Bending fatigue behaviors of thin Zr61Ti2Cu25Al12 bulk metallic glass beams for compliant mechanisms applications

Abstract In this work, bending fatigue behaviors of thinner Zr61Ti2Cu25Al12 (ZT1) BMG beams with thicknesses of 500 μm and 100 μm were systematically investigated in three-point bending (3PB), with the aim to evaluate the reliability and safety of this material used in compliant mechanisms under cyclic bending load. Fatigue endurance limits (FELs) of ZT1 beams with thicknesses of 500 μm and 100 μm were determined to be 470 MPa, and ∼0.3 of its tensile strength. Fatigue life and FEL of ZT1 beams in bending are substantially independent of the beam thickness, in the scale from 500 μm down to 100 μm. Fatigue cracks of all ZT1 beams initiated from the extrinsic microvoids, either indirectly derived from surface scratches during mechanical polishing or originated from casting process. However, fatigue crack propagation behavior is highly related to the magnitude of cyclic stress level. By interrupted fatigue tests, larger proportion about 60-70% of fatigue life was spent on crack propagation under high stress level, while crack initiation occupied larger proportion about 80-85% under low stress level. The medium stress level of about 590 MPa almost bisected the crack-initiation life and crack-propagation life. In addition, there are two crack deflection mechanisms during cracks steady propagation stage for ZT1 beams. One mechanism is crack deflection along the interacted shear bands ahead of crack-tip, the other mechanism is crack “jump” from one shear band to another shear band at certain angle. There two mechanisms alternate dominate the crack propagation process, generating typical staircase-like crack propagation trajectory. Fatigue endurance limits of ZT1 beams either in the form of stress amplitude or strain amplitude are superior to traditional candidate materials in the field of compliant mechanisms, thus, the better reliability and larger deflection deformation can be achieved for flexible members made by ZT1 BMG even under cyclic loading.