Fracture and fatigue crack propagation in a nickel-base metallic glass

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This work is an investigation of fracture and fatigue in thin ribbons of a nickel-base metallic glass: Ni78Si10B12. The fracture and fatigue crack propagation behavior of this high tensile strength and high toughness amorphous alloy is of interest for two reasons: (1) the alloy, has no normal microstructure, and (2) the alloy shows an unusual form of plastic deformation which proceeds by nucleation and propagation of localized shear bands. On uniaxial tensile loading, failure of uniform ribbons occurs instantaneously at the yield stress by shear rupture through an intense shear band inclined at 55 deg to the loading axis. The development of a local plastic zone at the crack tip in single edge-notched specimens under monotonic tensile loading has been studied by a replication technique. Under plane stress conditions, these plastic zones are dominated by shear bands elongated in the direction of crack extension. Dugdale's “strip yield” model offers a reasonable description of the plastic zone sizes and displacements at the crack tip. The relationship between fatigue crack growth per, cycle,da/dN, and the alternating stress intensity factor, ΔK, has been determined atR=0.1. For growth rates in the range 10−6 through 5×10−4 mm/cycle, the Paris law (with an exponentm∼-2) is obeyed. The mechanism of fatigue crack extension is shown to depend on the deformation microstructure of the alloy. At intermediate growth rates, the plastic zone consists of a number of shear bands similar in shape to the Prandtl slip line field for nonhardening materials. Decohesion along these bands produces undulating fracture morphologies. At near threshold values of ΔK, growth rates deviate from the Paris law producing an extremely low ΔK TH (=0.5 MPa {\(\sqrt m \)}). This is attributed to the ease of shear band nucleation, and a simple geometrical model of crack growth at low ΔK levels is proposed.