Atomic-scale observation of β′ and LPSO phase in Mg-Y-Ni alloy by HAADF-STEM

This paper reports on an atomic-scale investigation into the beta' precipitates and the long-period stacking ordered phase (LPSO) in Mg-5Y-2.5Ni-0.5Zr (at.%) alloy, using Cs-corrected high angle annular dark fieldscanning transmission electron microscopy (HAADF-STEM). The results displayed that the 18R-type and 14Htype LPSO phases coexisted in the as-cast and the solid solution states, and the 18R-type and 14H-type LPSO structures were thermal stable. After aging treatment, the aging peak hardness reached 138 HV at 225 degrees C for 48 h. The significant increase in hardness was attributed to the formation of the metastable beta' phase. The lattice parameters of a and b axes for beta' phases are a = 0.65 nm, b = 2.20 nm, and c = 0.52 nm by HAADF-STEM. The interaction between the LPSO phase and the beta' can be found. The atomic-scale interactions between the LPSO and beta' phases are divided into two parts: under-aging and peak- aging conditions between the building blocks.