Relationship between fracture toughness and stretched zone width, shear band length and crack tip opening displacement on specimen surface in bulk metallic glasses

Relationship between fracture toughness and shear band length (SBL), crack tip opening displacement (CTOD) on the specimen surface and stretched zone width (SZW) on the fracture surface in Zr-, Cu- and Ti-based bulk metallic glasses (BMGs) was investigated in order to examine the mechanisms that the BMGs show a high fracture toughness. Zr50Cu40Al10 and Zr50Cu30Ni10Al10 BMGs fabricated by an ark tilt casting method, and Cu60Zr30Ti10, Ti41.5Zr2.5Hf5Cu42.5Ni7.5Si1 BMGs fabricated by a high-frequency induction casting method were used. The fracture toughness test was conducted based on ASTM E399. Compact type specimen was used (1.0 mm, 2.0 mm and 2.3 mm in thickness, thickness : width = 1 : 4). Fatigue pre-crack was induced under Delta K less than 6 MPa center dot m(1/2). The crack length and SZW were measured on the fractographs by scanning electron microscope (SEM), and the SBL and CTOD were measured on the photographs of specimen surfaces by the SEM. Observed SBL and CTOD in each BMG showed the near values of the plastic zone size and CTOD calculated by the fracture mechanics under the plane stress condition, respectively. By the stereo-matching observation on the fracture surfaces, the crack tip inside the specimen clearly opened and a stretched zone area was shown. Average SZW in front of the fatigue crack in the Zr- and Cu-based BMGs was roughly proportional to the fracture toughness value.