On the Equilibrium Intermetallic Phase in Mg-Nd-Ag Alloys
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TRODUCTION
RARE-EARTH (RE) containing magnesium alloys have received considerable attention due to their excellent age hardening response and creep resistance.[1–4] Nd is one of the most important RE elements and Mg-Nd binary alloys exhibit remarkable age hardening response during isothermal aging process.[5–7] The decomposition of supersaturated solid solution of Mg-Nd alloys involves the formation of clusters, G.P. zones, b¢¢ (Mg3Nd), b¢ (Mg7Nd), b1 (Mg3Nd), b (Mg12Nd) and be (Mg41Nd5).[3,6,8] The b phase, with a Mg12Nd XIAOJUN ZHAO, ZHIQIAO LI, and HOUWEN CHEN are with the International Joint Laboratory for Light Alloys (Ministry of Education), College of Materials Science and Engineering, Chongqing University, Chongqing 400044, P.R. China. Contact e-mail: [email protected] RAINER SCHMID-FETZER is with the Institute of Metallurgy, Clausthal University of Technology, RobertKoch-Str. 42, D-38678 Clausthal-Zellerfeld, Germany. Contact e-mail: [email protected] JIAN-FENG NIE is with the International Joint Laboratory for Light Alloys (Ministry of Education), College of Materials Science and Engineering, Chongqing University and also with the Department of Materials Science and Engineering, Monash University, Clayton, VIC 3800, Australia. Contact e-mail: [email protected] Manuscript submitted August 30, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
composition and a tetragonal structure (I4/mmm, a = 1.031 nm, c = 0.593 nm), was initially accepted as an equilibrium phase in the Mg-Nd binary system.[9] But the subsequent work indicated the equilibrium intermetallic phase was Mg41Nd5 (tetragonal, I4/m, a = 1.474 nm, c = 1.040 nm), with the b phase actually being a metastable phase.[10–12] The age hardening response and creep resistance of Mg-Nd binary alloys could be further improved by Zn or Ag addition.[5,13–15] Instead of forming b series precipitates, the precipitation sequence in Mg-Nd-Zn ternary alloys involves the formation of G.P. zones, c¢¢ (Mg5(Nd,Zn)) and c (Mg3(Nd,Zn)).[5,16] The G.P. zones and c¢¢ are both formed as plates on the basal plane of the Mg matrix. The equilibrium phase c is reported to have a face-centered cubic structure, with lattice parameter a = 0.72 nm, which is isomorphous to Mg3Nd, although the lattice parameter is contracted from 0.74 to 0.72 nm with some Nd atoms replaced by Zn atoms.[16] Early in 1960s, Payne and Bailey found that the age hardening response of binary Mg-Nd alloys could be significantly improved by Ag addition, which subsequently led to the development of a commercial alloy designated QE22 (Mg-2.5Ag-2Nd-0.7Zr, wt pct).[14] The addition of Ag to other Mg-RE-based alloys, such as Mg-Gd-Zr alloys[17–21] and Mg-Y-Zn-Zr alloys,[22–24]
could also lead to a remarkable increase in age hardening response. QE22 alloy is well known for its good tensile properties at elevated temperature and creep resistance and has been used in the aircraft and aerospace industries.[3,4] The precipitation sequence in this alloy has not been well established. Two independent
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