Fatigue endurance limit and crack growth behavior of a high-toughness Zr61Ti2Cu25Al12 bulk metallic glass

We report the fatigue damage behavior of a Zr61Ti2Cu25Al12 (ZT1) bulk metallic glass (BMG), which was known to have a record-high fracture toughness. The ZT1 exhibits the highest fatigue endurance limit among all BMGs, with a normalized fatigue limit of sigma(a) = 440 MPa (in four-point bending at a loading ratio of 0.1), or similar to 0.27 of its tensile strength. The crack-growth resistance in the stress/life tests arises from crack-tip plastic shielding due to prolific shear-banding, as well as deflected crack path following a zig-zag pattern. The relation between the range of stress intensity factor (Delta K) and the crack-growth rate (crack extension per cycle, da/dN) was also determined. ZT1 shows a fatigue threshold, Delta K-th, of 2.8 MPa root m, which appears to be related to the onset of shear band nucleation. Three distinct regimes of fatigue crack growth were observed, with different slopes in the da/dN-Delta K curve, attributable to different micromechanisms. In the Paris regime, each fatigue striation on fracture surface is generated by a number of loading cycles, rather than by a single loading cycle. In general, the Delta K-th of a BMG approximately scales with its fracture toughness. At different levels of Delta K, shear bands appear to play different roles, either facilitating or blunting the crack growth. (C) 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.