Studies on fracture of bulk metallic glasses and their composites

• Main Author: Devashish Rajpoot
• Other Authors: Upadrasta Ramamurty
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2023
• Subjects: Engineering::Materials::Testing of materials; Engineering::Materials::Material testing and characterization; Engineering::Materials::Composite materials; Engineering::Materials::Mechanical strength of materials; Engineering::Mechanical engineering::Mechanics and dynamics
• Online Access: https://hdl.handle.net/10356/166493
• Institution: Nanyang Technological University

Bulk metallic glasses (BMGs) possess incredible mechanical properties such as superior yield strength, high elastic strain limit, corrosion resistance, and good thermal and electrical conductivity. However, poor fracture toughness and nil ductility at room temperature limits their utility in structural applications. In order to alleviate the above mentioned drawbacks, secondary phases in the form of dendrites (in situ BMG matrix composite (BMGC)), particles or fibres (ex situ BMGC), and pores (porous BMG) are introduced. These secondary phases restrict shear band propagation and lead to proliferation of multiple shear bands in the BMG derivatives mentioned above. In this thesis, in order to assess the structural applicability of in situ BMGCs, first, fracture experiments have been conducted. Fracture behavior of in situ BMGCs containing both transforming and non-transforming β-Ti dendrites under shear and opening modes are examined. Experimental results show that the fracture toughness of all the tested BMGCs is considerably lower in mode II than in mode I, due primarily to the shear dominant stress state of the former, which renders easy shear band initiation. However, stable crack growth in mode I is insignificant whereas it is considerable in mode II. The toughness of BMGCs reinforced with coarse but non-transforming dendrites in both the modes is higher than the respective values in BMGCs with transforming β-Ti dendrites inspite of their ability to strain harden and exhibit enhanced ductility. Fracture surface features and shear band patterns at notch tips of both mode I and mode II specimens indicate that despite the interactions of dendrites with the shear bands, the fracture criterion and mechanism of in situ BMGCs is identical to that of monolithic BMGs. The differences in the fracture behaviour of these BMGCs is rationalized by considering the effect of relaxation enthalpy of the amorphous matrices in addition to the length scale and transforming tendency of the dendrites on the transition of a shear band into a crack. Implications of these results in terms improving the fracture toughness of BMGCs with transforming dendrites is discussed. A β-Ti dendrite reinforced Zr-based BMGC is found to be brittle when cast in large size. The reasons for the embrittlement and effectiveness of cryothermal cycling (CTC) treatment in restoring the mode I fracture toughness are examined. Plasticity in all the CTC treated BMGCs is estimated from the distribution and occurrence of pop-ins in nanoindentation tests and by measuring the magnitude of enthalpy of relaxation (∆Hrel) via differential scanning calorimetry (DSC). This is further validated by examining the strain-to-failure ( f) in compression tests. Mode I fracture behaviour of as-cast embrittled BMGC and CTC treated BMGC, which exhibits maximum plasticity, is examined. Results show that both BMGCs are equally brittle and exhibit 5 times lower notch toughness (KQJ) than their tougher counterpart. Postfacto imaging of the side surfaces reveals the absence of notch-tip plasticity in both BMGCs. The lack of notch tip plasticity of CTC treated BMGC, despite exhibiting signatures of plasticity in nanoindentation and higher ∆Hrel is rationalized by reassessing the origin of pop-ins in nanoindentation tests and describing the variations in chemical and topological short range ordering during CTC, respectively. Implications of these results in terms of improving the fracture toughness of structurally relaxed BMGCs via CTC are discussed. Fracture tests using notched micro-cantilever (MC) specimens are increasingly being used to measure the fracture toughness of materials at the micro-scale. Detailed finite element analyses (FEAs) of loading of self-similar micro- and bulk cantilever beam fracture specimens using isotropic, elastoplastic constitutive models, are conducted to critically examine the validity of the toughness data obtained using MC specimens. From the simulated normalized load versus load point displacement and the corresponding normalized energy release rate versus load curves, the transition of the failure regimes from crack propagation to plastic collapse of the uncracked notch ligament are identified. While the crack propagation regime allows for the estimation of valid fracture toughness of the probed material, the plastic collapse regime does not. The effects of specimen aspect ratios, material hardening, and yield criteria on the master curves and transition point are examined. A method to interpret the failure regimes, assess the validity and size effects in micro-cantilever fracture tests is proposed. An expression is derived for the minimum size requirements of MC fracture specimens in order to get a valid fracture test. The data reported in the literature from fracture tests on bulk metallic glasses, nano-crystalline materials and ultra-fine grained materials are assessed using the proposed methodology. The present work has important implications for specimen design, interpretation of failure regimes, and potential size effects in MC fracture tests. In summary, in this thesis, implications of results obtained in terms of improving the fracture toughness of BMGCs with transforming dendrites and improving the fracture toughness of structurally relaxed BMGCs via CTC treatment are explored. Finally, the implications of the results for specimen design, failure regime interpretation, and potential size effects in the notched micro-cantilever fracture tests are explored.