Bending fatigue behavior of thin Zr61Ti2Cu25Al12 bulk metallic glass beams for compliant mechanisms application

In this work, bending fatigue behavior of thinner Zr61Ti2Cu25Al12 (ZT1) BMG beams with thicknesses of 500 mu m and 100 mu m were investigated with three-point bending (3PB), to evaluate the reliability and safety of this potentially material used in compliant mechanisms under cyclic bending load. Fatigue endurance limits (FELs) of ZT1 beams with thicknesses of 500 mu m and 100 mu m were determined to be 470 MPa, and similar to 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 mu m down to 100 mu m. Fatigue cracks of all ZT1 beams initiated at the extrinsic microvoids, either indirectly derived from surface scratches during mechanical polishing or originated from as-cast 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 shared the crack-initiation life and crack-propagation life. In addition, there are two crack deflection mechanisms during cracks steady propagation stage. One mechanism is crack deflection along the interacted shear bands ahead of crack-tip, the other one is crack jump from one shear band to another shear band at a certain angle. These two mechanisms alternate dominate the crack propagation process, generating typical staircase-like crack propagation trajectory. Fatigue endurance limits of ZT1 beams in the form of either stress amplitude or strain amplitude are superior to traditional candidate materials in the field of compliant mechanisms, showing that the better reliability and larger deflection deformation can be achieved for flexible members made by ZT1 BMG even under cyclic loading. (C) 2021 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.

Automated summary

The bending fatigue behavior of thinner Zr61Ti2Cu25Al12 (ZT1) BMG beams was investigated under cyclic bending load using three-point bending tests. The fatigue endurance limits (FELs) and fatigue life of ZT1 beams were found to be independent of beam thickness, with fatigue cracks initiating at extrinsic microvoids. Crack propagation behavior was influenced by cyclic stress level, with different mechanisms dominating crack propagation and generating unique crack trajectories. Fatigue endurance limits of ZT1 beams were superior to traditional materials for compliant mechanisms, demonstrating better reliability and larger deformations under cyclic loading.