Temperature-dependence of mode I fracture toughness of a bulk metallic glass

Abstract

Within the temperature range over which the shear band (SB)-mediated plastic deformation is dominant, metallic glasses exhibit an intermediate temperature ductility minimum (ITDM), which occurs at about 65% of the glass transition temperature, Tg. This ITDM is associated with a small number of SBs, with each band carrying large amount of plastic strain, which in turn leads to their easy transition to shear cracks, eventually leading to fracture. Some MGs are known to exhibit high room temperature (RT) fracture toughness, which has been associated with SB-mediated crack-tip plasticity. Hence, it is expected that ITDM would also correspond to a minimum in toughness. In order to ascertain this, temperature-dependence of mode I fracture toughness, Jc, of a bulk metallic glass (BMG), Vitreloy 105, was investigated by recourse to 4-point bend testing of single edge notched specimens within 298–475 K range, which corresponds to ∼0.44 and 0.7Tg of the tested BMG. Complementary finite element analyses were utilized to convert the critical load for fracture into Jc. Results confirm a minimum in Jc at ∼0.67Tg, which is in agreement with the results of unnotched 3-point bend experiments on unnotched bars that show ITDM at 0.65Tg. These observations are rationalized with the aid of notch plastic deformation and post mortem fractographic characterizations 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 gradients ahead of the notch tip. This study highlights the sensitive nature of BMGs' fracture toughness, even when they are nominally ductile, to temperature.