Fatigue behavior of a zirconium-based bulk metallic glass

Although the mechanical behavior of Zr-based bulk metallic glasses (BMGs) has been studied, fatigue studies in controlled environments and testing conditions have not been adequately performed or understood. Early fatigue examinations have led to substantial variability of reported fatigue-endurance limit ranges. In the present research, several investigations were engaged to better understand changeability in the fatigue behavior of a Zr-based BMG, Zr52.5Al 10Ti5Cu17.9Ni14.6 (at. %). The studies were primarily conducted to explain how the loading conditions, the sample preparation, the quality of the glass materials, the test environment, or the chemical composition affect the degradation behavior of BMGs. Samples were tested in environments such as air, vacuum, and a hydrogen-charged state. Some of the more outstanding findings are discussed. Charged hydrogen embrittles the material with increases in hardness values and lower fatigue lifetimes for cathodically-charged samples. Fatigue studies were performed on button head, uniaxial specimens with different surface finishes in order to better understand the influence the average surface roughness and/or critical surface defects may have on the fatigue behavior. Careful studies of surface conditions indicate that fatigue-endurance limits are greatly impacted by the average surface roughness with possible reductions over fifty percent. Four-point and three-point bending fatigue studies were conducted to observe the effect of variability in loading conditions versus uniaxial tension studies, and to observe any impact from testing volume on the fatigue life of BMG-11. Both, three point and four point bend results seemed to exhibit slightly better fatigue behavior compared to the uniaxial tests. Lastly, a study was performed to better understand the effect crystallinity has on the fatigue behavior of Zr-based bulk metallic glasses. A large volume fraction of crystallinity has been shown to dramatically lower the fatigue lifetime of a Zr-based BMG. This careful study of fatigue behavior leads to the conclusion that the detrimental effect crystallinity and geometrical surface defects have on the fatigue-endurance limit and the fatigue lifetime explain the variability in previously reported results.