Dynamic behaviors of body-centered cubic metals with ultrafine grained and nanocrystalline microstructures

We have investigated the dynamic mechanical behaviors of representative body-centered cubic (b.c.c.) refractory metals with ultrafine grained (UFG, grain size (d) smaller than 500 nm, but larger than 100 nm) and nanocrystalline (NC, d smaller than 100 nm) microstructures. The UFG/NC microstructures were produced by either bottom-up or top-down fabrication. The dynamic compressive behavior was evaluated by means of uni-axial high strain rate Kolsky bar tests. For some of the metals, due to the special technique used for their fabrication, the specimen dimensions were very small. Therefore, a miniaturized desk-top Kolsky bar system was used to measure the dynamic behavior. Our experimental results indicate that the majority of the UFG/NC b.c.c. metals are prone to localized plastic flow under uni-axial dynamic compression, with UFG-Ta being an exception. This tendency is a consequence of the combined effect of reduced strain hardening capacity and strain rate sensitivity (SRS), increased yield strength, and enhanced adiabatic heating. The experimental observations are discussed in connection with mechanics-based models.