Capture the early stage of shear banding for bulk metallic glass sheet for MEMS applications

Bulk metallic glasses (BMGs) are a class of promising candidate materials which applied in Microelectromechanical Systems (MEMS). However, a suitable mechanical parameter and an effective measurement approach are still lacked to characterize the onset of plastic deformation exactly for BMG sheets in bending. In this work, bending proof strength with permanent strain at the level of 0.005% (σ_p,0.005%) was proposed to represent the exactly onset of yielding for BMG sheets. This mechanical parameter was measured by a series of stepwise cyclic loading–unloading tests, which are particular suitable for timely capture the earlier stage of shear banding for BMG sheets within MEMS scale, thus showing the significant superiorities to monotonic loading test and conventional cyclic tests which needed to be equipped with the deflectometers. The σ_p,0.005% of Zr₆₁Ti₂Cu₂₅Al₁₂ (ZT1) BMG sheet with thickness of 100 μm was determined to be 1900±10 MPa, and physically correlates to the propagation stress of embryonic shear band instead of the nucleation stress. In addition, a quantitative correlation between permanent strain and shear offsets on BMG sheet surface has established, thus the permanent strain of thinner BMG sheets can be attained by a simple approach, and without relying on the conventional contact-type deflectometers with high accuracy. The σ_p,0.005% can be used as a more reliable and rational design parameter for thinner BMG sheets which applied in MEMS fields.