Stress-life fatigue behavior and fracture-surface morphology of a Cu-based bulk-metallic glass

The stress-life fatigue behavior and fracture morphology of a (Cu₆₀Zr₃₀Ti₁₀)₉₉Sn₁ bulk-metallic glass alloy was investigated under both three-point and four-point bending conditions. For all stress levels tested, the fatigue lifetimes tended to be higher for the three-point loading condition. The fatigue endurance limits (defined as 10⁷ cycles without failure), based on the applied stress range, for three-point and four-point loading conditions were approximately 475 MPa and 350 MPa, respectively. All fracture surfaces were found to be composed of four main regions: a crack-initiation site, a stable crack-growth region, an unstable fast-fracture region, and a melting region. Finely spaced parallel marks, similar to fatigue striations found in crystalline alloys, oriented somewhat perpendicular to the direction of crack propagation were observed in the stable crack-growth region. Analyses of these marks found that their spacing increased with increasing stress-intensity-factor range. Damage was found to initiate from preexisting defects present on or near the surface.