Effects of Rare Earth Element Additions on the Impression Creep Behavior of AZ91 Magnesium Alloy
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AN excellent combination of superior castability, good corrosion resistance, and acceptable mechanical properties is the main reason for the great attention paid to Mg-Al alloys. In these alloys, aluminum contents in excess of 2 wt pct result in the formation of a network of Mg17Al12 precipitates along grain boundaries.[1,2] Due to their low melting point, which ranges from 731 K maximum to the eutectic temperature between the precipitates and Mg, 710 K, these precipitates are thermally unstable and, thus, they can accelerate grain boundary diffusion, resulting in considerable instability of the microstructure in regions adjacent to grain boundaries.[3,4] This makes the high Al-content Mg alloys unsuitable for applications involving high temperature and load. Specifically, their susceptibility to creep limits their allowable temperature to less than 398 K. The AZ91 alloy containing 9 wt pct Al and 1 wt pct Zn is the most commonly used cast magnesium alloy for low-temperature applications. In this alloy, in addition to the coarsening and dissolution of Mg17Al12 precipitates at high temperatures, cellular discontinuous precipitation of this phase can also deteriorate creep resistance during aging. It is believed that this type of precipitation multiplies the grain boundary area, providing more surfaces for easy deformation regimes such F. KABIRIAN, Research Assistant, and R. MAHMUDI, Professor, are with the School of Metallurgical and Materials Engineering, University of Tehran, Tehran, Iran. Contact e-mail: mahmudi@ut. ac.ir Manuscript submitted January 3, 2009. Article published online July 14, 2009 2190—VOLUME 40A, SEPTEMBER 2009
as grain boundary sliding at high temperatures.[5] Most of the attempts for improving the creep resistance of this alloy have concentrated on suppressing discontinuous precipitation of Mg17Al12 and creating thermally stable intermetallics in the magnesium matrix. For example, the addition of 0.35 wt pct Sb to AZ91 results in the formation of rod-shaped, thermally stable Mg3Sb2 intermetallics at grain boundaries, causing significant increase in creep resistance of the alloy.[6] Similarly, Bi addition has been reported to form the thermally stable Mg3Bi2, which improves the creep resistance of the AZ91 alloy.[7] Although both Sb and Bi refine the microstructure and suppress the discontinuous precipitation of the b-Mg17Al12 phase during aging, they have almost no effect on the volume fraction of Mg17Al12 in the as-cast condition. In contrast to Sb and Bi, there are some other alloying elements such as Zr, Ca, and rare earth (RE), which reduce the amount of the Mg17Al12, by consuming the aluminum content of the alloy to form Al-containing intermetallic compounds. Kabirian and Mahmudi[8] studied the effects of Zr additions on the microstructural stability of cast AZ91 after long-time isothermal aging. They showed that the formation of thermally stable Al2Zr and Al3Zr2 particles is accompanied by significant reduction in the volume fraction of the Mg17Al12 phase in the as-cast condition. Thi
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