Low-Cycle Fatigue Behavior of Die-Cast Mg Alloys AZ91 and AM60
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IN recent years, magnesium alloys have gained renewed interest due to their low density and high specific strength. For automotive applications, decreasing the weight is important to improve fuel economy. For example, replacing steel and aluminum automotive components with magnesium could result in a 0.25 L and 0.1 L per 100 km reduction in fuel consumption, respectively.[1] According to one report, North American automotive manufacturers hope to substitute 154 kg of magnesium components for 286 kg of current ferrous and aluminum parts by 2020.[2] The Mg-Al-Zn-Mn alloy AZ91 (Mg-9Al-0.7Zn0.13Mn wt pct) has good castability, mechanical properties at ambient temperature, and corrosion resistance. It is the most commonly used magnesium die-casting alloy.[3] AM60 (Mg-6Al-0.13Mn wt pct) has slightly LUKE H. RETTBERG, Graduate Student, Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48105, is now PhD Graduate Student with the University of California, Santa Barbara, CA. Contact e-mails: [email protected]; rettberg@umail. ucsb.edu J. WAYNE JONES, Professor, is with the Materials Science and Engineering, University of Michigan. J. BRIAN JORDON, Assistant Professor, is with the Mechanical Engineering, University of Alabama, Tuscaloosa, AL 35487. MARK F. HORSTEMEYER, Chair Professor, is with the Department of Mechanical Engineering, Mississippi State University, Mississippi State, MS 39762. Manuscript submitted March 22, 2011. Article published online March 22, 2012 2260—VOLUME 43A, JULY 2012
lower yield and tensile strengths but greater ductility and toughness when compared with AZ91, making it more desirable for certain applications. However, before these and similar alloys can be used for structural components in automobiles, their fatigue behavior must be better understood. The development of more comprehensive fatigue models requires that the cyclic stress– strain properties be known and the impact of heat treatment, especially on fatigue behavior, be understood. In general, the establishment of design criteria to protect against component failure by fatigue relies on information obtained from fatigue testing. In a defect–free magnesium alloy, fatigue cracks initiate from the slip bands that are formed by the movement of dislocations during cyclic microplastic deformation.[4] Dislocation motion primarily occurs on basal slip planes (0001) in the h1120i directions at room temperature.[5] However, high–pressure, cast magnesium alloys contain porosity as a result of a large solidification range and turbulent flow during mold filling. These defects create regions of high stress concentration that make pores the predominant crack initiation sites.[6–8] It has been reported by Gall et al.[9] that pore size influences the likelihood of fatigue crack formation in a cast A356 aluminum alloy, whereas Wang et al.[10] reported that the presence of porosity decreases fatigue life by at least an order of magnitude when compared with a defect–free A356-T6. Mayer et al.[7] and Horstemeyer[11] concluded that fatigu
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