| As an emerging mechanism,compliant mechanisms(CMs)offer great promise in providing new and better solutions to many mechanical design problems.Bulk metallic glasses(BMGs)with amorphous structure exhibit some significant advantages in mechanical properties,typically showing high yield strength and large elastic strain limit(～2%),making BMGs potentially become a new generation of materials used for CMs.In the present work,high-toughness and high fatigue endurance limit Zr61Ti2Cu25Al12(ZT1)BMG was chosen to investigate its bending behaviors,including three main aspects.(ⅰ)Determination of bending proof strength for ZT1 BMG,and its correlation with shear bands initiation.(ⅱ)Characteristics of nonlinear large deflection bending were investigated for ZT1 beams,and bending proof strength for 100 μm thickness ZT1 beam was determined by nonlinear bending theory(ⅲ)Bending fatigue behaviors of ZT1 beams with thicknesses of 500 μm and 100 μm were investigated under three-point bending(3 PB)loading.To meet the requirement of small-deflection condition in mechanics,a range of span-to-thickness ratio was specified at 10-15 for the BMGs subjected to 3PB,which is one order of magnitude less than that of conventional polycrystalline metals.With monotonic bending tests,the modulus of elasticity in bending and flexural yield strength of the ZT1 were determined to be 84 GPa and 2070 MPa,respectively.Combining the stepwise loading-unloading with LOM observations alternatively,it was shown that apparent yielding response of ZT1 beam directly correlates with the event of shear-band initiation.With simplified cyclic loading-unloading tests,bending proof strength at a permanent strain of 0.005%for the ZT1 BMG was determined to be 1900±10 MPa.It is noticeable that bending proof strength of ZTI BMG is～16%higher than uniaxial tensile strength.In addition,large modulus of resilience(22 MJ/m3)in bending for this BMG manifested its significant advantage as candidate material of flexible components in engineering.Higher flexibility is achievable by increasing the span-to-thickness ratio,which is desired for flexible members subjected to larger-deflection performance.Moreover,yielding event of the MG beams in large-deflection situation can be captured via a conversion of load-deflection response into the non-dimensional form.Meanwhile,intrinsic sources responsible for the buckling events can be justified in terms of theoretical analytical solutions in mechanics.With monotonic bending and nonlinear bending theory,flexural yield strength of ZT1 MG with a thickness of 100 μm was determined to be 2050 MPa.Moreover,its bending proof strength at 0.005%permanent strain(σp,0.005％)was determined to be 1900 MPa,indicating that the beam-thickness effect on σp,0.005%is absent at least when the thickness was reduced from 1 mm down to 100 μm.The response at σp,0.005%physically correlates with the propagation stress of embryonic shear bands,instead of the nucleation stress.In comparison with the current candidate materials in compliant mechanisms,MGs exhibit significant advantages in the components that the larger-deflection elastic bending and high elastic-strain energy store are desired.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 several millimeters down to 100 μm.Fatigue cracks of ZT1 beams initiated from the extrinsic microvoids,either indirectly derived from surface scratches during mechanical polishing or originated from casting process.In addition,fatigue crack propagation behaviors depend on the magnitude of stress amplitude.Larger proportion about 60-70%of fatigue life was spent on crack propagation at high stress amplitude,while crack initiation occupied larger proportion about 80-85%at low stress amplitude.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.There two mechanisms alternate dominate the crack propagation process,generating typical staircase-like crack propagation trajectory.FELs of ZT1 beams are superior to traditional candidate materials in the field of CMs,thus,the better reliability and larger deflection deformation can be achieved for flexible members made by ZT1 BMG under cyclic loading. |
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