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The microstructure and mechanical properties of an extruded Mg-12Dy-1.1Ni alloy have been investigated in this paper. The microstructure of the as-extruded alloy consisted of α-Mg matrix, Mg₂Dy phase and a lamellar long-period stacking order (LPSO) phase distributed along the extrusion direction. Ageing the extruded alloy at 250 °C showed double peaks at 54 h and 108 h during ageing. The first ageing peak was due to a high number density of blocky 18R LPSO phases in conjunction with the precipitation of underaged 14H LPSO phase. The second ageing peak mainly arose from the precipitation of high volume fraction of 14H LPSO phases in the grain interior. Tensile testing indicated that the extruded alloy aged for 108 h exhibited the highest yield and ultimate tensile strengths from room temperature to 300 °C. The high tensile strengths of the alloy are mainly attributed to grain refinement and …