheets were 88.5% and 1.15 mm, respectively. An equiaxed fine microstructure of OFC sheets with a grain size of 6.0 ㎛ were obtained when the asymmetrically cold rolled OFC sheets were heat treated at 180℃ for 40 minutes. The tensile strength of the asymmetrically cold rolled specimen increased from 217.6 MPa to 396.1 MPa, while the elongation of the specimen asymmetrically cold rolled and heat treated increased from 29.0% to 66.9% along with an 8% increase of the tensile strength.

• The Korean Journal of Ceramics
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• v.2 no.1
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• pp.39-42
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• 1996

${\alpha}-SiC$ powder with or without the addition of 0.1 wt% of large ${\alpha}-SiC$ partices(seeds) was pressureless-sintered at $1950^{\circ}C$ for 0.5, 2, and 4 h using $Y_3Al_5 O_{12}$ (yttrium aluminum garnet, YAG) as a sintering aid. The materials without seeds had an equiaxed grain struture. In contrast, the materials with seeds sintered for 2 and 4 h had a duplex microstructure with large elongated grains and amall equiaxed grains. Addition of large ${\alpha}-SiC$ seeds into ${\alpha}-SiC$ accelerated the grain growth of some ${\alpha}-SiC$ grains during sintering and resulted in the increased fracture toughness of the sintered materials. The fracture toughnesses of materials with or without seeds sintered for 4 h were 6.6 and $5.2 MPa \;m^{12}$, respectively.

• Journal of the Korea Institute of Military Science and Technology
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• v.7 no.4 s.19
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• pp.70-78
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• 2004

A study has been made to investigate the high temperature deformation behavior of Ti-6Al-4V alloyand to predict the final microstructure under given forming conditions. Equiaxed and $Widmanst\ddot{a}tten$ microstructures of Ti-6Al-4V alloys were prepared as initial microstructures. By performing the compression tests at high temperatures$(700\~1100^{\circ}C)$ and at a wide range of strain rates$(10^{-4}\~10^2/s)$, various parameters such as strain rate sensitivity(m) and activation energy(Q) were calculated and used to establish constitutive equations. When the specimens were deformed up to strain 0.6, equiaxed microstructure did not show any significant changes in microstructure, while $Widmanst\ddot{a}tten$ microstructure revealed considerable flow softening, which was attributed to the globularization of a platelet at the temperature range of $800\~970^{\circ}C$ and at the strain rate range of $10^{-4}\~10^{-2}/s$. To predict the final microstructure after forming, finite element analysis was performed considering the microstructural evolution during the deformation. The grain size and the volume fraction of second phase of deformed body were predicted and compared with the experimental results.

• Metals and materials international
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• v.24 no.6
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• pp.1346-1358
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• 2018

A novel additive manufacturing method with TIG-MIG hybrid heat source was applied for fabricating 5356 aluminum alloy component. In this paper the microstructure evolution, mechanical properties and fracture morphologies of both as-deposited and heat-treated component were investigated, and how these were affected by different heat-treated temperature. The as-deposited microstructure showed dominant equiaxed grains with second phase, and the size of them is coarse in the bottom region, medium in the middle region and fine in the top region owing to different thermal cycling conditions. Compared with as-deposited microstructure, the size of grain becomes large and second phases gradually dissolve in the matrix as heat-treated temperature increase. Different microstructures determine the mechanical properties of component. Results show that average ultimate tensile strength enhances from 226 to 270 MPa and average microhardness increases from 64.2 to 75.3 HV0.1 but ductility decreases from 33 to 6.5% with heat-treated temperature increasing. For all components, the tensile properties are almost the same in the vertical direction (Z) and horizontal direction (Y) due to equiaxed grains, which exhibits isotropy, and the mechanisms of these are analyzed in detailed. In general, the results demonstrate that hybrid arc heat source has the potential to fabricate aluminum alloy component.

• Journal of the Korean Society for Heat Treatment
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• v.18 no.1
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• pp.18-23
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• 2005

Effect of cold rolling on microstructural changes has been investigated for a Zn-15%Al alloy to elucidate the reason for its work softening behavior. Fully annealed microstructure of the Zn-15%Al alloy is characterized by ${\eta}$ grains and (${\eta}+{\alpha}$) lamellar colonies, where ${\eta}$ and ${\alpha}$ are Zn-rich HCP and Al-rich FCC phases, respectively. The hardness decreases continuously with increasing cold rolling degree, exhibiting work softening behavior. It is revealed that during the cold rolling, (${\eta}+{\alpha}$) lamellar colonies gradually change into equiaxed ${\eta}$ and ${\alpha}$ grains due to dynamic recrystallization at room temperature, while pre-existing ${\eta}$ grains are only deformed without recrystallization. Furthermore, cold rolling causes the precipitation of dissolved Al solutes in ${\eta}$ grains. In view of these results, change of (${\eta}+{\alpha}$) phases from lamellar to equiaxed morphology, which results in structural softness and increase in equiaxed ${\eta}/{\alpha}$ grain boundaries with higher mobility, and deterioration of solution hardening by precipitation of Al solutes from ${\eta}$ grains, are thought to contribute to the work softening of Zn-15%Al alloy.

• Journal of the Korean Ceramic Society
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• v.34 no.11
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• pp.1129-1138
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• 1997

Al2O3/20 vol%YAG composite containing equiaxed grains and Al2O3/20 vol%LaAl11O18 composite containing elongated grains were fabricated using Al2O3-Y2O3 composition and Al2O3-La2O3 composition, respectively, by hot-pressing. In order to investigate the influence of microstructural control of second phase on toughening effect of toughened ceramic composites, AE (acoustic emission) measurements have been coupled with fracture toughness experiments(SENB and SEPB method). A separation of the fracture toughness and analysis of toughening mechanism was possible using the AE technique. The fracture toughness of hot-pressed materials was estimated to be 3.2 MPam0.5 for monolithic alumina, 4.7 MPam0.5 for Al2O3/20 vol%YAG composite and 6.2 MPam0.5 for Al2O3/20 vol%LaAl11O18 composite. In monolithic Al2O3, toughening does not occur as a result of either microcracking or grain bridging, whereas, composites exhibit toughening effects by both microcracking in the frontal zone and gain bridging in the wake zone, resulting in an improvement of fracture toughness as compared with monolithic Al2O3. The fracture toughness of Al2O3/20 vol%LaAl11O18 composite is higher than that of Al2O3/20 vol%YAG composite. It may be attributed to the elongated microstructure of Al2O3/20 vol%LaAl11O18 composite, resulting relatively greater bridging effect.

• Journal of Korea Foundry Society
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• v.37 no.2
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• pp.38-44
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• 2017

Given that the conventional extrusion of high-strength Al alloys such as 7075 aluminum alloys is difficult due to their very low extrudability as compared to that of 6061 aluminum alloys, thixo-extrusion can be used to obtain a high-strength material easily at a lower extrusion pressure as compared to conventional extrusion. In this study, hot- and thixo-extruded 7075 aluminum alloys are prepared by a vertical forward extrusion process and their microstructures, hardness levels, and compressive properties are investigated. Hot-extruded alloy bars are assessed to obtain a microstructure elongated in the extrusion direction, whereas with thixo-extruded alloy bars, it was possible to obtain a microstructure having fine and equiaxed grains by dynamic recrystallization. The resulting isotropy and improved formability at the hot deformation temperature of the thixo-extruded alloy were attributed to the fine and equiaxed grains formed by the thixo-extrusion process.