Precipitation in a Ag-Containing Mg-Y-Zn Alloy
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THE development of magnesium alloys with excellent creep resistance and sufficiently high strength at elevated temperature is of the highest importance to meet the requirement of the automobile, aerospace, and aircraft industries.[1,2] Rare-earth (RE) containing magnesium alloys have been recognized as such candidates and thus have received considerable attention in recent years.[3] Most of Mg-RE-based alloys are precipitation hardenable, and the metastable precipitate phases improve the strength and creep resistance of the alloys. Typical commercial and experimental examples are based on the Mg-Y, Mg-Nd, Mg-Gd, Mg-Y-Gd, Mg-Y-Nd, and Mg-Gd-Nd systems.[4–11] Their precipitation sequences have been relatively well established. In a recent study,[12] ternary additions of Zn to Mg-Y alloys were found to yield little improvement in age-hardening response. The microstructure of the ternary Mg-Y-Zn alloys usually contains intermetallic particles or precipitates with 18R and 14H structures.[13–19] The crystal structures of the 18R and 14H remain controversial. Our recent work[20] indicates that Y.M. ZHU, Post-Doctoral Fellow, and J.F. NIE, Professor, are with the Department of Materials Science and Engineering, Monash University, Melbourne, VIC 3800, Australia. Contact e-mail: jianfeng. [email protected] K. OH-ISHI, formerly Post Doc with the National Institute for Materials Science, 1-2-1 Sengen, Tsukuba 305-0047, Japan, is now Researcher with Toyota Central R&D Labs, Nagakute, Japan. N.C. WILSON, Research Scientist, is with CSIRO Manufacturing, Clayton, VIC 3168, Australia. K. HONO, NIMS Fellow and Managing Director of Magnetic Materials Center, is with the National Institute for Materials Science. A.J. MORTON, Senior Research Fellow, is with the Department of Materials Science and Engineering, Monash University, and also with CSIRO Manufacturing. Manuscript submitted July 29, 2015 Article published online December 14, 2015 METALLURGICAL AND MATERIALS TRANSACTIONS A
both 18R and 14H are made of building blocks that have a hexagonal structure with an ABCA stacking sequence of its close-packed planes. The Y and Zn atoms mainly enrich in two inner atomic layers. However, Yokobayashi et al.[21] and Egusa et al.[22] pointed out recently that the enrichment of Y and Zn atoms occurred in the consecutive four layers of the ABCA building blocks. It seems that the degree of chemical ordering of the 18R and 14H phases depends on the composition of the alloys and heat treatment conditions.[23] Precipitation hardening is the most effective way to improve alloy strength. In particular, it was found more than fifty years ago that the addition of Ag could considerably increase the age-hardening response and hence mechanical properties of Mg-Nd alloys and this led to the development of the commercial alloy QE22 (Mg-2.5 wt pctAg-2 wt pctNd-0.7 wt pctZr).[24] The precipitation process was reported to involve Guinier– Preston zones (G.P. zones), two metastable phases designated b and c which are responsible for the peak aging, and an equilibrium
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