Advancing the Understanding of Bulk Metallic Glasses as Structural Materials through Exploration of Mechanical Properties

Bulk metallic glasses (BMGs) offer a great deal of potential through their near-theoretical strength, potentially high toughness and unique forming and molding traits. However, there are still a number of outstanding issues that bar their widespread use. This thesis describes the investigation of three research questions, each designed to further the scientific community’s understanding of BMGs. First, the strong fatigue properties of Zr52.5CU17.9Ni14.6Al10Ti5 BMG were explored as a function of the ambient environment by testing an ambient air (20 to 40 % relative humidity) and a dry inert nitrogen environment. It was found that in both environments Zr52.5CU17.9Ni14.6Al10Ti5 exhibited a relatively high fatigue threshold of ~2 MPa√m and lacked the environmentally influenced stress insensitive fatigue crack growth rate plateaus observed in other Zr-based BMGs tested in ambient air. Second, the effect of sample size upon the fracture properties of BMGs was explored. SE(B) and C(T) BMG test samples of composition Zr52.5CU17.9Ni14.6Al10Ti5 and varying un-cracked ligament sizes were tested. It was found that all samples that conformed to the KIC standards found in ASTM E399 regardless of un-cracked ligament size had statistically similar values of KIC. However, comparing these results to samples only conforming to the J-integral standards found in ASTM E1820 showed higher values of KJ by a statistically significant margin. Additionally, sub-sized samples (those which did not conform to either KIC or J-integral requirements) were found to show an increase in conditional stress intensity factor, KQ. These results are of particular interest as they are in contrast to reported results for crystalline metals. Finally, the mechanical effect of the slow β relaxation in BMGs was explored. Au49Ag5.5Pd2.3Cu26.9Si16.3 BMG samples were tested by dynamic mechanical analysis (DMA), isothermal aging near the slow β relaxation temperature and compression testing. The results of DMA testing revealed the slow β relaxation temperature for Au49Ag5.5Pd2.3Cu26.9Si16.3 to be ~50 oC. Isothermal aging revealed that at the slow β relaxation temperature, two specific effects are taking place; an initial quick drop in elastic moduli followed by a slow rise in elastic moduli. Compression testing was performed using as-cast and heat treated samples at a minimum observed elastic modulus. These compression tests revealed a statistically insignificant decrease in compressive strength with heat treatment. These results hope to further the understanding of BMGs and enhance their use in scientific, commercial and industrial applications. Monday, June 10 th 2013 10:00 AM, Covell 117 School of Mechanical, Industrial, and Manufacturing Engineering