For most structural applications, materials are selected by their ability to resist fracture, rather than their strength. A canonical example of the selection criteria of fracture toughness are bulk metallic glasses (BMGs). Fracture toughness, its wide variation, and its mechanistic origin is, however, not well understood in BMGs, which encompass alloys spanning from close to ideal brittle to remarkably tough behavior. Hence, a deeper understanding of fracture toughness is needed, which could eventually be used to develop novel BMGs with higher resistance to fracture to unleash their broad potential applications.

Several challenges have prevented a deeper understanding of fracture toughness and accurate measurements in the past. Requirements of fast cooling to vitrify the liquid BMG former can generate external contributions such as internal stresses, cavitation, voids, and other casting defects. Such contributions may overshadow the intrinsic fracture behavior of a BMG. In addition, sample geometries and concepts, which have been widely used in fracture toughness studies of BMGs in the past, have been originally developed for strain hardening crystalline metals, in sharp contrast to the strain softening behavior of BMGs.

We have developed an alternative method to measure fracture toughness in BMGs, based on thermoplastic forming of BMGs. This fabrication method reduces most of the previously identified extrinsic contributions. As a consequence, we have increased the repeatability and relative accuracy in fracture toughness measurements of BMGs from a scatter of ~500 % prior to our work, down to ~3%. This highly accurate, controlled, and repeatable method suggest itself to tackle major open questions of fracture toughness of BMGs. These include the origin of the large range of fracture toughness in metallic glasses, what determines the length scale for a flaw tolerance behavior in metallic glasses, and how does fracture toughness depends on chemical composition of the BMG.

Our study will build on our research during the previous funding periods. We will measure fracture toughness for a broad range of BMG forming alloys. These data, together with results from the previous funding period, where we determined the effect of the structural state of the BMG on fracture toughness, will be used to develop a mechanistic understanding of fracture toughness. Specifically, we hypothesis that fracture toughness in BMGs is ultimately limited by the normal, cavitation strength and not by the shear strength. Furthermore, we hypothesize and will increase in the mechanistic description, that the low barrier events, likely shear transformation events, are involved in the fracture toughness of BMGs. Our theory will be describing a complete picture from microscopic events to the fracture toughness of BMGs.