Microstructural Evolution and Mechanical Response of Equal-Channel Angular Extrusion-Processed Al-40Zn-2Cu Alloy
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TRODUCTION
ZINC-ALUMINUM (Zn-Al) alloys are widely known as bearing materials with their excellent castability, high resistance to wear, and low cost.[1] However, mechanical properties of as-cast or wrought Zn-Al alloys such as tensile strength, ductility, and impact toughness are not adequate for various other engineering applications. Past works showed that the tensile strength of Zn-Al alloys could be improved by thermal processing,[2] the addition of alloying elements such as Cu[3,4] and Si,[5,6] and conventional plastic forming methods. Unfortunately, their ductility remained insufficient for structural applications even after these processes. Equal-channel angular extrusion (ECAE) has emerged as a widely known procedure for the formation of ultrafine-grained materials in bulk form.[7,8] The ECAE-processed materials usually demonstrate extraordinary mechanical properties such as high strength and good ductility.[9] In addition, other technologically important properties including fatigue strength, wear resistance, and superplastic forming capability were notably improved via ECAE processing.[10–13] G. PURCEK, Associate Professor, and O. SARAY, Ph.D. Student, are with the Department of Mechanical Engineering, Karadeniz Technical University, 61080 Trabzon, Turkey. Contact e-mail: [email protected] I. KARAMAN, Associate Professor, and M. HAOUAOUI, Doctor, are with the Department of Mechanical Engineering, Texas A&M University, College Station, TX 77843. Manuscript submitted October 26, 2008. Article published online August 20, 2009 2772—VOLUME 40A, NOVEMBER 2009
Previous studies on the ECAE processing of Zn-Al alloys have mainly focused on eutectic (Zn-8Al),[14] eutectoid (Zn-22Al),[15] and monotectoid (Zn-40Al)[16] compositions. The ECAE was also applied successively to commercial Zn-Al alloys, namely, ZA-8,[11] ZA-12,[17] and ZA-27[18] (where the digits in the alloy designations indicate their approximate Al content in weight percent). These studies showed that ductility and superplasticity under high strain rates and at relatively low temperatures can be improved with some increase or slight decrease in the strength. Elimination of dendritic as-cast microstructure, reduction in the volume fraction of microporosities, and deformation-induced chemical and microstructural homogenization have been proposed to be the responsible mechanisms for such mechanical property evolution.[11,16] Softening of binary Zn-Al alloys was attributed to the processing at relatively high temperatures due to their poor roomtemperature ductility except eutectic and eutectoid compositions.[11,14–20] Processing at an elevated temperature, however, may cause recrystallization of the microstructure, especially at high deformation rates. Thus, microstructural refinement and mechanical strengthening can be suppressed. It is well known that room-temperature ductility and the melting point of binary Zn-Al alloys increase with increasing Al content.[21,22] Thus, Al-rich Zn-Al alloys can be subjected to ECAE at lower homologous temperatures. As a re
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