High strain rate properties of metals and alloys

Abstract

The high strain rate dependence of the flow stress of metals and alloys is described from a dislocation mechanics viewpoint over a range beginning from conventional tension/compression testing through split Hopkinson pressure bar (SHPB) measurements to Charpy pendulum and Taylor solid cylinder impact tests and shock loading or isentropic compression experiment (ICE) results. Single crystal and polycrystal measurements are referenced in relation to influences of the crystal lattice structures and nanopolycrystal material behaviours. For body centred cubic (bcc) metals, the strain rate sensitivity (SRS) is in the yield stress dependence as compared with the face centred cubic (fcc) case of being in the strain hardening property. An important consequence is that an opposite ductility influence occurs for the tensile maximum load point strain that decreases with strain rate for the bcc case and increases with strain rate for the fcc case. Different hexagonal close packed (hcp) metals are shown to follow either the bcc or fcc case. A higher SRS for certain fcc and hcp nanopolycrystals is explained by extrapolation from conventional grain sizes of an inverse square root of grain size dependence of the reciprocal activation volume determined on a thermal activation strain rate analysis (TASRA) basis. At the highest strain rates, additional deformation features enter, such as deformation twinning, adiabatic shear banding and very importantly, for shock induced plasticity, transition from plastic flow that is controlled by the mobility of the resident dislocation density to plasticity that is controlled by dislocation or twin generations at the shock front. The shock description is compared with the very different high rate shockless ICE type loading that occurs over nanoseconds and leads to higher compressive strength levels because of dislocation drag resistance coming into play for the originally resident mobile dislocation density. Among the high strain rate property, concerns are the evaluation of ductile to brittle transition behaviours for bcc and related metals and also, projectile/target performances in ballistic impact tests, including punching. Very complete metallographic and electron microscope observations have been reported in a number of the high rate deformation investigations.