• Journal of the Korean Society of Mechanical Technology
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• v.20 no.6
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• pp.818-823
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• 2018

In this study, the effect of pre-aging treatment for inhibition of natural aging of Al-4.8Zn-1.3Mg alloy by extrusion process was investigated. Firstly, the as-cast microstructure of Al-4.8Zn-1.3Mg alloy billet and its evolution during homogenization($460^{\circ}C$, $4h+510^{\circ}C$, 5h) were investigated by means of optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), hardness analysis. The as-cast microstructures of Al-4.8Zn-1.3Mg alloy reveal $Mg_2Zn$, $Al_5Cu$, $Al_{13}Cu$ formed between dendrities. After homogenization, MgZn, $Al_4Cu$, $Al_{13}Cu$ phases precipitated into the matrix. In addition, standard deviation of homogenized billet was improved than as-cast billet from 2.62 to 0.99. According to pre-aging($100^{\circ}C$, 1h) Al-4.8Zn-1.3Mg alloy by extrusion process, yield strength and tensile strength deviation improved more than condition by natural aging.

• Journal of the Korean Society of Industry Convergence
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• v.4 no.3
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• pp.237-242
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• 2001

Ti-6Al-4V alloy can be obtained the stability of organization and product treasure, the evaluation of the cutting ability and the mechanical characteristics after the heat treatment of annealing, solution and aging. The difficulty in machining titanium alloy is how treat the heat generated during the process. Because the heat conductivity of titanium alloy is regardfully low, the heat generated during grinding is accumulated in workpiece. and it causes the increasing of grinding wheel grits' wear and makes the ground surface rough. So, these characteristics in grinding of titanium alloy will change the mechanical properties of the titanium alloy. From this study. the mechanical characteristics of annealed one and solution and aging one treated Ti-6Al-4V alloy after grinding was concerned with checking out the bending strength and hardness. For the result, both of bending strength and hardness were increased at the burned area on the surface. Roughness value was remarkably high at the table speed of 10m/min.

• Journal of the Korean Society for Heat Treatment
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• v.9 no.2
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• pp.91-102
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• 1996

Microstructural control to produce multiphase structure has received much attention to improve the high temperature strength as well as low temperature ductility of intermetallics. Transmission electron microscopic investigation has been carried out concerning the effect of Cr-precipitation on the mechanical properties of B2-ordered NiAl containing 4 to 8 mol% of Cr. By aging at temperatures around 973 K after solution annealing, fine spherical precipitates took place homogeneously in the NiAl matrix and the alloys hardened appreciably. Selected area electron diffraction (SAED) patterns have not revealed any additional extra-spots during aging, because the Cr-particles show cube-cube orientation relationship and keep a perfect coherency with the ordered matrix lattice. Dislocations were confirmed to bypass the particles during deformation. Although the dispersion of Cr-particles increased the yield strength of NiAl at intermediate temperature, the strength decreased appreciably at higher temperatures.

• Journal of the Korean Society for Nondestructive Testing
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• v.34 no.3
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• pp.241-247
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• 2014

This study presents the ultrasonic nonlinearity of AISI316 austenitic stainless steels subjected to longterm isothermal aging. These steels are attractive materials for use in industrial mechanical structures because of their strength at high-temperatures and their chemical stability. The test materials were subjected to accelerated heat-treatment in an electrical furnace for a predetermined aging duration. The variations in the ultrasonic nonlinearity and microstructural damage were carefully evaluated through observation of the microstructure. The ultrasonic nonlinearity stiffly dropped after aging for up to 1000 h and, then, monotonously decreased. The polygonal shape of the initial grain structures changed to circular, especially as the annealing twins in the grains dissolved and disappeared. The delta ferrite on the grain boundaries could not be observed at 1000 h of aging, and these continuously transformed into their sigma phases. Consequently, in the intial aging period, the rapid decrease in the ultrasonic nonlinearity was caused by voids, dislocations, and twin annihilation. The continuous monotonic decrease in the ultrasonic nonlinearity after the first drop resulted from the generation of $Cr_{23}C_6$ precipitates and ${\sigma}$ phases.

• Journal of the Korean Society for Heat Treatment
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• v.21 no.1
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• pp.20-25
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• 2008

Structural studies have been performed on precipitation hardening and microstructural variations found in Ti-Al-Cr ternary $L1_0$- and $L1_2$-phase alloys using transmission electron microscopy. Both the $L1_0$ and $L1_2$ phase alloys harden by aging at 973 K after solution annealing at higher temperatures. The amount of age hardening of the $L1_2$ phase alloy is larger than that of the $L1_0$ phase alloy. The phase separation between $L1_0$ and $L1_2$ phase have not been observed by aging at 973 K. But $Al_2Ti$ was formed in each matrix alloy during aging. The crystal structure of the $Al_2Ti$ phase is a $Ga_2Zr$ type in the $L1_0$ and a $Ga_2Hf$ type in the $L1_2$ phase, respectively. At the beginning of aging the fine coherent cuboidal $Al_2Ti$-phase are formed in the $L1_0$ phase. By further aging, two variants of $Al_2Ti$ precipitates grow along the two {110} habit planes. On the other hand, in the $L1_2$ phase, the $Al_2Ti$ phase forms on the {100} planes of the $L1_2$ matrix lattice. After prolonged aging the precipitates are rearranged along a preferential direction of the matrix lattice and form a domain consisting of only one variant. It is suggested that the precipitation of $Al_2Ti$ in each matrix alloy occurs to form a morphology which efficiently relaxes the elastic strain between precipitate and matrix lattices.

• Journal of the Korean Society for Heat Treatment
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• v.24 no.6
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• pp.311-317
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• 2011

The effect of strain rate on the yield stress of an Al-Li alloy has been investigated at temperatures between 77 and 523 K and over the strain rate range from $1.77{\times}10^{-4}s^{-1}$ to $1.77{\times}10^{-2}s^{-1}$. At testing temperatures below 373 K, the yield stress is almost independent of strain rate at any aging stage. At testing temperatures above 373 K, the yield stress increases linearly with the logarithm of strain rate, and the strain rate dependence increases with increasing testing temperature. The yield stresses of under-aged alloy at temperatures between 373 and 473 K at high strain rates are greater than the yield stress at 77 K. For the alloy under-aged or aged nearly to its peak strength, the temperature range within which the positive temperature dependence of yield stress appears expands to the higher temperature side with increasing strain rate. The strain rate dependence of the yield stress is slightly negative at this aging stage. The yield stress of the over-aged alloy decreases monotonically with decreasing strain rate and with increasing testing temperature above 373 K. The modulus normalized yield stress is nearly constant at testing temperatures below 373 K at any strain rate investigated. And, strength depends largely both on the aging conditions and on the testing temperature. The peak positions in strength vs. aging time curves shift to the side of shorter aging time with increasing testing temperature. For the specimens aged nearly to the peak strength, the positive temperature dependence of yield stress is observed in the temperature range. The shift of peak positions in the aging curves are explained in terms of the positive temperature dependence of cutting stress and the negative temperature dependence of by-passing stress.

• Proceedings of the Korean Nuclear Society Conference
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• 2001.05a
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• pp.328.1-328
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• 2001

• Korean Journal of Metals and Materials
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• v.49 no.1
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• pp.9-16
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• 2011

To develop a 6061 aluminum alloy with low residual stress and high tensile strength, a cryogenic treatment process was investigated. Compared to the conventional heat treatment process for precipitation hardening with artificial aging, the cryogenic treatment process has two additional steps. The first step is cryogenic quenching of the sample into liquid nitrogen, the second step is up-hill quenching of the sample into boiling water. The residual stress for the sample was measured by the $sin^2{\psi}$ method with X-ray diffraction. The 6061 aluminum alloy sample showed 67% relief in stress at the cryogenic treatment process with artificial aging at $175^{\circ}C$. From this study, it was found that the optimum cryogenic treatment process for a sample with low residual stress and high tensile strength is relatively low cooling speed in the cryogenic quenching step and a very high heating speed in the up-hill quenching step.

• Korean Journal of Materials Research
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• v.9 no.9
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• pp.905-912
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• 1999

Tensile properties of powder metallurgy (PM) processed Cu-7.5Ni-5Sn alloys, either as-received or additionally solution heat treated, were examined as a function of aging time. It was found that the as-received Cu-7.5Ni-5Sn alloys showed an abrupt increase in tensile strength after aging at $350^{\circ}C$ for 20 minutes, due to the metastable ${\gamma}$\\` precipitation and a marginal Spinodal decomposition. The resolutionized PM Cu-7.5Ni-5Sn alloys, on the other hand, showed a gradual increase in tensile strength from the very early stage of aging. The overall tensile strength of resolutionized PM Cu-7.5Ni-5Sn alloys, however, was lower than that of the as-received and aged counterparts, due to the grain growth during resolutionization. Afterprolonged aging for the as-received PM Cu-7.5Ni-5Sn alloys, a considerable amount of discontinuous precipitates formed along the grain boundaries. The formation and growth kinetics of such discontinuous precipitates appeared to be dependent on the heat treatment conditions, and affect the mechanical properties greatly.

• Journal of the Korean Society for Heat Treatment
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• v.35 no.5
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• pp.239-245
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• 2022

The purpose of this study is to evaluate the degree of microstructural change of materials using ultrasonic nonlinear parameters. For microstructure change, isothermal heat-treated ferritic 2.25Cr steel and low-cycle fatigue-damage copper alloy were prepared. The variation in ultrasonic nonlinearity was analyzed and evaluated through changes in hardness, ductile-brittle transition temperature, electron microscopy, and X-ray diffraction tests. Ultrasonic nonlinearity of 2.25Cr steel increased rapidly during the first 1,000 hours of deterioration and then gradually increased thereafter. The variation in non-linear parameters was shown to be coarsening of carbides and an increase in the volume fraction of stable M6C carbides during heat treatment. Due to the low-cycle fatigue deformation of oxygen-free copper, the dislocation that causes lattice deformation developed in the material, distorting the propagating ultrasonic waves, and causing an increase in the ultrasonic nonlinear parameters.