Low-Cycle Fatigue of Ultra-Fine-Grained Cryomilled 5083 Aluminum Alloy
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INTRODUCTION
ULTRA-FINE-GRAIN (UFG) and nanocrystalline (NC) materials have generated both practical and scientific interest because of the novel properties that these materials exhibit. Research on bulk UFG metals, formed by severe plastic deformation (SPD) methods, has been driven by the desire to make alloys with superior strength consistent with the Hall–Petch relationship.[1] Research has shown that various UFG materials exhibit different hardening and deformation mechanisms when subjected to monotonic loading.[2,3] However, the application of UFG materials in mechanical or structural designs is not only dependent on strength but also on properties such as corrosion resistance, ductility, fatigue resistance, formability, machinability, and welding characteristics. Fatigue resistance is a measure of a material’s ability to resist damage accumulation and crack initiation under cyclic loading. Since fatigue resistance is dependent on the microstructural mechanisms controlling plastic deformation and damage accumulation, prior research has raised questions of whether the fatigue resistance of these novel UFG alloys would be adequate for many applications.[4] In addition, damage accumulation is a J.L. WALLEY, Graduate Student, is with the Department of Materials Science and Engineering, The Ohio State University, Columbus, OH 43210. E.J. LAVERNIA and J.C. GIBELING, Professors, are with the Department of Chemical Engineering and Materials Science, University of California, Davis, CA 95616. Contact e-mail: [email protected] Manuscript submitted July 25, 2008. Article published online September 12, 2009 2622—VOLUME 40A, NOVEMBER 2009
function of plastic deformation, and as such, low-cycle fatigue testing at high stress levels, under constant plastic strain control, would be advantageous for further study into the active deformation mechanisms in UFG alloys. Methods for creating UFG alloys include but are not limited to SPD by equal channel angular extrusion (ECAE), high pressure torsion, and high energy ball milling, also known as mechanical alloying.[5] Although limited research has been conducted into the fatigue behaviors of these materials, most published studies describe work conducted with ECAE materials.[4,6–12] Low-cycle fatigue testing of ECAE metals has led to the conclusion that the fatigue lives of these UFG metals are only 20 to 50 pct of their coarse grain conventional counterparts, although a recovery heat treatment of these metals increases their fatigue lives.[4] Research into monotonic properties has shown that processing methods greatly change the final mechanical properties of UFG materials,[12,13] so it is not possible to make generalized predictions regarding the fatigue behavior of all UFG materials based solely on data collected for ECAE materials. Therefore, the goal of this study was to examine the fatigue resistance and deformation mechanisms active during low-cycle fatigue testing of two UFG AA5083 alloys created by the high-energy ball milling process known as cryomilling. The cryomillin
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