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

Abstract

Cast Mg 85 Y 9 Zn 6 has an 18R-type LPSO structure. However, Mg 85 Y 9 Zn 6 recovered after being subjected to a loading pressure of 7 GPa at 973 K 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 MgYZn 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. [

Formation and Stability of Solute Enriched Stacking Fault in the Mg–Zn–Y, Mg–Co–Y and Mg–Zn–Ca Ternary Systems

“…Hillert proposed how the segregated composition of solute elements in the grain boundary could be calculated by so-called grain boundary phase model, in which the grain boundary was regarded as a proper phase with certain thickness and volume. 25,26) This model was extended to the segregation on the fcc-SF in the hcp-Mg matrix in the MgZnY system 27,28) and was applied to the MgMX ternary systems in the present study. Figure 5 shows schematic illustrations of the extended Hillert's model.…”

Small-angle X-ray scattering analysis on the in-plane cluster distribution during heating amorphous Mg85Y9Cu6 alloys

“…[1][2][3][4][5][6] Detailed LPSO structures with nearly perfect order have been examined using electron microscopes, [7][8][9][10][11][12][13][14] first-principles calculations, [15][16][17][18] and other techniques. [19][20][21][22][23] For such ideal cases, LPSO structures are characterized by periodic stacking faults in the c-planes of the hcp Mg lattice, where Zn 6 Y 8 -Mg, so-called L1 2 clusters, 7,[10][11][12][13][14][15][16] are aligned as 2√3 × 2√3 two-dimensional superstructures. However, in MgYZn alloys, less-ordered samples with well-defined signatures of LPSO structures, that is, sharp diffraction peaks of 18R or 10H stacking and diffuse six-hold cluster arrangements, are commonly observed, suggesting that the average cluster-cluster distance can be loose and change smoothly during annealing.…”

Research for Long-Period Structure in Mg Alloy using High Pressure

“…The gray (■) indicates the value estimated from the (0002) of hcp, i.e., the (00018) of 18R. The dotted line indicates the extrapolated c-axis length below 500 K [15]. Fig.…”

“…6. The temperature dependence of the calculated (a) free-energy (F) difference between the Mg(2H) and Mg(18R) structures [15]. Fig.…”

“…7. The free-energy surface of the hcp and fcc phases in the Mg Y Zn ternary system at a temperature of 500 K at ambient pressure [15].…”