High Energy Synchrotron X-ray Study of Fatigue Damage in Amorphous and Polycrystalline Engineering Alloys

The dissertation summarizes a series of studies on the fatigue damage of both amorphous and crystalline engineering alloys. The attention focuses on the utilization of synchrotron x-ray scattering related techniques for advanced material characterizations. In the first part of the research, critical issues regarding the mechanical response and structural evolution of Bulk-Metallic Glasses (BMGs) in the elastic region are addressed. The effects of cyclic-loading on the microstructures of an amorphous alloy are investigated, aiming to provide better mechanistic understandings of fatigue damage in BMGs. The second part of the research focuses on the characterization method based on two-dimensional X-ray diffraction to better predict the fatigue life of Ni-based superalloys.

Bulk-amorphous metallic alloys are a new class of materials that exhibit superior material properties. X-ray pair-distribution function (PDF) analysis is used to study the deformation of BMGs on the microscopic scale in the elastic region. The results show that the deformation behavior of BMGs is fundamentally visco-elastic.

The effect of “fatigue” on the fatigue behavior and atomic structure of Zr-based BMGs has been investigated. Fatigue experiments on the failed-by-fatigue samples indicate that the remnants generally have similar or longer fatigue life than the as-cast samples. Meanwhile, the pair-distribution-function (PDF) analysis of the as-cast and post-fatigue samples showed very small changes of local atomic structures. These observations suggest that the fatigue life of the 6-mm in-diameter Zr-based BMG is dominated by the number of pre-existing crack-initiation sites in the sample

For the study of fatigue damage in Ni-based superalloys, the correlation between the microstructure, from the x-ray diffraction point of view, and fatigue life is established. The development of residual strain/stress, can be measured accurately by in-situ two-dimensional (2D) x-ray diffraction. The size of the compressive strain zone ahead of a notch tip increases with fatigue life and is most sensitive during the initial cycles and final stage. However, the estimation of fatigue damage is qualitative, not quantitative. Finally, the strain variation possibly caused by the intergranuler stresses is large at the beginning of the fatigue life, but decrease with increasing fatigue cycles, which indicates more and more grains were plastically deformed.