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

By atomic-scale high-angle annular dark field scanning transmission electron microscopy, we characterized a long period stacking ordered (LPSO) phase, 654R, in an Mg88Co5Y7 (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(T3T¯3)14]3, with lattice parameters of a = 0.321 nm, c = 170.4 nm, and a space group of R3¯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. [Display omitted] •654R long period stacking ordered (LPSO) phase was characterized at atomic level.•654R LPSO phase is an ordered intergrowth of 15R and 12H LPSO phase.•A new notation was introduced to describe stacking sequence of 654R LPSO phase.•The possible formation process of 654R LPSO structures was proposed.