A Two-Stage Physical-Based Model for Predicting Flow Stress of As-cast TiAl Alloy Under Hot Deformation Conditions
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A Two-Stage Physical-Based Model for Predicting Flow Stress of As-cast TiAl Alloy Under Hot Deformation Conditions Jingyuan Shen, Zhanglong Zhao, Zekun Yao, Yongquan Ning, Yuhang Xiong, and M.W. Fu (Submitted December 4, 2017; in revised form March 19, 2018) The hot deformation behavior of Ti-30Al-4.2Mn-4.5Nb-0.2B alloy was investigated using the isothermal compression experiment at temperatures of 1020-1200 °C and strain rates of 0.001-1 s21. The flow stress was sensitive to the deformation parameters like temperature and strain rate, which decreases with the increase in temperature and decrease in strain rates. Based on the true stress-true strain data, a two-stage physical-based model was proposed to describe the flow stress curve of as-cast TiAl alloy during hot deformation process. For establishing the model, at first, the flow curves of dynamic recovery (DRV) were modeled by employing stress-dislocation relation and adjusting dislocation annihilation coefficient X. Then, the flow curves of dynamic recrystallization (DRX) were modeled by considering the dynamic softening behavior into Avrami equation. Finally, the flow curves in the entire deformation stages could be described by embedding the predicted data of DRV model (i.e., flow stress before the critical strain) into the predicted data by DRX model (i.e., flow stress after the critical strain). The critical strain for initiation of DRX was determined by the double-differentiation method. To evaluate the applicability and effectiveness of DRX kinetics equation, the DRX curves were calculated and were consistent with the microstructure observation. Comparison between the experimental and predicted data shows that the proposed physical-based model can well forecast the flow stress under a wide working domain. Keywords
dynamic recrystallization, dynamical recovery, hot deformation, physical-based model, TiAl alloy
1. Introduction The gamma TiAl alloys are presently considered as one of the most potential structural materials for aerospace industry application because of the excellent properties, such as low density, high specific strength, high specific stiffness, good corrosion resistance and creep properties at elevated temperature (Ref 1-5). However, the low room temperature ductility and poor plastic deformability of TiAl alloys severely restrict their wide application (Ref 6-9). Thermo-mechanical processing, such as hot extrusion and hot forging, is an effective and feasible method to improve the toughness and formability of TiAl alloys by converting as-cast coarse-grain microstructure to refined and homogeneous microstructure (Ref 4, 10). In order to produce defect-free and homogeneous TiAl alloys, it is essential to optimize thermo-mechanical processing parameters (Ref 11-14). Constitutive model is one of the basic steps to understand the flow behavior of materials during hot deform-
Jingyuan Shen, Zhanglong Zhao, Zekun Yao, Yongquan Ning, and Yuhang Xiong, School of Materials Science and Engineering, Northwestern Polytechnica
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