Improved mechanical performance of double-pass extruded Mg-Gd-Er-Zr alloys with various rare earth contents

The present work focuses on the effects of rare earth (RE = Gd and Er) contents on the microstructural evolution and mechanical performance of Mg-Gd-Er-Zr alloys. Three different alloys, i.e., Mg-12Gd-2Er-0.4Zr, Mg-14Gd-1Er-0.4Zr, and Mg-14Gd-2Er-0.4Zr, are fabricated and subjected to double-pass extrusion. Incomplete dynamic recrystallization (DRX) resulted in a non-homogeneous grain structure with coarse and refined equiaxed grains during single-pass extrusion. In contrast, the grain size decreased dramatically after double-pass extrusion, and a high number of fine Mg5RE particles precipitated in the Mg-14Gd-2Er-0.4Zr alloy than the other two extruded alloys. Moreover, the Mg5RE particles larger than 1 mu m contributed to increase the DRX fraction, whereas the finer ones exerted a strong pinning effect on grain boundaries. The volume fraction of beta' precipitates increased with increasing RE content in the matrix. A large volume fraction of beta' precipitates in the alloy will primarily result in higher hardness and strength in the peak-aged condition. After peak-aging treatment, the double-pass extruded Mg-14Gd-2Er-0.4Zr alloy exhibited a good yield strength of 481 +/- 3.7 MPa and an adequate elongation of 3.2 +/- 0.6%. The influence mechanisms of grain refinement, fine particles, and the strengthening of beta' precipitates are discussed.