Variations of Microsegregation and Second Phase Fraction of Binary Mg-Al Alloys with Solidification Parameters
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THE Mg-Al alloys like AZ (Al and Zn), AS (Al and Si), AE (Al and RE), and AM (Al and Mn) are common cast Mg alloys used for automotive and aerospace applications.[1] Typically, die casting and gravity casting followed by little or no heat treatments are used for the production of cast Mg components. As second phase formation is an inevitable phenomenon during any casting process, many researchers[2–7] have related the secondary phase fraction with the final mechanical properties of Mg-Al-based alloys. Specifically, the Mg17Al12 (b) phase is found to be thermally unstable at elevated temperatures and, therefore, has been found responsible for poor creep resistance.[1] Another issue that is concurrent with any casting process is the chemical inhomegeneity or microsegregation of the solute elements in the solidified microstructure. The extent of microsegregation may directly affect the mechanical properties of the final products and would require post-solidification processes like annealing and homogenization. The solidification conditions in a casting process are directly related to the extent of microsegregation and total amount of secondary phases in the cast microstructure. The control of second phase fraction during solidification is one of the key research areas for Al alloys. Eskin et al.[8] have summarized the previous studies for Al alloys on the dependence of second phase fractions MANAS PALIWAL, Ph.D. Student, and IN-HO JUNG, Associate Professor, are with the Dept. Mining and Materials Engineering, McGill University Montreal, Montreal, QC, Canada. Contact e-mail: [email protected] DAE HOON KANG, formerly Post-doctoral Fellow with Dept. Mining and Materials Engineering, McGill University Montreal, is now Researcher with Novelis Global Research and Technology Center, Kennesaw, GA. ELHACHMI ESSADIQI, formerly Scientist with Canmet Materials, Ottawa, ON, Canada, is now Professor with the Universite International de Rabat, Rocade de Rabat, Morocco. Manuscript submitted August 15, 2013. Article published online March 20, 2014 3308—VOLUME 45A, JULY 2014
with cooling rates. However, no conclusive reason has been offered on the trend of second phase fraction with cooling rate observed in all the previous studies. For example, several key previous studies on Al solidification are summarized in Table I. Most of the significant works were performed for Al-Cu alloys. An increasing tendency of secondary phase (Al2Cu) fraction with cooling rate was observed by Taha et al.[9] and Michael and Beaver[10] up to several 10 K/s. Sarreal and Abbaschian[11] reported that the amount of secondary phase increased until the cooling rate of 190 K/s and then decreased. The results by Du et al.,[12] Novikov and Zoloterveskii,[13] and Kasperovich et al.[14] showed the same trend as Sarreal and Abbaschian,[11] but the maximum in the second phase fraction was observed at the cooling rate of 1 to 3 K/s instead of 190 K/s. Liu and Kang[15] investigated the solidification of Al-Mg alloys up to the cooling rate of 167 K/s and reported the maximu
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