Investigation of two-phase Mg-Gd-Ni alloys with highly stable long period stacking ordered phases

• LPSO/Mg two-phase Mg-Gd-Ni alloys with up to ∼40 vol.% LPSO phase were obtained.
• LPSO phase volume fraction was directly proportional to Ni content.
• LPSO phase could hardly dissolve into the Mg matrix at 500 °C for 100 h.
• Structure of LPSO phase transformed from 18R to 14H upon heat treatment at 500 °C.
• Nearly zero solid solubility of Ni in Mg plays key roles in LPSO phase formation.

The effects of the Ni addition on the microstructures of Mg-2Gd-xNi (at.%, x = 0, 0.25, 0.5 and 1) alloys were systematically investigated. A series of two-phase Mg-Gd-Ni alloys consisting of long period stacking ordered (LPSO) phase and α-Mg were obtained. The LPSO phase presented block-like shape and was distributed between α-Mg dendrite arms. Its volume fraction was found to be proportional to the Ni content, ranging from 0 to ∼40% with the Ni content increasing from 0 to 1 at.%. After a heat treatment at 500 °C for 100 h, the LPSO phase experienced a structure transformation from 18R to 14H but was hardly dissolved into the α-Mg matrix, which evidenced a high thermal stability. The mechanism for the effects of the Ni addition on the formation of the LPSO phase was discussed.