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

Abstract 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 →  β 1 (Mg 3 Ce; BiF 3 -type) →  β (Mg 12 Ce; Mn 12 Th-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 β 1 - and the β -phase come into sight in the matrix with definite crystallographic orientation relations of [0 0 1] m //[1 1 0] β 1 //[1 0 0] β and (1 1 0) m //(1 1 1) β 1 //(0 1 0) β , finally completing the aging effect with full of coarse β -precipitates.