Microstructure and strengthening mechanism of high strength Mg-10Gd-2Y-0.5Zr alloy

Microstructure and mechanical properties of Mg-10Gd-2Y-0.5Zr (wt.%) alloy in a series of tempers, including as-cast, cast-T4, cast-T6 and extruded-T5 conditions, have been investigated. The evolution of the microstructure from as-cast to cast-T4 to cast-T6 involves solid solution + eutectic compound → supersaturated solid solution + cuboid-shaped compound → solid solution + β′ precipitates + cuboid-shaped compound. Zirconium cores exist in all these conditions. Effective grain refinement is attained by hot extrusion with a small extrusion ratio (∼9.3). A good combination of high strength and sufficient ductility at room temperature is achieved for the cast-T6 alloy by optimizing the heat treatment parameters and for the extruded-T5 alloy extruded at 673 K, whose ultimate tensile strengths, tensile yield strengths and elongations are 362, 239 MPa and 4.7%, and 403, 311 MPa and 15.3%, respectively. Moreover, the strengths decrease gently from room temperature to 200 °C with a gradual increase of elongation. Existing traditional strengthening theories, together with data from microstructural characterization and mechanical properties are used to determine the magnitude of individual contribution. Strengthening due to precipitation is the largest contribution to alloy strength, either in cast-T6 condition or in extruded-T5 condition. The grain boundary strengthening also contributes significantly after hot extrusion.