| Plasticity and flow in metallic glasses (MGs) is believed to be the result of the activation of defects in the amorphous structure. These defects are regions of the glass, thought to be comprised of 10--100 atoms, which can undergo a localized rearrangement in response to a shear stress (tau) and are called shear transformation zones (STZs). This dissertation is an investigation strain rate-dependence of the hardness in three, Zr-based bulk metallic glasses (BMGs) to back out information about the STZs. The primary characterization technique is nanoindentation, which is capable of determining both the static materials properties, such as hardness and elastic modulus, as well as the time-dependent properties, which include information on the activation of flow defects. Other experimental techniques, such as differential scanning calorimetry and X-ray diffraction, were applied to measure the thermophysical properties of the BMGs. The low-stress activation energies (Delta G0*) for defect rearrangement are in the range 3.5--10 eV, and the stress-dependence of the activation energy is a power-law of the form DeltaG*(tau) ∼ taun , where n is measured to be in the range 1.5--2.0. From the measurements it is possible to estimate the volume of the STZ, O*, which has not been measured previously. , O* , is found to vary anywhere from 100--300 atomic volumes. In addition to the experiments conducted on the as-cast alloys for the three different Zr-Cu-Al compositions, similar tests were carried out on the same materials after they had been subjected to heat-treatment near the glass transition temperature. Not only are the characteristics of the STZs sensitive to the composition of the BMG, but they also exhibit substantial changes upon annealing; the quantities DeltaG0* and O* decrease by 10--30%. These latter results are in good agreement with expected trends and provide some insight into the relationship between the structure and dynamics in the MG. |
