The Serrated Flow Behavior of Mg-Gd(-Mn-Sc) Alloys
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MAGNESIUM alloys containing rare-earth elements are attractive lightweight materials for aerospace and aircraft applications due to their high specific strength and excellent mechanical properties at elevated temperatures.[1–6] The Mg-Gd system is one of the most promising candidates for developing precipitation hardenable magnesium alloys. Mg-Gd alloys with Gd contents greater than 10 wt pct exhibit tensile properties and creep resistance that are comparable to, or better than, those of commercial WE54 and WE43 alloys.[7–13] In recent years, Mordike and Smola et al.[14–17] developed a new group of creep-resistant Mg-Gd materials microalloyed with Mn and Sc. These newly developed Mg-Gd-Mn-Sc alloys have superior creep resistance, which reportedly results from the formation of a thermally stable Mn2Sc phase. This Mn2Sc phase forms as fine platelets on the basal plane of the Mg matrix and does not dissolve at temperatures up to 570 C. While these Mg-Gd based casting alloys exhibit excellent creep resistance, their microstructures and deformation behavior have not been studied in detail. An improved understanding of microstructures and deformation behavior of the Mg-Gd based alloys is of critical importance for any further enhancement in mechanical properties at ambient and elevated temperatures. In this study, a Mg-0.8 at. pct Gd alloy and a Mg0.8 pct Gd-0.5 pct Mn-0.45 pct Sc (at. pct) alloy were selected as model alloys. These two alloys were hot X.Y. FANG, Postgraduate Student, is with the Department of Materials Engineering, Monash University, Victoria 3800, Australia, and the School of Materials Science and Engineering, Central South University, Changsha 410083, P.R. China. D.Q. YI, Professor, is with the School of Materials Science and Engineering, Central South University. J.F. NIE, Professor, is with the Department of Materials Engineering, Monash University, Victoria 3800, Australia. Contact e-mail: [email protected] Manuscript submitted March 22, 2009. Article published online August 27, 2009 METALLURGICAL AND MATERIALS TRANSACTIONS A
extruded and the tensile deformation behavior of the extruded samples was systematically characterized. It is found that both alloys exhibit serrated flow, i.e., the Portevin–Le Chatelier (PLC) effect, when they are tested in certain temperature and strain rate regimes. The original reports on serrated flow of magnesium alloys, Mg-0.05 at. pct Th (Mg-0.5 wt pct Th)[18,19] and Mg-2.41 at. pct Ag (Mg-10 wt pct Ag) alloy,[20,21] were published in the 1960 and 1970s. However, the serration phenomenon in magnesium alloys has not attracted any attention until recent years. In the past few years, the serrated flow has been reported to occur in several magnesium alloys, including WE54, Mg-Ca-Zn-Zr, AZ91, Mg-Nd, Mg-Li(-Al), ZE43, and Mg-Y.[22–28] The dynamic strain aging (DSA) model is used to explain the serration in the WE54, Mg-Ca-Zn-Zr, and ZE43 alloys. These three alloys exhibit serrations when they are tested in the peakaged condition. It was pointed out[22,23,27] that ther
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