• Journal of the Korean Chemical Society
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• v.23 no.4
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• pp.217-236
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• 1979

The author's theory for plastic deformation was applied to superplastic alloys (Zn-Al eutectoid, Al-Cu, Pb-Sn, Sn-Bi, Mg-Al eutectics). The plastic deformation of the superplastic alloys could be described by two Maxwell models connected in parallel which represent two grain boundary flow units. The flow units are characterized by the two parameters $X_{gj}/{\alpha}_{gj}\;and\;{\beta}_{gj}$ (j=l or 2, g signifies the grain boundary) the values of which were obtained by applying our flow equation [Eq. (5)] to experiment. We confirmed that our flow equation describes the superplasticity very well. The curve of strain rate sensitivity m (=${\partial}\;In\;f/{\partial}\;In\;\dot{s})\;vs.\;-In\dot{s}$, where f and s are stress and strain rate, respectively, showed two peaks corresponding to flow unit gl and g2, the separation of the two peaks is determined by the difference between ${\beta}_{g1}\;and\;{\beta}_{g2}$. The condition of superplasticity is also determined by ${\beta}_{gj}$, which satisfies $\dot{s}_{mj}{\leqslant}1.53}{\beta}_{gj}$ [Eq.(13)], where $\dot{s}_{mj}$ is the s of the jth unit at the peak. The grain size dependence of ${\beta}_{gj}$ is described by $ln({\beta}_{gj})^{-1}$=alnx+b [Eq. (16)], where x is the grain size, and a and b are constants. The activation enthalpy for each flow unit, ${\Delta}H_{gj}^{\neq}$ was also determined from the temperature dependence of ${\beta}_{gj}$ which is proportional to the relaxation time of the j th unit. Since the superplasticity is determined by Eq. (13), and since ${\beta}_{gj}$ and ${\Delta}H_{gj}^{\neq}$ are related, we obtained the conclusion that superplasticity occurs in the system having small ${\Delta}H_{gj}^{\neq}$ values. The Aej values were equal to the activation enthalpies of grain boundary self-diffusion of the component atoms of the alloys, this accords with our proposed flow mechanism. The ${\Delta}H_{gj}^{\neq}$ value increases with grain size as expected from Eq. (16).

• Journal of the Society of Naval Architects of Korea
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• v.39 no.1
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• pp.82-89
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• 2002

The inherent strain method is known to be very effective in predicting the plate deformation by line heating. Traditionally the inherent strain regions have been determined from the temperature distribution and the phase transformation regions(Ac3) of welding experiments. Since the phenomena of line heating are similar to those of welding, the experimental results under the same welding conditions have been applied directly to line heating analysis. The results cannot, however, reflect the effect of heating pattern and plate thickness. Besides, water-cooling in the actual heating process can alter the steel's phase to martensite and shear plastic deformation occurs during the transformation. In this study, the experimental measurement of temperature distribution was substituted with a transient heat transfer analysis using FEM so that we could obtain the temperature distribution according to heat flux models of the heating pass. In order to consider plastic strains occurring additionally under phase transformation, inherent strain regions were assumed to be limited to the eutectoid temperature(Ac1). Using the regions, plate deformations could be predicted to validate our method and the results were in good agreement with the experimental ones

• Journal of the Korean Society for Heat Treatment
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• v.17 no.6
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• pp.346-355
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• 2004

The flame quenching process has been employed to modify the surfaces of commercial marine propeller material, aluminum bronze alloy (Cu-8.8Al-5Ni-5Fe), and the microstructure, hardness and wear properties of the flame-quenched layers have been studied. The thermal history was accurately monitored during the process with respect to both the designed maximum surface temperature and holding time. The XRD and EDX analyses have shown that at temperatures above $T_{\beta}$, the microstructure consisting of ${\alpha}+{\kappa}$ phases changed into the ${\alpha}+{\beta}^{\prime}$ martensite due to an eutectoid reaction of ${\alpha}+{\kappa}{\rightarrow}{\beta}$ and a martensitic transformation of ${\beta}{\rightarrow}{\beta}^{\prime}$. The ${\beta}^{\prime}$ martensite phase formed showed a face-centered cubic (FCC) crystal structure with the typical twinned structure. The hardness of the flame-quenched layer having the ${\alpha}+{\beta}^{\prime}$ structure was similar to that of the ${\alpha}+{\kappa}$ structure and depended sensitively on the size and distribution of hard ${\kappa}$ and ${\beta}^{\prime}$ phases with depth from the surface. As a result of the sliding wear test, the wear resistance of the flame-quenched layer was markedly enhanced with the formation of the ${\beta}^{\prime}$ martensite.

• Korean Journal of Metals and Materials
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• v.49 no.2
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• pp.93-103
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• 2011

Precipitation behavior of high-nitrogen duplex Fe-24Cr-7Mn-4Ni-4Mo-0.43N stainless steel aged at $850^{\circ}C$ was investigated using scanning transmission electron microscopy. Based on the analyses of selected area diffraction patterns, four kinds of precipitates (intermetallic sigma (${\sigma}$) and chi (${\chi}$), $Cr_2N$ and secondary austenite) were identified. At the ferrite/austenite phase boundary, the ${\sigma}$ phase and secondary austenite were formed via ${\alpha}{\rightarrow}{\gamma}+{\sigma}$ eutectoid reaction. The precipitation of $Cr_2N$ occurred at the austenite grain boundary as well as the interior of the ferrite. The intermetallic ${\chi}$ phase also formed within the ferrite and showed a cube-cube orientation relationship with the ferrite. Further aging produced a lamellar structure composed of $Cr_2N$ and austenite along the ferrite/austenite boundary and enhanced the precipitation of the ${\chi}$ phase. The crystallographic features of the precipitates were also examined in terms of the orientation relationship with the austenite or ferrite matrix.

• Journal of Conservation Science
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• v.27 no.4
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• pp.421-430
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• 2011

Thermal conditions in practice at the traditional bronze workshop of the Korean Folk Village in Yongin were examined along with the microstructures of some high tin bronze objects made there. Laboratory experiments approximating the conditions of the workshop were also carried out and the results were compared. The operating temperature of the workshop furnace was measured to range from $750^{\circ}C$ to $850^{\circ}C$ while the surface temperature of an object, upon its removal from the furnace for additional thermo-mechanical treatments, was generally in the range of $600^{\circ}C$ to $685^{\circ}C$. This variation in working temperatures was reflected in varying microstructures developed upon quenching. The products of the Folk Village were found to consist of microstructures where the ${\alpha}$ grains of the Cu-Sn system were distributed in the background of different phases including the ${\beta}$-martensite phase, retained ${\gamma}$ phase, ${\alpha}+{\delta}$ eutectoid or their mixtures. This variability, which is also identified in objects made in ancient times as well as in our laboratory experiments, suggests that the actual thermal conditions given during the quenching treatments are much more complicated than is inferred from the temperature measurements. This paper will present detailed accounts of the thermo-mechanical treatments as observed in the high tin bronze workshop of the Korean Folk Village and discuss the evolution of varying microstructures in terms of the substantial variability involved in the implementation of the traditional forged high tin bronze technology of Korea.