Key Factor for the Transformation from hcp to 18R-Type Long-Period Stacking Ordered Structure in Mg Alloys

Cast Mg85Y9Zn6 has an 18R-type LPSO structure. However, Mg85Y9Zn6 recovered after being subjected to a loading pressure of 7 GPa at 973K shows a fine dual-phase structure composed of a face-centered cubic (fcc) structure showing a superlattice (D0(3)), as well as a hexagonal close-packed structure (hcp:2H). The D0(3)/hcp structure transformed to 18R-type LPSO during heating at ambient pressure. In this research, the transformation process from the D0(3)/hcp structure to 18R-type LPSO structure was discussed by means of in situ XRD and first-principles calculation. At first, lattice volume of 2H increased with an increase in the temperature, because of the Zn and Y emitted from the D0(3) phase into the 2H lattice. After the volume expansion of 2H lattice, the structure collapsed due to insert of random stacking faults (SFs). Then, a formation of 18R-type LPSO structure occurred. Based on a first-principles calculation for pure Mg, volume expansion of the 2H lattice causes the transformation to an 18R structure. Furthermore, the results of free energy calculations for the hcp and fcc structures in the Mg-Y-Zn ternary system show that the segregation of Y and Zn atoms on SFs occurs by the Suzuki effect. These segregated Y and Zn atoms in SF layers, which have a local fcc structure, create a synergy between the stacking and chemical modulations. Present result insists that the volume increase of 2H lattice takes place first, and then the transformation from the hcp structure to 18R stacking occurs.