Size-dependent fracture toughness of bulk metallic glasses

The fracture toughness is a critical material property that determines engineering performance. However, as is well known for crystalline materials, if certain sample geometry and size requirements are not met, test results become sample-size dependent and difficult to compare between different studies. Here, the room-temperature fracture toughness of the Zr-based bulk metallic glass (BMG) Zr52.5Cu17.9Ni14.6Al10Ti5 (Vitreloy 105) was evaluated using compact-tension, as well as single-edge notched-bend, specimens of different sizes to measure KIc values according to ASTM standard E399 and JIc values according to ASTM standard E1820. It is concluded that the ASTM standard E399 sample-size requirements should be cautiously accepted as providing size-independent (valid) KIc results for BMGs; however, it is also concluded that small-sized samples may result in a wider scatter in conditional toughness KQ values, a smaller yield of valid tests and possibly somewhat elevated toughness values. Such behavior is distinct from crystalline metals where the size requirements of ASTM standard E399 are quite conservative. For BMGs, KQ values increase and show a larger scatter with decreasing uncracked ligament width b, which is also distinct from crystalline metals. Samples smaller than required by ASTM standards for KIc testing are allowed by the J-integral-based standard E1820; however, in this study on BMGs, such tests were found to give significantly higher toughness values as compared to valid KIc results. Overall, the toughness behavior of BMGs is more sensitive to size requirements than for crystalline metals, an observation that is likely related to the distinct size-dependent bending ductility and strain softening behavior found for metallic glasses. It is concluded that toughness values measured on BMG samples smaller than that required by the KIc standard, which are common in the literature, are likely sample size- and geometry-dependent, even when they meet the less restrictive valid JIc requirements.