Transformation of long-period stacking ordered structures in Mg-Gd-Y-Zn alloys upon synergistic characterization of first-principles calculation and experiment and its effects on mechanical properties

Based on experiments and first-principles calculations, the microstructures and mechanical properties of as-cast and solution treated Mg-10Gd-4Y-xZn-0.6Zr (x = 0, 1, 2, wt.%) alloys are investigated. The transformation process of long-period stacking ordered (LPSO) structure during solidification and heat treatment and its effect on the mechanical properties of experimental alloys are discussed. Results reveal that the stacking faults and 18R LPSO phases appear in the as-cast Mg-10Gd-4Y-1Zn-0.6Zr and Mg-10Gd-4Y-2Zn-0.6Zr alloys, respectively. After solution treatment, the stacking faults and 18R LPSO phase transform into 14H LPSO phase. The Enthalpies of formation and reaction energy of 14H and 18R LPSO are calculated based on first-principles. Results show that the alloying ability of 18R is stronger than that of 14H. The reaction energies show that the 14H LPSO phase is more stable than the 18R LPSO. The elastic properties of the 14H and 18R LPSO phases are also evaluated by first-principles calculations, and the results are in good agreement with the experimental results. The precipitation of LPSO phase improves the tensile strength, yield strength and elongation of the alloy. After solution treatment, the Mg-10Gd-4Y-2Zn-0.6Zr alloy has the best mechanical properties, and its ultimate tensile strength and yield strength are 278.7 MPa and 196.4 MPa, respectively. The elongation of Mg-10Gd-4Y-2Zn-0.6Zr reaches 15.1, which is higher than that of Mg-10Gd-4Y0.6Zr alloy. The improving mechanism of elastic modulus by the LPSO phases and the influence on the alloy mechanical properties are also analyzed.