On the Precipitation in an Ag-Containing Mg-Gd-Zr Alloy
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rength Mg alloys is significant to the wider application of Mg alloys.[1,2] In this regard, Ag-containing Mg-rare earth (RE) alloys, especially Mg-Gd-Ag-Zr alloys, have attracted much attention in recent years for achieving the highest yield strength (YS) and ultimate tensile strength (UTS) in Mg casting alloys.[3–10] Although the appreciably high strength achieved in the Mg-Gd-Ag-Zr alloys is mainly attributed to precipitation hardening arising from the simultaneous presence of prismatic and basal precipitates,[5–7] neither prismatic precipitates nor basal
YU ZHANG is with the National Engineering Research Center of Light Alloy Net Forming and State Key Laboratory of Metal Matrix Composite, Shanghai Jiao Tong University, Shanghai, 200240, P.R. China and also with the Department of Materials Science and Engineering, Monash University, VIC, 3800, Australia. YUMAN ZHU, JIAN-FENG NIE, and NICK BIRBILIS are with the Department of Materials Science and Engineering, Monash University. Contact e-mail: [email protected] WEI RONG, YUJUAN WU, and LIMING PENG are with the National Engineering Research Center of Light Alloy Net Forming and State Key Laboratory of Metal Matrix Composite, Shanghai Jiao Tong University. Contact e-mail: [email protected] Manuscript submitted May 24, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
precipitates in the Mg-Gd-Ag-Zr alloys have been systematically investigated in the literature thus far. The b-type precipitation sequence of the Mg-Gd-Ag-Zr alloys can refer to the precipitation in Mg-Gd(-Zr) alloys. The generally accepted precipitation sequence of the Mg-Gd alloy system includes the formation of b¢¢, b¢, b1, and b phases.[11,12] The b¢¢ phase has a D019 structure (a = 0.641 nm, c = 0.521 nm) and an orientation relationship (OR) with respect to the a-Mg matrix: [0001]b¢¢ // [0001]a and 2110 b¢¢ // 2110 a. It was originally reported that the b¢¢ precipitates formed with the habit planes almost as plates [13] parallel to 2110 a. The b¢ phase is the predominant precipitate phase in peak-aged Mg-Gd(-Zr) alloys,[14,15] which has a base-centered orthorhombic Bravais lattice: a = 0.650 nm, b = 2.272 nm, and c = 0.521 nm and an OR with respect to the a-Mg matrix: [001]b¢ // [0001]a and (100)b¢ // 1210 a. The b¢ precipitates usually show a lenticular shape with the broad faces parallel to 2110 a. The b1 phase has a face-centered cubic (fcc) structure (a = 0.731 nm) and the OR between the b1 phase and the a-Mg matrix is such that ½110b1 // [0001]a and ð112Þb1 // ð1100Þa.[16] The b1 precipitates show a plate-like morphology with two ends inevitably in contact with the b¢ precipitates. The equilibrium b phase has a fcc structure (a = 2.23 nm) its and OR with respect to the a-Mg matrix is 111 b // 2110 a and
ð110Þb // ð0001Þa.[11,12] The b precipitates also form as plates with the broad faces parallel to 1010 a. Although the structures, orientations, and morphologies of b¢¢, b¢, b1, and b phases have been documented in the literature,[11,12] experimental and theoretica
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