Ultra-high cycle fatigue properties in Zr₅₅Al ₁₀Ni₅Cu₃₀ metallic glassy alloy using ultrasonic fatigue testing machine

In this work, the ultra-high cycle fatigue behavior of Zr ₅₅Al₁₀Ni₅Cu₃₀ metallic glass was investigated by using ultrasonic fatigue testing method. Test alloy rods with a diameter of 8mm were prepared by tilt-casting with copper mold. Test specimens were machined to hourglass shape type (the minimum diameter ; 2.0mm), and after machining the surface of specimens was manually polished with polishing paste. The specimens were tested at a frequency of 20kHz under a stress ratio of -1. The fatigue limit ( s w) (half a total stress amplitude) and fatigue ratio ( s w/ s B) of the alloy showed 893MPa and 0.53, respectively. Furthermore, we did not see any fish-eye like crack-growth morphology on fracture surfaces. The initiation region of fatigue crack was always observed at the side surface of the specimens. A viscous-flow fracture region with large dimples between the fatigue crack-growth region and the final unstable fracture region was observed. This peculiar fracture morphology may be due to adiabatic heating of the metallic glass specimens during the fracture, resulting in transition to supercooled liquid, under extremely high frequency of fatigue stress using ultrasonic loading.