Fatigue and Tensile Behavior of Cast, Hot-Rolled, and Severely Plastically Deformed AZ31 Magnesium Alloy
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NTRODUCTION
SUFFICIENT fatigue resistance is an important requirement on engineering materials used for components subjected to cyclic loading. The demand for improving the efficiency of automotive vehicles, particularly by weight reduction, results in tougher requirements on fatigue resistance of Mg alloys as candidate materials for automotive parts. To that end, a better knowledge of fatigue behavior and damage processes in relation to the microstructure of Mg alloys is necessary. The microstructure of Mg alloys and the attendant mechanical properties strongly depend on the method of production. The Mg alloy AZ31 is often produced as a high pressure cast material, because it exhibits excellent castability. Squeeze casting (SC) consisting of two casting steps is one of the most frequently applied procedures. Plastic forming of magnesium alloys is difficult because of their hexagonal close-packed structure and limited number of slip systems, but forming is possible at elevated temperatures. Wrought alloys Z. ZU´BEROVA´, T.T. LAMARK, Cand. Scient. Phys., and Y. ESTRIN, Professor, are with the Institute of Materials Science and Technology, Technical University Clausthal, 38678 ClausthalZellerfeld, Germany. Contact e-mail: [email protected] L. KUNZ, Professor, is with the Institute of Physics of Materials, Academy of Sciences of the Czech Republic, 616 62 Brno, Czech Republic. M. JANECˇEK, Associate Professor, is with the Faculty of Mathematics and Physics, Charles University, 121 16 Prague 2, Czech Republic. This article is based on a presentation made in the symposium entitled ‘‘Ultrafine-Grained Materials: from Basics to Application,’’ which occurred September 25–27, 2006 in Kloster Irsee, Germany. Article published online April 25, 2007. 1934—VOLUME 38A, SEPTEMBER 2007
exhibit improved mechanical properties, namely, higher tensile strength and yield stress, as compared to the conventionally cast alloys.[1,2] Another often used production process is hot rolling (HR), which provides substantially better basic mechanical properties, including the fatigue life. Wrought Mg alloys have had a much more limited use than the high-pressure die castings. To date, the knowledge of the fatigue behavior of wrought alloys is surprisingly limited. During the last 10 years, some attempts have been made to process Mg alloys by means of equal-channel angular pressing (ECAP). The microstructure produced by severe plastic deformation by ECAP strongly depends on the temperature regimes and the details of the ECAP process, e.g., on the die geometry. Low ECAP processing temperature results in smaller grain size as compared to the pressing at higher temperatures.[3,4] Considerable grain refinement by ECAP was found for AZ31. The resulting grain size after four ECAP passes was about 2 lm.[1,5] By comparison, the extruded alloy usually has a grain size of about 50 lm. This observation is in agreement with the results published in References 6 through 9. ECAP leads to the grain refinement through recrystallization, and this process ma
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