Influences of residual stresses on the serrated flow in bulk metallic glass under elastostatic four point bending A nanoindentation and atomic force microscopy study
Abstract The effects of residual stress on the deformation behavior of a Zr based bulk metallic glass (BMG) during nanoindentation were studied by atomic force microscopy. The residual stress was introduced by elastostatically preloading a beam shaped BMG sample by four point bending up to tensile and compressive stress levels of 2.0 GPa for up to 14 days. Strain rate controlled nanoindentations were performed on the four point bent samples at various times during loading and after unloading to analyze the serrated flow during indentation. The hardness of the alloy, the pile up behavior as well as the serrations strongly depend on the magnitude and sign of the applied residual stresses. Tensile stresses suppress pile up formation, decrease the hardness but increase the jump width of the serrated flow during nanoindentation. In contrast, increased pile up formation with increased hardness occurs along with a successive serrated flow behavior on the compression side. The discrepancy of pile up and serrated flow is explained by a difference in the shear banding mechanism. The results suggest that for compressive stress individual shear planes are successively activated, leading to localized shear steps on the surface. For tensile residual stresses, the plastic volume is more widely spread, leading to vanishing pile up together with an intermittent activation of a big number of shear events, causing big serrations. Due to the widely varying pile up behavior, a hardness correction was performed. This strongly reduced the apparent hardness variations across the beam. For this specific testing arrangement, only reversible mechanical property variations with time due to long time prestraining at high elastostatic stresses were observed.