Shear Processes in Pd$_{40}$Ni$_{40}$P$_{20}$ Bulk Metallic Glasses

Depending on the imposed deformation rate, plastic deformation in metallic glasses can be either Newtonian or non-Newtonian. To investigate the influence of deformation history on non-Newtonian plastic flow in bulk Pd$_{40}$Ni$_{40}$P$_{20}$ glass we deformed the same volume of the glass specimen along differently oriented glide planes. We found that the glass has a memory of its previous plastic deformation, but this memory is largely independent of the previous glide direction. Loss of the memory follows first-order kinetics with a time constant of 1260 s at 553 K. The transition from Newtonian to non-Newtonian flow is rather abrupt and occurs at a Deborah number, \textit{De = }$\dot {\gamma }\,\cdot \tau $ = 0.5, where $\dot {\gamma }$ is the plastic shear strain rate and $\tau $is the time constant for the exponential annihilation of the flow defects. This value of \textit{De} is consistent with the value of \textit{De} $\approx $ 1 observed at the onset of flow instabilities in liquids. The abruptness of the transition, together with the strong stress-sensitivity of the viscosity in the non-Newtonian regime, suggests that the plasticity agents in the Newtonian and non-Newtonian flow regimes are not the same.