Correlation of Microstructure and Electrochemical Corrosion Behavior of Squeeze-Cast Ca and Sb Added AZ91 Mg Alloys
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TRODUCTION
MAGNESIUM (Mg) alloys with superior specific strength have the potential to replace the dominant aluminum alloys and steels in the automobile sector leading to improved fuel efficiency. The potential expansion fields for Mg alloys are the powertrain applications in automobiles where resistance to creep deformation over the temperature range of 423 K to 573 K (150 C to 300 C) is a foremost prerequisite.[1] Several Mg alloys are currently being used in the automotive industry. The most widely used and cost-effective commercial Mg alloy for automotive applications is AZ91. This alloy exhibits superior die-castability in combination with excellent mechanical and corrosion properties at room temperature. Nevertheless, it undergoes significant creep deformation beyond 393 K (120 C), and thus, it is not suitable for powertrain applications. To make the AZ91 alloy suitable for this application, attempts have been made to incorporate thermally stable intermetallics in its microstructure by the addition of various alloying elements. The addition of bismuth (Bi), calcium (Ca), holmium (Ho), lead (Pb),
A.K.S. BANKOTI, A.K. MONDAL, and B.C. RAY are with the Department of Metallurgical and Materials Engineering, National Institute of Technology, Rourkela-769008, India. CHANDRA S. PERUGU and S. KUMAR are with the Department of Materials Engineering, Indian Institute of Science, Bangalore-560012, India. Contact e-mails: [email protected], [email protected] Manuscript submitted December 14, 2016. Article published online August 2, 2017 5106—VOLUME 48A, OCTOBER 2017
antimony (Sb), silicon (Si), strontium (Sr), and rare earth elements (RE) to AZ91 alloy have been reported to enhance its creep and elevated temperature tensile properties to a considerable extent.[2–6] Yet the corrosion resistance of the AZ91 alloy is significantly affected as a result of the presence of intermetallic phases resulting from elemental additions. The studies on corrosion response of the AZ91 alloy following elemental additions are limited. Wu et al.[3] studied the corrosion behavior of the AZ91 alloy with the additions of RE and Ca. They reported that the additions of RE and Ca in the AZ91 alloy resulted in an improved corrosion response by formation of the reticular Al2Ca phase that acted as an effective barrier against corrosion. Fan et al.[4] investigated the role of cerium (Ce) on the modification of the microstructure, mechanical, and corrosion properties of the AZ91D alloy. They concluded that the addition of Ce refined the b-Mg17Al12 phase and formed a new Al4Ce phase, which significantly improved the corrosion behavior of the AZ91D alloy. The corrosion behavior of Ho added AZ9l alloy in 3.5 pct NaCl was examined by Zhou et al.[6] They reported that the Ho addition enhanced the corrosion resistance of the AZ9l alloy as a result of the easy oxidation and passivation of the Ho, Al, Mg, and Mn elements present in the Ho containing phase. Furthermore, the addition of Ho diminished the concentration of Fe and fraction of the b-Mg17Al12 phase
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