Effects of overload and frequency on fatigue crack propagation in nanocrystalline Zr-based bulk metallic glass

A nanocrystalline (NV) bulk glass Zr₅₅Al₁₀Cu₃₀Ni₅(at%) containing nano-scale crystals embedded uniformly in a glassy matrix has both high tensile strength of 1.7 GPa and high ductility. The new alloy is therefore expected to have practical applications in machines and structures. The influences of frequency and overload on fatigue crack propagation behavior of the NC bulk glass were examined. The fatigue crack propagation rate da/dn less than 3 × 10^-5 mm/cycle was independent of frequency in the frequency range of 0.1 to 50 Hz at the stress ratio of 0.1 under sine and triangular waves. When the overload ratio (overload/baseline load) was large a complete crack arrest occurred and the ΔK value just before a crack regrowth was three times larger than the threshold stress intensity factor range Δ K_th. The reason for the crack arrest was not explained by the crack closure effect. The overloading induced the kinking and branching of the crack. The stress reduction near the crack tip due to the kinking and branching of the crack and the crack closure effect gave an appropriate explanation for the complete crack arrest and the larger threshold stress intensity factor range. When the overload ratio was small, a temporary crack arrest occurred and the kink and branching of cracks occurred intermittently at the crack front.