Stress-Relaxation Behavior of Magnesium-3Gadolinium-2Calcium-Based Alloys at Elevated Temperatures
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INTRODUCTION
DUE to their lightness and abundance, magnesium (Mg) alloys, and cast Mg alloys in particular, have attracted increased interest from industry sectors where mass reduction is critical.[1–3] One of the major issues that limit the wider engineering application of magnesium alloys is their relatively low strength and high-temperature creep resistance.[4] In recent years, considerable effort has been made to develop high-strength and high-creep-resistant cast Mg alloys. In general, rareearth elements (REs), with gadolinium (Gd) in particular,[5] are regarded as the most effective additions to enhance creep resistance.[6,7] The strengthening has been ascribed to the formation of metastable coherent precipitates with aging.[8] Binary Mg-Gd-based alloys have been reported to have a creep resistance that is superior to the WE43 and QE22 alloys.[5] However, binary Mg-Gd alloys that contain less than 10 wt pct (1.69 at. pct) Gd show little precipitation-hardening response after aging.[9] A further enhancement of strength and creep resistance at elevated temperature requires a Gd addition of up to 20 wt pct and/or elemental scandium, yttrium (Y), or neodymium (Nd)
NING MO, QIYANG TAN, and MING-XING ZHANG are with the School of Mechanical and Mining Engineering, The University of Queensland, St Lucia, QLD, 4072, Australia. Contact e-mail: [email protected] BIN JIANG and FUSHENG PAN are with the College of Materials Science and Engineering, Chongqing University, Chongqing, 400030, China. Manuscript submitted January 29, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
addition,[10–13] which leads to an unaffordable cost and an increased density. Hence, there is commercial interest to decrease the Gd content. Nie et al.[7] reported a Mg-6Gd-0.6zirconium (Zr) alloy with 2 wt pct Zn addition, and a lower Gd content (0.98 at. pct) with a remarkably enhanced age-hardening response and creep strength. Recently, Abaspour and Caceres[14] investigated the high-temperature compression and stress-relaxation behaviors of a number of solution-treated cast binary Mg alloys with dilute concentration, and concluded that the improvement by the solutes on the creep resistance of Mg alloys relies on the relative tendency of the solutes to develop a short-range order (SRO).[14,15] According to their tendency to develop a SRO, the solutes involved were ranked as[16]: Y, Gd > Nd, Ca > Zn, silver, tin (Sn) > aluminum (Al) > antimony, strontium According to this order, Y and Gd have the highest preference to develop a SRO, and therefore to enhance the creep resistance most effectively, followed by Nd and Ca. Among these alloying elements, Ca has the lowest cost and a lower density than Mg. Thus, Ca is a promising element that could replace REs in the development of new creep-resistant Mg alloys with a lower cost and that are compatible with an improved performance. The aim of this work was to investigate the stress-relaxation behavior of a number of Mg-3Gd-2Ca-based alloys, which were designed based on the Mg-6Gd-2Zn-0.6Zr alloy[7
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