Effect of pre-stress on the onset of yielding in bulk metallic glasses

Abstract To investigate the intrinsic effect of the pre-stress (σpre) on the onset of yielding, nanoindentation experiments are performed on two metallic glasses (MGs) which are elastically-bent and constrained within a home-made apparatus. Statistical calculations for the first pop-ins are made and noticeable decrease in maximum shear stress (τmax) is observed under tensile pre-stress, but only slight increase under compressive one, without considering the pre-stress effect on the ratio (0.29) of τmax to the maximum contact pressure (p0). This asymmetry phenomenon is related to the anharmonic and asymmetric nature of the interatomic interaction potential caused by the applied pre-stress. Moreover, finite-element analysis is conducted to obtain the elastic stress field of the pre-stressed region upon indentation, which becomes anisotropy and quarter-symmetric. τmax for the specimen is defined as 0.29p0 + 0.50σpre for the tensile-stressed region and 0.29p0 for the compressive-stressed region within elastic strain limit of MGs, which requires the correction for the intrinsic τmax with tensile σpre, but not for compressive σpre. The corrected τmax and activation energy barrier of shear transformation zone are found to be independent on σpre, suggesting that the intrinsic shear stress is a constant and the yielding of MGs obeys the maximum shear stress criterion.