Electron Microscopy Study Of Long Period Stacking Structure In Mg₉₇Zn₁Y₂ Alloy

As a new type of effective strengthening phase in a magnesium-based alloy, the long-period-stacking structure (LPS) formed in the hcp matrix can contribute greatly to strength enhancement of the alloy.In this dissertation, formation, structural feature and changes for various LPSs in the Mg₉₇Zn₁Y₂ alloys, subjected to different solidification and heat treatment processes, have been studied by means of TEM, SEM and XRD techniques, etc. The information about these aspects is essential for controlling the growth and distribution of LPS, and further for designing and developing new high-performance magnesium alloys.A long-period-stacking structure with 6-layered period (called 6H'-type) has been found to form in the Mg₉₇Zn₁Y₂ alloy prepared by the induction melting method. This 6H'structure is different from the typical 6H type in a closely-packed structure. It is stable at 300; and does change when annealed at this temperature for 50 hours. By annealing at 525℃for 7 hours, however, this 6H'structure turn to transform to the structure of the 14H type.For the rapidly-solidified Mg₉₇Zn₁Y₂ ribbon, two series of heat treatments have been applied to study formation processes of various LPS phases during isothermal annealing at a fixed temperature, and, on the other hand, to investigate transformation between them when annealed at different temperaturesHeat treatment series I: Isothermally annealing for different periods of time After the rapidly-solidified Mg₉₇Zn₁Y₂ ribbon was annealed at 300℃for 1 hour, LPS phases of 18R-type were found to form extensively and they can growth across whole grains. When annealing at 300℃for 5 hrs, LPS of the 6H'type was found to be formed, while the remnant LPS of 18R-type is structurally faulted in this annealing stage. Elongating the annealing time to 20 hrs, the LPS structure formed is still of the 6H'type. Our results indicate that the Mg-Y-Zn LPS can undertake an 18R-to-6H'structural transition during annealing at 300℃. It is believed that LPS of the 6H'type in the Mg-Zn-Y system is structurally stable at this temperature.Heat treatment series II: Isochronically annealing at different temperatures Annealing the rapidly-solidified Mg₉₇Zn₁Y₂ ribbon at 300℃for 20 minutes leads to the formation of the 18R-type LPS. This 18R phase was found to grow into grain from the networked second phases containing high contents of solute atom. After annealing at 395℃for 20 minutes, those networked second phases decompose and dissolve completely. Three types of LPS structures belong to 18R, 14H and 6H'types, respectively, are found to coexist after the annealing of the Mg₉₇Zn₁Y₂ ribbon at 500℃. These results show that solute atoms containing in the second phases will return to matrix by elevating the annealing temperature and this makes formation of various LPS and their structural transformation more easy.Formation of LPS has also been examined and studied in the other as-solidified alloys which were prepared with the purpose of replacing the alloying component"Zn"in Mg₉₇Zn₁Y₂ with"Ni"and"Ag", respectively. The existence of LPS in the Mg -Ni-Y system had been found for the first time. The LPS phases of 18R-type and those of 6H'-type coexist in the as-solidified Mg₉₇Ni₁Y₂ alloy. We found that the lattice distortion in the LPS structures occurs more frequently in the Mg-Ni-Y than in the Mg-Zn-Y systems. In contrast, no LPS structure is found to be formed in the Mg-Ag-Y sysyem. In addition, the diffrence of heat of mixing for alloying elements is compared and the factor affecting the LPS formation in magnesium alloys is also discussed.