Crystallographic account of an ultra-long period stacking ordered phase in an Mg88Co5Y7 alloy

Abstract By atomic-scale high-angle annular dark field scanning transmission electron microscopy, we characterized a long period stacking ordered (LPSO) phase, 654R, in an Mg 88 Co 5 Y 7 (at.%) as-cast alloy, which is an ordered intergrowth of 15R and 12H LPSO phases. The unique stacking sequence of 654R LPSO phase can be described as [(T2) 10 (T3 T ¯ 3) 14 ] 3 , with lattice parameters of a  = 0.321 nm, c  = 170.4 nm, and a space group of R 3 ¯ m . T/ T ¯ refers to AB′C/AC′B building blocks, while normal and subscript number respectively represent the number of Mg layers sandwiched between T/ T ¯ and the number of sub unit cells. The formation mechanism of 654R LPSO phase is also proposed.