Effects of Zr Additions on the Microstructure and Impression Creep Behavior of AZ91 Magnesium Alloy
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TRODUCTION
THE need of aerospace and automobile industries to light materials has led to considerable attention of materials researchers into magnesium alloys. Some key properties that cause this great interest in magnesium alloys are low density and good room-temperature mechanical properties.[1,2] Among magnesium alloys, those based on Mg-Al are the most promising for more development, especially in the automobile industry because of acceptable mechanical properties, good corrosion resistance, and excellent castability.[3] Although increasing the aluminum content of Mg-Al alloys results in a significant increase of room-temperature mechanical properties, there is some evidence that this would cause a reduction of creep resistance at T > 398 K (125 C).[3–5] The suitable room-temperature mechanical properties of Mg-Al alloys is generally attributed to the formation of large amounts of Mg17Al12 precipitates along grain boundaries in addition to the solid solution hardening of Al solute atoms in the Mg matrix. The low melting point of the Mg17Al12 phase (about 710 K (437 C)), however, leads to its coarsening and dissolution at temperatures lower than 0.5Tm of Mg. This results in the instability of microstructure, especially in regions near grain boundaries at temperatures above 398 K (125 C) and, thus, deterioration of creep properties of the Mg-Al alloys.[6] The use of alloying elements, which are capable of forming thermally stable intermetallics rather than the F. KABIRIAN, Research Assistant, and R. MAHMUDI, Professor, are with the School of Metallurgical and Materials Engineering, University of Tehran, PO Box 11155-4563, Tehran, Iran. Contact e-mail: [email protected] Manuscript submitted February 16, 2010. Article published online September 2, 2010 3488—VOLUME 41A, DECEMBER 2010
unstable b-Mg17Al12, is known as the major solution to develop Mg-Al alloys for high-temperature applications. Although some alloying elements such as Bi and Sb, respectively, can establish thermally stable compounds of Mg3Bi2 and Mg3Sb2 in the AZ91 alloy and improve creep resistance, they cannot suppress the formation of Mg17Al12 particles.[7,8] In contrast to Sb and Bi, there are some other alloying elements such as Ca and RE, which reduce the amount of Mg17Al12, by consuming the aluminum content of the alloy to form the Al-containing Al2Ca and Al11RE3 intermetallic compounds, respectively. The high melting point of these compounds and, thus, their high thermal stability are believed to be responsible for the improved creep resistance of the Mg-Al-Ca[9–11] and Mg-Al-RE alloys.[12,13] Similar to Ca and RE, some transition elements such as Zr have high affinity for Al to form Al-Zr compounds. The high thermal stability of these intermetallics may make them a suitable choice for hightemperature applications. However, the use of Zr in Mg alloys has been confined to pure Mg and Mg-Zn based alloys, mainly aiming at improving mechanical properties by grain refinement for superplasticity[14,15] and identifying creep mechanisms.[16,17] Concerning
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