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Abstract

In this study, we present a new method for obtaining the parameters of the Johnson-Mehl-Avrami-Kolmogorov equation for dynamic recrystallization grain size. The method consists of finite-element analysis and optimization techniques. An optimization tool iteratively minimizes the error between experimental values and corresponding finite-element solutions. Isothermal backward extrusion of the AA6060 aluminum alloy was used to acquire the main parameters of the equation for predicting DRX grain size. We compared grain sizes predicted using optimized and reference parameters with experimental values from the literature and found better agreement when the optimized parameters were applied.
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Abstract

Isothermal hot compression experiments were carried out using the Gleeble-1500D thermal mechanical simulator. The flow stress of the Cu-1%Zr and Cu-1%Zr-0.15%Y alloys was studied at hot deformation temperature of 550°C, 650°C, 750°C, 850°C, 900°C and the strain rate of 0.001 s–1, 0.01 s–1, 0.1 s–1, 1 s–1, 10 s–1. Hot deformation activation energy and constitutive equations for two kinds of alloys with and without yttrium addition were obtained by correlating the flow stress, strain rate and deformation temperature. The reasons for the change of hot deformation activation energy of the two alloys were analyzed. Dynamic recrystallization microstructure evolution for the two kinds of alloys during hot compression deformation was analyzed by optical and transmission electron microscopy. Cu-1%Zr and Cu-1%Zr-0.15%Y alloys exhibit similar behavior of hot compression deformation. Typical dynamic recovery occurs during the 550-750°C deformation temperature, while dynamic recrystallization (DRX) occurs during the 850-900°C deformation temperature. High Zr content and the addition of Y significantly improved Cu-1%Zr alloy hot deformation activation energy. Compared with hot deformation activation energy of pure copper, hot deformation activation energy of the Cu-1%Zr and Cu-1%Zr-0.15%Y alloys is increased by 54% and 81%, respectively. Compared with hot deformation activation energy of the Cu-1%Zr alloy, it increased by 18% with the addition of Y. The addition of yttrium refines grain, advances the dynamic recrystallization critical strain point and improves dynamic recrystallization.
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Abstract

In the current study, the hot deformation of medium carbon V-Ti micro-alloyed steel was surveyed in the temperature range of 950 to 1150°C and strain rate range of 0.001 to 1 s–1 after preheating up to 1200°C with a compression test. In all cases of hot deformation, dynamic recrystallization took place. The influence of strain rate and deformation temperature on flow stress was analyzed. An increase in the strain rate and decrease in the deformation temperature postponed the dynamic recrystallization and increased the flow stress. The material constants of micro-alloyed steel were calculated based on the constitutive equations and Zener-Hollomon parameters. The activation energy of hot deformation was determined to be 458.75 kJ/mol, which is higher than austenite lattice self-diffusion activation energy. To study the influence of precipitation on dynamic recrystallization, the stress relaxation test was carried out in a temperature range of 950 to 1150°C after preheating up to 1200°C. The results showed no a stress drop while representing the interaction of particles with dynamic recrystallization.
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