Microstructures and mechanical properties of a Mg–9Gd−3Y−0.6Zn−0.4Zr (wt.%) alloy modified by Y-rich misch metal

Microstructural evolutions and mechanical properties of a Y-rich misch metal modified Mg–9Gd−3Y−0.6Zn−0.4Zr (wt.%) alloy were investigated. In the as-cast sample, the intermetallic phases are Mg₅RE, Mg₃RE and 14H-type long-period stacking ordered (LPSO) phase. After solution, all Mg₃RE and most Mg₅RE were dissolved while the 14H-LPSO plates were coarsened. The as-extruded alloy has a bimodal structure of ultra-fine dynamically recrystallized (DRXed) grains and coarse un-recrystallized grains, and is with a typical non-fiber texture. Disintegrated Mg₅RE and 14H-LPSO particles aggregate in extrusion stringers while fine dynamically precipitated Mg₅RE particles distribute at grain boundaries. After peak-aging, ultra-thin basal γ’’ and prismatic β’ precipitated in DRXed grain and un-recrystallized regions, respectively. The yield strength of the as-extruded and peak-aged alloys reaches to 385 MPa and 481 MPa, respectively, at room temperature, and to 320 MPa and 350 MPa, respectively, at 250 °C. Grain boundary strengthening from both grain boundaries and sub-grain boundaries in DRXed and un-recrystallized regions, respectively, and precipitation strengthening were revealed as the dominant strengthening mechanisms.