Effect of Ag Addition on the Hot Deformation, Constitutive Equations and Processing Maps of a Hot Extruded Mg-Gd-Y Alloy
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RODUCTION
MAGNESIUM alloys are considered the lightest structural metals with high specific strength, being suitable candidates for weight-saving purposes in automobile and aerospace industries.[1] The HCP crystal structure of Mg imposes limited slip systems that bring about poor formability at ambient temperature.[2] This drawback of Mg alloys has restricted their industrial applications. As a result, hot working of magnesium alloys through proper thermomechanical processes seems to be an adequate solution to overcome this A. REZAEI and R. MAHMUDI are with the School of Metallurgical and Materials Engineering, College of Engineering, University of Tehran, North Kargar Street, 14395-515 Tehran, Iran. Contact e-mail: [email protected] R. LOGE is with the Laboratory of Thermomechanical Metallurgy - PX Group Chair, Ecole Polytechnique Fe´de´rale de Lausanne (EPFL), 2002 Neuchaˆtel, Switzerland. Manuscript submitted April 20, 2020.
METALLURGICAL AND MATERIALS TRANSACTIONS A
adversity due to activation of non-basal slip systems.[3] Conventional magnesium alloys, however, suffer from low strength and low thermal stability at elevated temperature conditions.[4] Consequently, addition of rare earth elements (RE) has been suggested in order to enhance high-temperature properties, e.g., microstructural stability,[5,6] creep resistance[7,8] and strength.[9,10] Among Mg-RE alloys, those based on the Mg-Gd-Y system are prominent for their remarkable high strength because of the solubility of both Gd and Y atoms in magnesium and the presence of thermally stable precipitates with a wide range of varieties. Previous studies[11–15] have reported that a tensile strength of > 400 MPa can be easily achieved in the Mg-Gd-Y alloys through the aforementioned strengthening mechanisms. Hence, hot working of the Mg-Gd-Y alloys would be of great importance because of improving the post-deformation properties and performance of this class of materials. Accordingly, understanding the underlying mechanisms of hot deformation behavior and tuning the processing parameters such as strain rate and temperature are essential.
There are several studies on correlating the flow stress with hot deformation parameters of Mg-Gd-Y alloys through constitutive equations, most of these models being established on the basis of the hyperbolic-sine relationship. Moreover, developing processing maps based on the dynamic material model (DMM) has become a useful tool to investigate the hot workability and identify the safe deformation windows according to microstructural evolutions.[16] Many attempts have been made to develop the processing maps for assessing the safe deformation zones of the cast Mg-Gd-Y alloys through DMM.[3,17–19] For instance, Li and Zhang[19] studied the occurrence of flow instability domains in processing maps at low temperatures and high strain rates during hot compression of a cast Mg-9Gd-4Y-0.6Zr alloy, where the formation of elongated deformation bands was recognized as the key reason for the unstable flow. Nevertheless, there are limited stu
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