Advancing the understanding of bulk metallic glasses as structural materials through exploration of mechanical properties Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/4q77fv29n

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  • 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 in 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. Additionally, the author hypothesized that the root cause of the fatigue plateaus in Zr-based BMGs is a reactive species in the ambient air, though not water itself. 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 K[subscript IC] standards found in ASTM E399 regardless of un-cracked ligament size had statistically similar values of K[subscript IC]. However, comparing these results to samples only conforming to the J-integral standards found in ASTM E1820 showed higher values of K[subscript J] by a statistically significant margin. Additionally, sub-sized samples (those which did not conform to either K[subscript IC] or J-integral requirements) were found to show an increase in conditional stress intensity factor, K[subscript Q]. These results are of particular interest as they are in contrast to reported results for crystalline metals. It was concluded that there are no size effects in samples that conform to the K[subscript IC] standards; however, size does affect results for samples analyzed by the J-integral method. The author hypothesized that the size dependence of the J-integral method is the result of the strain softening behavior of BMGs which is in contrast to the strain hardening behavior expected in the J-integral method. Additionally, the author hypothesized that the increase in K[subscript Q] in sub-sized samples is the result of the size dependent plasticity reported for BMGs at small sample sizes. 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 modulus followed by a slow rise in elastic modulus. 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. The author hypothesized that the microscopic cause of the two phase slow β relaxation is similar to a previously reported process in silicate oxide glass. The author hypothesized that the two phases corresponded to; first, the low density flow units in the BMG spontaneously collapsing, locally stressing the surrounding high density matrix, second, with continued thermal energy, the local stresses being uniformly released into the collapsed flow unit, stressed matrix and surrounding high density matrix allowing the BMG to microscopically form shear transformation zones. These results and hypothesis hope to further the understanding of BMGs and enhance their use in scientific, commercial and industrial applications.
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