Fatigue crack growth in bulk metallic glasses.

摘要

With high strength-to-density ratios, large elastic limits, good corrosion resistance, exceptional formability, and other attractive properties, bulk metallic glass (BMG) alloys are being considered for use in a range of high-tech industries. For load-bearing applications where exposure to alternating loads is expected, the fatigue behavior of BMGs must be critically assessed before they can be considered for use. Recent studies have shown that fatigue life in BMGs is controlled by growth, rather than initiation, of cracks. The objective of this research was to understand the mechanisms and behavior of growing fatigue cracks in BMGs, both under steady-state and variable-amplitude fatigue loading conditions. The role of temperature, free volume and glass relaxation on fatigue crack-growth properties of these metastable solids was also examined.; Steady-state fatigue crack growth in BMGs has been proposed to be due to cyclic crack tip blunting and resharpening. Implications of and deviations from this theory are discussed. Results from elevated temperature steady-state fatigue crack growth tests showed an increase in fatigue threshold for temperatures approaching the glass transition temperature. Inspection of elevated temperature near-threshold fatigue-fractured surfaces revealed the presence of a regular pattern of ridges running parallel to the crack-growth direction, indicative of a destabilized non-planar crack front. The shape of these ridges was predicted using a Mohr-Coulomb yield criterion. Ridge wavelengths were found to increase with stress intensity and temperature. Ridge formation conditions are discussed in terms of planar crack stability.; Various types of variable-amplitude load testing were performed on BMG, resulting in transient fatigue crack-growth behavior. Crack-closure arguments were found to be unsatisfactory in explaining the transient fatigue behavior, especially given the lack of distributed plasticity in BMGs. Despite the lack of plasticity, near-tip damage during fatigue crack growth was found to be primarily homogeneous in nature, rather than concentrated heterogeneously in shear bands. Transient fatigue crack-growth results are explained on the basis of free volume accumulation near the fatigue crack tip and its attendant effects on crack-tip shielding levels. The model results were used to accurately predict transient fatigue crack-growth rates following variable-amplitude loading.