Cast defects induced sample-size dependency on compressive strength and fracture toughness of Mg–Cu–Ag–Gd bulk metallic glass

In this work, Mg59.5Cu22.9Ag6.6Gd11 bulk metallic glass (BMG) is selected to investigate the specimen-size dependency of the compressive fracture strength and notch toughness. As the diameter of cylindrical sample increases, apparent fracture strength is reduced by 25% from ∼950 MPa at 1-mm down to ∼710 MPa at 10-mm sample, which is associated with the population and size of as-cast porosity in the BMG rods as indicated by X-ray computed tomography (CT). Meanwhile, it is inappropriate to describe the size dependence of fracture strength using Weibull or Gumbel statistics owing to the inhomogeneous distribution of cast defects in different sized rods. Similar to the compressive fracture strength, cast defects in Mg59.5Cu22.9Ag6.6Gd11 BMG also yield specimen-size dependency of the measured notch toughness, when the plate-specimen thickness varies within the range of 1–8 mm. With three-parameter Weibull analysis, threshold toughness Kμ of as-cast Mg59.5Cu22.9Ag6.6Gd11 BMG can be estimated, which represents a minimum toughness value below which the cracks arrest. Using finite element analysis, it is confirmed that the specimen-thickness dependency of notch toughness does not result from the transition from plane-stress to plane-strain in the specimen, but from the extrinsic flaws.