Elevated Temperature Fatigue Crack Propagation of a Zr-Ti-Cu-Ni-Be Bulk Metallic Glass
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CC9.10.1
Elevated Temperature Fatigue Crack Propagation of a Zr-Ti-Cu-Ni-Be Bulk Metallic Glass Peter A. Hess and Reinhold H. Dauskardt Department of Materials Science and Engineering, Stanford University, Stanford, CA 94305 ABSTRACT The sub-Tg elevated temperature fatigue propagation behavior of a Zr-based bulk metallic glass was examined. Fatigue crack-growth rates as a function of the applied stress intensity range, ∆K, are reported. Increased testing temperature was found to increase ∆KTH. Examination of crack surfaces at near-threshold growth rates revealed the presence of elongated ridges perpendicular to the crack front at higher temperatures; these ridges were absent in specimens tested at lower temperatures. The size of the features was found to increase with temperature and applied ∆K, finally breaking up at higher growth rates. Mechanisms responsible for the ridge formation are described, and modeled in terms of fluid meniscus instabilities that occur at the fatigue crack tip.
INTRODUCTION While Zr-based bulk metallic glasses have been shown to have remarkable monotonic mechanical properties [1-6], there is significant evidence that they are quite susceptible to fatigue processes [7, 8]. A low fatigue crack-growth threshold, ∆KTH, and endurance limit, σe, under cyclic loading are among the foremost concerns for structural applications. Although the mechanisms of fatigue crack initiation and propagation in bulk metallic glasses are not wellunderstood, fatigue crack growth has been explored to some extent in Ni-Si-B thin ribbon metallic glasses [9]. This study used fractographic evidence in order to propose that fatigue cracks propagate along fractured ligaments between shear bands at higher growth rates, but at lower growth rates the cracks instead propagate within shear bands. Subsequent studies on bulk amorphous Zr41.25Ti13.75Cu12.5Ni10Be22.5 (Vitreloy 1) have proposed a striation-based mechanism in which the crack propagation rate, da/dN, was proportional to the change in crack tip opening displacement, δ [3, 8, 10]. The present study examines the effect of sub-Tg elevated temperature on Mode I fatigue crack propagation of Vitreloy 1. The fatigue threshold, ∆KTH, has been reported in this material to be from 1-3 MPa-m1/2 [8, 10]. Near-threshold morphology has been shown to be mirror-like in macroscopic appearance, and nearly featureless when analyzed microscopically. As the vast majority of fatigue crack lifetimes are typically spent near or below the fatigue threshold, particular attention is paid to this regime of crack growth. The premature contact of mating crack faces, otherwise known as crack closure, can have a large effect on crack propagation in this near-threshold regime. Therefore, the influence of crack closure on high-T fatigue is also examined, as well as the fracture surface morphologies from low- to mid-range growth rates.
CC9.10.2
EXPERIMENTAL Compact tension fatigue specimens were machined from ~3 mm thick plates with dimensions in general accordance with the ASTM-E647 standard fo
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