| dc.description.abstract | Bulk metallic glasses (BMGs) possess very attractive mechanical properties like high yield strength (~ 2 GPa), high elastic strain limit (2 %), and good corrosion resistance. BMGs display fracture toughness values (in mode I) which are ranging from being extremely brittle (less than 5 MPa√ ) to very tough (150 MPa√ ). Further the fracture toughness of BMGs is highly sensitive to intrinsic and extrinsic factors like composition, temperature of testing, thermal history, impurity content, sample size, fracture testing mode and crack tip dimensions. Thorough understanding of the above issues is essential for their safe deployment in structural applications. Hence, finite element and experimental studies on fracture behavior of BMGs are undertaken in this thesis.
In order to understand the effect of testing temperature on the fracture toughness and ductility of the BMG a temperature dependence study is undertaken. Complimentary finite element analyses were utilized to convert the critical load to fracture toughness. Temperature dependence mode I fracture toughness and three point bend experiments study reveals that toughness and ductility minimum occurs at same temperature range which is about 65% of glass transition temperature. These observations are rationalized with the aid of notch plastic deformation and post mortem fractographic characterization and in terms of the influence of temperature on factors such as the number of shear bands, the barrier for their conversion into shear cracks, and hydrostatic stress gradient ahead of the notch tip. This study highlights the sensitive nature of BMGs fracture toughness, even when they are nominally ductile, to temperature.
It is well recognized qualitatively, that shear band mediated plasticity ahead of crack or notch tips is the reason for the high fracture toughness of 'ductile' bulk metallic glasses (BMGs), quantitative connection between those two material properties is yet to be established. In an attempt to study this, we examine if mode I fracture initiation toughness,
iii
KIc, of a number of BMGs can be related to the shear band number, Ni, which is a discretized measure of plasticity in MGs, around spherical indentation impressions that are made to a fracture mechanism based predetermined indentation strain. Results show that KIc scales with (Ni)3/2. Then, the relation between the shear band density in the notch tip plastic zone, Nn, and KIc is examined, which shows that a power law: KIc (Nn)1/2, captures the data reported in literature for a number of BMGs. This result confirms that it is indeed the notch tip plasticity that determines KIc of BMGs. The power law exponent of 0.5 is rationalized by recourse to elasto-plastic fracture mechanics. Possible connections between Ni and Nn, ways of enhancing the latter so as to increase KIc, and the central role played by the relative density of MGs in determining both elastic, plastic, and fracture responses are discussed.
Finally, we studied notched pure torsion as well as tension-torsion experiments, coupled with FEA, on notched Zr-based BMG bars. The notch tip deformation and fracture initiation processes is studied in detail. Further, we have compared the deformation and fracture responses of shallow and sharp notched specimens. It helped in understanding the effect of change in the notch acuity on shear band deformation patterns as well as effect on the hydrostatic stress distributions at the notch root. A comparison of the fracture toughness values in all the three modes and mixed mode (I and III) is carried out. Results show that mode II fracture toughness is lower in comparison of mode I and mode III. Fracture toughness obtained from mode II tests may serve as a conservative baseline for using BMG in designing structural applications. | en_US |
