Enhancing strength and ductility of Mg-12Gd-3Y-0.5Zr alloy by forming a bi-ultrafine microstructure

► A bi-ultrafine microstructure (BU) was formed in an Mg-RE alloy after 4-pass ECAP. ► Tensile properties of the Mg-RE alloy with the BU microstructure were improved. ► Formation of BU was due to the interactions of matrix and particles during ECAP. ► BU effects dislocation accumulation, shear band propagation and fracture behavior. ► Fracture mechanisms for the Mg-RE alloy with and without BU were proposed.