The Microstructure of Mg_98.5Zn_0.5Y₁ Alloy with Long-Period Stacking Ordered Structure

Abstract

The microstructure of an Mg_98.5Zn_0.5Y₁ 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)_α-Mg habit plane and grew along [1210]_α-Mg direction in Mg_98.5Zn_0.5Y₁ 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 Mg_98.5Zn_0.5Y₁ 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°. Based on the present study, an atom model Mg₁₄₂Zn₁₂Y_16, which considered as the characteristic structure of 14H-type LPSO in Mg_98.5Zn_0.5Y₁ alloy prepared by DS, was constructed. The atomic model were verified by simulation. The simulated diffraction patterns were consistent with the experimental pattern.