In this work, the microstructural evolutions and mechanical properties of an as-cast Mg-10Gd-4Y-1.5Zn-0.5Zr (wt %) alloy during successive multi-pass equal channel angular pressing (ECAP) were systematically investigated by X-ray diffractometer, scanning electron microscopy, transmission electron microscopy, and compression test. The obtained results show that the microstructure of as-cast alloy consists of α-Mg grains, Mg3
Gd island phase, few Y-rich particles, and lamellar 14H LPSO (long period stacking ordered) phase located at the grain boundaries. During ECAP, the Mg3
Gd-type phase is crushed and refined gradually. However, the refined Mg3
Gd particles are not distributed uniformly in the matrix, but still aggregated at the interdendritic area. The 14H phase becomes kinked during the early passes of ECAP and then broken at the kinking bands with more severe deformation. Dynamic recrystallization of α-Mg is activated during ECAP, and their average diameter decreases to around 1 μm, which is stabilized in spite of increasing ECAP passes. Moreover, nano-scale γ′ phases were dynamically precipitated in 16p ECAP alloy. Compression tests indicate that 16p ECAP alloy exhibits excellent mechanical property with compressive strength of 548 MPa and fracture strain of 19.1%. The significant improvement for both strength and ductility of deformed alloy could be ascribed to dynamic recrystallization (DRX) grains, refined Mg3
Gd-type and 14H particles, and dynamically precipitated γ′ plates.
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