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 σ_a = 440 MPa (in four-point bending at a loading ratio of 0.1), or ∼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 "zigzag" pattern. The relation between the range of stress intensity factor (ΔK) and the crack-growth rate (crack extension per cycle, da/dN) was also determined. ZT1 shows a fatigue threshold, ΔK_th, of 2.8 MPa√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-ΔK 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 ΔK_th of a BMG approximately scales with its fracture toughness. At different levels of ΔK, shear bands appear to play different roles, either facilitating or blunting the crack growth.