Crystalline defects in bulk metallic glasses: consequences on fracture toughness determination and ductility

Dating back to the late 1980s, bulk metallic glasses (BMGs) are relatively new materials that exhibit exceptional mechanical properties (strength, hardness, fracture toughness, stored elastic energy …), compared to those of most crystalline metallic alloys. Their apparent brittleness under uniaxial loading, however, is still a major obstacle to their industrialization. Moreover, BMGs often contain crystalline defects developed, intentionally or not, during their complex and delicate elaboration. These flaws are known to affect their fracture toughness and their plastic behavior. This paper reviews twenty years of works about this subject on Zr-based BMGs that may contain a low volume fraction of crystalline defects of different natures, e.g. dendrites or spherulites, depending on the synthesis method. Dedicated experimental set-ups, mainly bending tests on notched beams, were developed to create in the specimen a proper pre-crack by fatigue and then load it monotonically up to fracture. The measured fracture toughness and the fractographic observations allow to conclude that these crystalline defects facilitate pre-cracking, but result in an embrittlement that is more or less significant depending on their type. The loading mode of the crack - mode I, II or mixed - as well as the temperature were shown to play a key role in crack initiation and propagation, whether steadily or catastrophically, in the BMG. By means of finite element computations analyses, explanations on how the crystalline flaws presence can affect fracture toughness and perturbate crack growth, under mode I and mode II, were proposed. Finally, the relevance of these experimental techniques as well as the link between crystalline defects, fracture toughness and their consequences on the ductility of a structural component are discussed.