The Microstructure of Mg98.5Zn0.5Y1 Alloy with Long-Period Stacking Ordered Structure

The microstructure of an Mg98.5Zn0.5Y1 alloy prepared by directional solidification (DS), synchronized with a long-period (LPSO) structure were systematically investigated using high-resolution transmission electron microscopy (HRTEM)and high-angle annular dark-field scanning transmission electron microscopy. The formation of 14H-type LPSO was observed to be accompanied by stacking faults. The lamellar 14H-type LPSO structure and stacking faults were both formed on (0001)(a-Mg) habit plane and grew along [(1) over bar2 (1) over bar0] a-Mg direction in Mg98.5Zn0.5Y1 alloy prepared by DS. These findings demonstrated that the stacking faults have a great effect on crystal growth in LPSO structure. The stacking faults nucleation were proposed to explain the growth of LPSO in DS. For the DS polycrystals, the LPSO exhibited a lamellar shape, whose unit cell was composed of two twin-related building blocks (ABCA-type and ACBA-type) which determined the physical and chemical properties of Mg98.5Zn0.5Y1 alloy. There were three atomic layers between two building blocks. The clusters were observed in the 14H-type LPSO which played an important role on building blocks. Furthermore, the two building blocks arranged in opposite shear direction is 68.7 degrees. Based on the present study, an atom model Mg142Zn12Y16, which considered as the characteristic structure of 14H-type LPSO in Mg98.5Zn0.5Y1 alloy prepared by DS, was constructed. The atomic model were verified by simulation. The simulated diffraction patterns were consistent with the experimental pattern.