Bernard C, Keryvin V
Univ. Bretagne Sud, UMR CNRS 6027, IRDL, F-56100 Lorient, France.
J Phys Condens Matter. 2020 Sep 4;32(48). doi: 10.1088/1361-648X/abaa7f.
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.
块状金属玻璃（BMG）可追溯到20世纪80年代末，是一种相对较新的材料，与大多数晶态金属合金相比，它具有优异的机械性能（强度、硬度、断裂韧性、储存弹性能……）。然而，它们在单轴加载下表现出的明显脆性仍是其工业化的主要障碍。此外，BMG在其复杂而精细的制备过程中，常常有意无意地产生晶体缺陷。众所周知，这些缺陷会影响它们的断裂韧性和塑性行为。本文综述了二十年来关于含不同性质（如枝晶或球晶）、不同体积分数晶体缺陷的锆基BMG这一主题的研究工作，这些晶体缺陷取决于合成方法。开发了专门的实验装置，主要是对带切口梁进行弯曲试验，通过疲劳在试样中产生适当的预制裂纹，然后对其进行单调加载直至断裂。测量得到的断裂韧性和断口观察结果表明，这些晶体缺陷促进了预制裂纹的形成，但根据其类型不同，会导致不同程度的脆化。裂纹的加载模式——I型、II型或混合型——以及温度在BMG中裂纹的起始和扩展（无论是稳定扩展还是灾难性扩展）过程中都起着关键作用。通过有限元计算分析，提出了关于晶体缺陷的存在如何在I型和II型模式下影响断裂韧性并扰动裂纹扩展的解释。最后，讨论了这些实验技术的相关性以及晶体缺陷、断裂韧性及其对结构部件延展性影响之间的联系。
