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Correlation between fracture surface morphology and toughness in Zr- based bulk metallic glasses

Fracture surfaces of Zr-based bulk metallic glasses of various compositions tested in the as-cast and annealed conditions were analyzed using scanning electron microscopy. The tougher samples have shown highly jagged patterns at the beginning stage of crack propagation, and the length and roughness of this jagged pattern correlate well with the measured fracture toughness values. These jagged patterns, the main source of energy dissipation in the sample, are attributed to the formation of shear bands inside the sample. This observation provides strong evidence of significant " plastic zone " screening at the crack tip. Bulk Metallic Glasses (BMG's) fail without detectable plasticity when loaded in the absence of geometrical confinement, as, for example, in tension or in bending of plates with thicknesses greater than the material characteristic length scale [1,2]. For fracture toughness measurements, typical test geometries are compact tension (CT) and single edge notched bending (SENB). The loading geometry of both methods is bending of plates which are several millimeters thick. Considering that typical characteristic process zone sizes for BMG's are below 1 millimeter [3], one would expect BMG to exhibit poor toughness. However, fracture toughness data for Vitreloy 1 (Zr 41.2 Ti 13.8 Cu 12.5 Ni 10 Be 22.5) [4], the first commercial BMG alloy, taken from several reports suggest a range of fracture toughness that extends to values comparable to those of conventional crystalline metals, although the scatter associated with these data is significant. Specifically, the reported fracture toughness of Vitreloy 1 varies between 16 and 55 MPa⋅m 1/2 [5-9]. In an additional study however in which single edge notched tension (SENT) was employed, the fracture toughness of Vitreloy 1 was reported to be in excess of 130 MPa⋅m 1/2 [10]. Such an extremely high toughness is surprising, given that the tension loading geometry of SENT is geometrically less confined than the bending loading geometry of CT and SENB. The unusual combination of zero ductility but high fracture toughness of BMG's was also pointed out by Ashby and Greer [3]. The perceived high fracture toughness of metallic glasses has been attributed to the formation of a high density of shear bands at the crack tip [11]. However, this argument has only been supported by finite element analysis [12] and observation of shear band networks that evolved on the outer surfaces of specimens [5,10], although it is known that such surface shear bands only reflect the stress state …