Effect of secondary extrusion on the microstructure and mechanical properties of a Mg-RE alloy

The effect of secondary extrusion on the microstructure and mechanical properties of a single-extruded Mg-8Gd-4Y-1Nd-0.5Zr alloy has been investigated. The microstructural results from scanning electron microscopy (SEM) show that irregular-shaped β phases in the single-extruded and pre-heated alloy are severely fragmented into small phases during secondary extrusion. Secondary extrusion increases the content of rare-earth (RE) elements in the matrix by dissolving the small phases. Transmission electron microscopy (TEM) analysis confirms that after secondary extrusion the β′ strengthening phase formed during ageing is more homogeneously dispersed than that after single extrusion. Precipitate strengthening makes a dominant contribution to a high ultimate tensile strength of 428 MPa with a ductility of 2.8% after peak ageing in the secondary-extruded alloy. In particular, electron backscatter diffraction (EBSD) analysis reveals an unusual texture component after secondary extrusion in which the majority of grains distribute c-axes along the extrusion direction (ED).