Studies on fracture of bulk metallic glasses and their composites

Abstract

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.iii First and foremost, I am very grateful to my supervisors, Prof. Parag Tandaiya and Prof.Upadrasta Ramamurty, for their unwavering support, motivation, and guidance during my PhD.Both have my heartfelt gratitude for their keen interest in this work and passionate participation in every aspect of it. Prof. Parag Tandaiya's prompt direction in carrying out the finite element analysis and experiments is greatly appreciated. His in-depth technical knowledge and enthusiasm for research helped me a lot. Prof. Upadrasta Ramamurty deserves special thanks for providing me an opportunity to join NTU Singapore and conduct experiments. I express my sincere gratitude to Prof. Lakshmi Narayan Ramasubramanian and Prof. Long Zhang for their valuable support in procuring the materials. Without the continuous support of Prof. Lakshmi Narayan Ramasubramanian, preparing the papers would ha...