Fracture toughness in direct extruded Mg-Al-Zn alloys

Fracture toughness and its deformed structures were investigated on a Mg-3AI-lZn (AZ31) alloy processed by direct extrusion. The average grain sizes of the alloy after extrusion at temperatures of 473 and 573 K were 4.0 and 14.5 mu m, respectively (i.e., fine-grained and coarse-grained alloys). The plane-strain fracture toughness, K-IC, as determined by stretched zone analysis in fine-grained and coarse-grained alloys was estimated to be 24.9 and 22.7 MPam(1/2), respectively. Microstructural observations of the fine-grained alloy after fracture toughness testing showed that non-basal slip and grain-boundary sliding was activated even at room temperature. In addition, the {10-12} deforrilation twins were observed despite the fine-grained Structures. However, the fraction of deformation twins was reduced with grain refinement. which related to the enhancement of fracture toughness with grain refinement.