Fig. 1. Geometrical configuration of structural unit [Gd-Mg12]Mg6 in hcp lattice, where the twinned octahedral cluster is centered by Gd (green ball), shelled by twelve nearest-neighbor Mg atoms (red balls), and glued by six next outer-shell Mg atoms (blue balls).

Fig. 2. Schematic configurations of fcc- and bcc-like lattices formed by solute-containing stable units [Gd-Mg12]Mg6 (simplified as {Gd}, blue balls at cube vertices), with their octahedral interstices being occupied by stable solvent unit [Mg-Mg12]Mg3 ({Mg}, red balls at face centers and green ones at edge centers): (a) fcc-like lattice with fully filled octahedral interstices, {Gd}1{Mg}1; (b) bcc-like lattice with octahedral interstices half-filled {Gd}2{Mg}3; (c) fcc-like lattice with fully filled octahedral interstices {Gd}1{Mg}3.

Fig. 3. Ultimate tensile strength (σb) as a function of elongation (δ) of prevailing Mg-Gd-based industrial alloys (including the four alloys of the present work, encircled and arrowed). All the alloys follow the three solute-homogenization formulas, with their atomic percent deviations from the ideal formulas in the center site being represented by color depth. The red-shaded zone covers the {Gd}1{Mg}1 alloys, the green one {Gd}2{Mg}3, and the blue one {Gd}1{Mg}3.

Fig. 4. Elongation (δ) vs deviation in the center site of {Gd} formula. High elongations fall near the zero deviation line. The designed compositions are encircled.

Fig. 5. Schematic of position in the mold and size of rectangle-shaped specimens for tensile tests.

Fig. 6. XRD patterns of Mg-Gd-Y-Zr alloys in as-cast F state (a), solutioned T4 state (b) and peak-aged T6 state (c). The major secondary phase is of Mg24(Gd,Y)5 type in the as-cast F state, as indicated by the strongest peak (the arrows in (a)). This phase is basically dissolved after solutioning treatment (b). After ageing (T6), the precipitation of some unknown phase is remarked (c). The unknown peaks are all labeled by question marks.

Fig. 7. OM images of as-cast (F state) Mg-Gd-Y-Zr alloys with their ideal formulas shown in the upper-left inlets.

Fig. 8. OM images of solution treated Mg-Gd-Y-Zr alloys (T4 state, 525 °C for 6 h).

Fig. 9. Mechanical properties of the four designed Mg-Gd-Y-Zr alloys, formulated by {Gd}1{Mg}1, {Gd}2{Mg}3, {Gd}1{Mg}2, and {Gd}1{Mg}3 in the as-cast, solutioning T4 and ageing T6 states: (a) ultimate tensile strength; (b) yield strength; (c) elongation; (d) yield ratio; (e) Young’s elastic modulus.

Fig. 10. Ultimate tensile strength and elongation properties of the four designed Mg-Gd-Y-Zr alloys in the aged T6 state. The red shadow marks the irregular behavior of the {Gd}1{Mg}2 alloy that does not follow the solute homogenization model. Those alloys that are designed according to the solute homogenization models show normal behavior (the irregular break of strength evolution is amended by a dotted line).