Fracture toughness in Zr-based bulk metallic glass

Fracture toughness (K-IC) was investigated for bulk metallic glass (BMG) materials Zr55Cu30Ni5Al10 at % fabricated by a suction casting method (material Su) and squeeze casting method (material Su), and Zr65Cu15Ni10Al10 at % fabricated by a powder extrusion method (material P). The K-IC value in the specimen with a notch plane normal to the boundary plane of molten metal was larger than that parallel to the boundary one. In the specimens with the notch plane normal to the boundary plane, the K-IC of a full glassy material Su. and a partially crystallized material Su. showed a large value about 40 MPa root m and a little small value about 31 Mpa root m, respectively. The K-IC of a nearly full-crystallized material Sq showed a small value about 11 MPa root m. The K-IC of a full glassy material P showed a value about 17MPaFm. In the material Su, long bent slip lines (length; similar to several hundred mu m) at the fatigue crack tip were observed on the specimen surface. The Plane-strain type slip lines and the discrete crack tip openings (width; about 5- 1 mu m) like the stretch zone in crystalline alloys were observed along the fatigue crack front on the fracture surface. Near the specimen surfaces, cracks propagated along the long bent slip lines. The crack length was about two times longer than the plastic zone size calculated under the plane-stress condition. There was a tendency that the K-IC value in the 2.3mm thickness plate was larger than that in the 1mm thickness plate. This tendency was reverse to the results of crystalline alloys.