Papers by Author: Horng Yu Wu

Abstract: The flow behavior and associated microstructural changes of wrought 6069 Al alloy deformed in tension were analyzed in this work. Tensile tests were conducted on an extruded tube with a thickness of 1.6 mm in the temperature range of 300–500 oC, with initial strain rates from 0.001 to 0.1 s^-1. The true stress–true strain curves exhibited a peak stress at a critical strain. The overall level of the flow curve increased when the strain rate was increased and/or the temperature was decreased. The flow curves exhibited a typical flow behavior with dynamic softening and showed that the softening degree after reaching the peak stress was dependent on the deformation conditions. This could be related to the softening mechanism. The main softening mechanism of the alloy was dynamic recovery (DRV) at low temperatures; dynamic recrystallization (DRX) occurred as deformed at high temperatures.

Abstract: The compression deformation behavior and constitutive relation of Hastelloy X superalloy were investigated in the temperature range of 950 °C to 1150 °C and strain rate range of 0.001 s^-1 to 1 s^-1 on a Gleeble-3500 thermo-simulation machine. The obtained experimental stress-strain data were used to establish strain-dependent constitutive equations. The correlation between the strain-dependent constitutive parameters and flow behavior was analyzed. Comparisons between the experimental and predicted results showed that the developed constitutive equation could be used to simulate numerically the flow stresses of Hastelloy X superalloy at any strains under hot deformation.

Abstract: Hot deformation characteristics of 316 stainless steel were investigated at elevated temperatures. Hot compressive tests were carried out in the temperature and strain rate ranges from 900 to 1100 °C and 1 × 10⁻¹ to 1 s^–1, respectively. Correlation between the flow behavior and the microstructural evolution was analyzed. The flow behavior showed that the softening mechanisms were related to the dynamic recovery (DRV), dynamic recrystallization (DRX), and grain growth. Flow behavior analyses and microstructural observations indicated that DRV was the major softening mechanism at high strain rates and low temperatures. Dynamic softening proceeded via a combination of DRV and DRX at intermediate strain rates and temperatures. The contribution of DRV to the softening effect decreased with decreasing strain rate (or increasing temperature). Grain growth was the major softening effect at low strain rates and high temperatures.

Abstract: Hot deformation characteristics of Hastelloy X Ni-base superalloy were investigated at elevated temperatures. Hot compressive tests were carried out in the temperature and strain rate ranges from 900 to 1150 °C and 0.001 to 1 s–1, respectively. The constitutive equation relating flow stress, temperature, and strain rate was obtained based on the peak stresses. The flow behavior showed that the softening mechanisms were related to the dynamic recovery (DRV) and dynamic recrystallization (DRX). The flow stress of Hastelloy X was fitted well by the constitutive equation of the hyperbolic sine function. The constitutive analysis suggested that the hot deformation mechanism of the Hastelloy X was dislocation creep.

Abstract: The flow behavior and associated structural changes of an Inconel 600 superalloy were analyzed by using hot compression tests in the temperature and strain rate ranges of 850–1200 °C and 0.001 to 10 s^–1, respectively. The stress–strain curves exhibited the trend typical of materials in which deformation is recovery-controlled at high strain rates and low temperatures, while at low strain rates and high temperatures, the flow curves exhibited a softening typical of recrystallization phenomena. Constitutive analysis was carried out to investigate the hot deformation mechanism using the hyperbolic sine law.

Abstract: The flow behavior and associated structural changes of an AZ61 Mg alloy were analyzed by using hot compression tests in the temperature and strain rate ranges of 250–400 °C and 0.001 to 1 s^–1, respectively. The stress–strain curves exhibited the trend typical of materials in which deformation is recovery-controlled in the high Z regime (Z is the Zener–Hollomon parameter), while at low strain rates and high T, the flow curves exhibited a softening typical of recrystallization phenomena. Microstructure analysis has been performed to correlate the microstructure changes to the flow behaviors.

Abstract: This work examined the effects of Li content on the strain-hardening behaviors of three varieties of Mg−Li−Zn alloys containing approximately 6 wt%, 9 wt%, and 10 wt% of Li. Tensile tests were carried out on specimens in the directions of 0, 45 and 90° to the rolling direction. Kocks–Mecking type plots were constructed to illustrate different stages of strain-hardening. The cold-rolled Mg−6Li−1Zn (designated as LZ61) alloy sheet showed stage II and stage III strain-hardening behaviors at room temperature. The specimens of Mg−9Li−1Zn (designated as LZ91) and Mg−10Li−1Zn (designated as LZ101) alloy sheets did not show stage II strain-hardening. Higher initial strain-hardening rates were observed in the 90° direction for these alloys as a result of the cold-rolled fibrous structure affording stronger barriers to dislocation movements in this direction.