Guiding and Deflecting Cracks in Bulk Metallic Glasses to Increase Damage Tolerance

Materials with high strength and toughness are desirable for lightweight and high-performance engineering structures. Although bulk metallic glasses (BMGs) typically have the highest strength among bulk alloys, the lack of tensile ductility and low toughness of somemetallic glasses will limit their use in structural applications. Composition optimization has been used to develop tough BMG matrix composites and monolithic BMGs. Ductile dendrites in BMG matrix composites can stabilize BMGs against the catastrophic fracture associated with extension of shear bands, producing graceful failure. To design a tough BMG matrix composite, several criteria must be satisfied, otherwise the materials may become more brittle. Robust theories that are able to predict new compositions that are able to satisfy so many different criteria are still unavailable. However, BMGs with high Poisson ratio (n) appear to encouragemultiple shear band formation at both notches and sharp cracks, thereby relieving local stress concentrations and producing an increase in toughness. This has become one criteria that has beenused to search for tough(er) BMGs. Recently, damage-tolerant Ti40Zr25Cu12Ni3Be20 glass, [10] Pd79Ag3.5P6Si9.5Ge2 glass, [8] and Zr61Ti2Cu25Al12 glass [12,13] were developed. Although the Pd79Ag3.5P6Si9.5Ge2 glass has the same n (0.42) as that of many other Pdand Pt-based glasses, its toughness appears to be much higher. However, sample size limitations and test details may contribute to inflated toughness numbers. A scaling law has been developed that takes the cavitation activation barrier into account in order to predict the toughness of BMGs. That work postulated that the toughness of metallic glasses should be proportional to n and the glass transition temperature(Tg). While thisappears toexplain the toughnessof many BMGs, neither Ti40Zr25Cu12Ni3Be20 nor Zr61Ti2Cu25Al12 glass has a very high n or high Tg, yet their fracture toughness values (i.e., notched and fatigue pre-cracked) are comparable to the highest toughness BMGs. Thus, the intrinsic toughening mechanism that produce high toughness in monolithic BMGs is still unclear. The present work utilizes pre-deformation of notched samples to significantly increase the notch toughness of both brittle and tough BMGs. This strategy utilizes pre-compression of notched components to locally change the deformation and fracture behavior in the vicinity of the notch, and results in significant increases to the notch toughness of both brittle and tough BMGs. In this aspect, toughening of various BMG components(e.g.,screws,fillets,etc.)canberealizedbychanging the deformation/fracture behavior exactly where it is needed (e.g., at the tip of a stress concentration), rather than requiring predeformation of the whole sample or component. It is noted that there ismuchpreviouswork thathasusedcold rolling to improve the mechanical behavior of a variety of BMGs. However, theseapproachesmayrequireveryhighloadsonbulk samples (i.e., that are subsequentlymachined into components) due to the veryhigh strengths ofmost BMGs. The novelty of the present work is that local deformation produced near a stress concentration (e.g., notch, fillet, threads, etc.) via simple precompression can be very effective in increasing the subsequent crack resistance of a component that contains a stress concentration. This work builds on very recent other work where this approach has also been shown to produce notch toughness increases under dynamic conditions. The approach developed presently somewhat mimics the toughening strategy that is effective in many natural biomineral materials. The excellent combination of strength and toughness of tooth tissues andmollusk shells stems from the fact that these biological materials incorporate strong-yetbrittle minerals and soft organic phases with sophisticated micro-architectures. The strong sub-micrometer building blocks provide strength while thin layers of soft organic phases encompass the mineral building blocks, maintaining integrity and dissipating energy during fracture. Fundamental to the enhanced toughness of these natural materials is the ability to channel and deflect cracks through the soft organic phases. The present approach to increase the notch toughness of BMGs attempts to mimic these biological materials by creating other mechanisms of crack deflection. A brittle Zr56Co28Al16 glass (ZC) and a tough Zr61Ti2Cu25Al12 glass (ZT) are chosen *[*] Dr. J. Yi, Prof. J. J. Lewandowski Department of Materials Science and Engineering, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, OH 44106, USA E-mail: jxy305@gmail.com Prof. W. H. Wang Institute of Physics, Chinese Academy of Sciences, Beijing 100080, China [**] The financial support of this work was provided by AROW911NF-12-1-0022 with partial support from DTRA-1-111-0064. W.H.W. appreciates the financial support from NSF of China (51271195). The authors appreciate Chris Tuma (Advanced Manufacturing and Mechanical Reliability Center) for help in mechanical tests. Supporting Information is available from the Wiley Online Library or from the author.