Difference in extrusion temperature dependences of microstructure and mechanical properties between extruded AZ61 and AZ91 alloys

The effects of extrusion temperature on the microstructure and tensile properties of extruded AZ61 and AZ91 alloys are investigated by subjecting them to hot extrusion at 300 and 400 °C. Although the average grain size of the extruded AZ61 alloy slightly increases from 9.5 to 12.6 µm with increasing extrusion temperature, its resultant microstructural variation is insignificant. In contrast, the average grain size of the extruded AZ91 alloy significantly increases from 5.7 to 22.5 µm with increasing extrusion temperature, and the type of Mg₁₇Al₁₂ precipitates formed in it changes from fine dynamic precipitates with a spherical shape to coarse static precipitates with a lamellar structure. As the extrusion temperature increases, the tensile yield strength of the extruded AZ61 alloy increases from 183 to 197 MPa while that of the extruded AZ91 alloy decreases from 232 to 224 MPa. The tensile elongations of the extruded AZ61 and AZ91 alloys decrease with increasing extrusion temperature, but the degree of decrease is significant in the latter alloy. These different extrusion temperature dependences of the tensile properties of the extruded AZ61 and AZ91 alloys are discussed in terms of their microstructural characteristics, strengthening mechanisms, and crack initiation sites.