Fatigue endurance limit and crack front evolution in metallic glass

Fatigue endurance limit is a threshold stress-amplitude under which a solid subjected to a cyclic loading can sustain infinite life. Such a limit has been confirmed in ferrous materials but remains controversial in many advanced new materials, including bulk metallic glasses with superb strength. By using a combination of ultrasonic fatigue (USF) test and conventional fatigue test, we obtain the stress amplitude vs. loading cycle curve of the Vitreloy 1TM metallic glass, with loading cycles up to 109. There is a clear fatigue endurance limit (FEL) which is about 320 MPa, and is 17% of the strength of Vitreloy 1TM. The residual strength of survivals after 109 cycles is nearly the same as those of intact samples. We demonstrate that all fatigue cracks initiate from spherical pores which are inevitable and intrinsic amid casting and fast cooling, and the fatigue crack front resembles an ellipse arc till final rapid rapture of all tested samples. The size and position of a pore can be utilized to extract the fatigue initiation threshold K0th, and the elliptical fatigue crack front is used to obtain the mode I fracture toughness KIC, which are K0th=2.0±0.5MPam and KIC=18-20MPam for Vitreloy 1TM, respectively. The fatigue endurance limit and its intrinsic origin of Vitreloy 1TM pave the way to understand fatigue in other metallic glasses.