TEM study of real precipitation behavior of an Mg–0.5at%Ce age-hardened alloy

• The precipitation in an Mg–Ce alloy are presented as Mg-solution → GP-zone → β1 → β.
• The GP-zones have an ordered structure, contributing to the age-hardening effect.
• After the top-aging, β1- and β-phase coexist with definite orientation relations.

Precipitates formed in an Mg–0.5at%Ce alloy during age-hardening at certain temperatures ranging between 180 °C and 250 °C have been thoroughly investigated by a combined technique of TEM and HAADF-STEM. The precipitation sequence can be presented as Mg-solid solution → GP-zone → β1 (Mg3Ce; BiF3-type) → β (Mg12Ce; Mn12Th-type). At an early stage of aging (180 °C for 2 h), fine precipitates of planar GP-zone with an ordered structure appear in parallel to (1 0 0)m planes of the Mg-matrix, having a thickness of sub-nm and an extension of 5–15 nm. With an advance of aging, the GP-zones increasingly grow larger and combine with the neighbors. When the GP-zones have reached as large as approximately 20 nm in diameter, the age-hardening effect is maximized (180 °C for 70 h). After the top-aging, the GP-zones begin to decompose and disappear, and instead precipitates of two other phases, i.e. the β1- and the β-phase come into sight in the matrix with definite crystallographic orientation relations of [0 0 1]m//[1 1 0]β1//[1 0 0]β and (1 1 0)m//(1 1 1)β1//(0 1 0)β, finally completing the aging effect with full of coarse β-precipitates.