Fracture toughness in Zr-based bulk metallic glass

Fracture toughness (K_IC) was investigated for bulk metallic glass (BMG) materials Zr₅₅Cu₃₀Ni₅Al ₁₀ at % fabricated by a suction casting method (material Su) and squeeze casting method (material Sq), and Zr₆₅Cu₁₅Ni ₁₀Al₁₀ 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√ m and a little small value about 31 MPa√m, respectively. The K_IC of a nearly full-crystallized material Sq showed a small value about 1 lMPa√m. The K_IC of a full glassy material P showed a value about 17MPa√m. In the material Su, long bent slip lines (length; ~ several hundred μ 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 5-10μm) 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 lmm thickness plate. This tendency was reverse to the results of crystalline alloys.