Mechanical Behavior of Nanocrystalline Palladium at High Strains and High Strain Rates

Abstract

We suggest a simple method to study the mechanical behaviour of nanocrystalline (nc) samples in compression-torsion mode. High applied pressure prevents the fracture of sample, and quantitative parameters of sample response during torsion test can be compared with developed microstructure. Here we present and discuss the results of systematic investigation of mechanical behaviour of nc Pd with a mean grain size of 12 nm in a wide range of shear strains (0<γ<200) and at strain rates γ& = 3 10-1 s-1 and 3 10-2 s-1. We show that in the studied shear strain range the notable changes in the microstructure, namely a strain induced grain growth occurs, and that controls the relevant deformation mechanisms. For lower strains when the grain size is still small enough, the plastic flow governs by twinning and probably grain boundary sliding. The flow stresses are lower as compared with the later stages of deformation, when the grain size becomes larger and deformation is controlled exceptionally by dislocation glide. Finally, a steady stage is achieved, when the grain size, dislocation density and flow stress are saturated.