Papers by Author: Young Won Chang

Abstract: Texture evolution and superplastic deformation behavior of a quasi-single phase Zn-0.3wt%Al have been investigated. It was attempted to produce a stable and fine-grained microstructure in a dilute Zn-Al alloy through a proper thermomechanical treatment process (TMTP). The grain size of about 1 µm was obtained in the Zn-0.3 wt.% Al alloy and a relatively coarse grain size of 10 µm was also obtained through a subsequent aging treatment. The fine-grained material showed typical rolling texture with basal poles tilted about 30 degrees away from the ND toward RD, while the coarse-grained material showed a typical recrystallization texture with basal poles parallel to ND. A series of load relaxation and tensile tests were conducted at room temperature. According to the internal variable theory of structural superplasticity, the grain boundary characters of fine and coarse-grained materials were different from each other. A large elongation of about 1400% was obtained in fine-grained material at room temperature.

Abstract: The superplastic deformation behavior of a fine-grained 7075 Al has been investigated to clarify the issue of threshold stress. A series of mechanical tests has been conducted at various temperatures for the specimens with various grain sizes. The quantitative constitutive parameters have been determined from load relaxation test by applying the internal variable theory of structural superplaticity (SSP) proposed by Chang et al. The GBS flow could be formulated as a viscosity-type equation, characterized by the Newtonian exponent of 1.0. The unresolved issue of threshold stress is clarified and identified as a critical stress required for the GBS. The micro-mechanical roll of grain size refinement has also been manifested in terms of proposed constitutive parameters.

Abstract: A series of tensile and load relaxation behaviors in B2 type β-CuZn alloy have been examined to clarify the strain rate & the temperature dependence of yield stress anomaly. As strain rate increased, the anomalous peak moved to a higher stress and temperature region. The anomalous yielding region below peak temperature (Tp) revealed slightly positive strain rate dependence, while the region above Tp showed considerably positive strain rate dependence. Based on the different tendency of temperature and strain rate dependence of anomalous yielding, two types of deformation modes were suggested and incorporated into the yield stress vs. temperature curve analysis, which was in accord well with the experimental results. The less sensitive strain rate dependence of yield stress anomaly indicated that the yield stress anomalous behavior in β-CuZn alloy mainly seemed to be attributed to the vacancy hardening.

Abstract: The spray forming process was used to produce Al-25Si alloy in this study. The microstructure of the hypereutectic Al-25Si alloy appeared to consist of Al matrix and equiaxed Si particles with average diameter of 5~7 µm. To deduce the extrusion condition, a series of compression tests has been conducted at temperatures ranging from RT to 500oC. The strain rate sensitivity parameter (m) of this alloy has been found to be very low ( ≤0.1) below 300oC and reached maximum value of about 0.13 at 500°C. The extrusion has been successfully conducted at the temperatures of 300°C and above with the ratio of area reduction of 28 and 40. The processing map for hot working has also been constructed.

Abstract: In the present study, PIM process has been employed to produce sintered specimens with the various porosities from the initial stage of sintering and to the stage of nearly full density. A series of tensile tests has been conducted on these specimens at room temperature to elucidate the effect of a wide range of porosity. The material used in this study was 17-4 PH stainless steel. Based on the results of tensile tests on PIMed specimens with the various porosity, a new approach to predict the strength and the elongation of sintered materials has been carried out and a new framework combining neck growth model and ideal pore model has been established. By applying this new model to not only the results obtained in this study but also to previously published data, its generality and validity were sufficiently verified.