Rare-earth (RE) element addition can remarkably improve the mechanical properties of magnesium alloys through pre-cipitation hardening. The morphology, distribution and crystal structure of precipitates are regarded as major strengthening mecha-nisms in the Mg-RE alloys. In order to understand the formation of precipitates during aging at 225 oC in a Mg-10Gd-3Y-0.4Zr alloy (GW103K) with high strength and heat resistance, a high-resolution transmission electron microscopy (HRTEM) was employed to characterize the microstructural evolution. It was found that three types of precipitates were observed in the alloy at the early stage, named as: single layer D019 structure, one single layer D019 structure and one layer of Mg, two parallel single layers (containing RE) and Mg layer in between, which was regarded as ordered segregation of RE, precursors to formβ′ andβ′ phase, respectively. Both ofβ′ andβ′ phase were transformed from the precursors. It was also found that large size ofβ′ phase and the small size ofβ′ phase were constantly existent in the whole aging process.β′ phase played a major role in the strengthening of the GW103K alloys and the de-crease of the hardness was caused by the coarsening ofβ′ phase.
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