Mechanical Behavior of Ta at Extreme Strain-Rates

The compressive stress-strain response of a commercially pure (99.98%) Ta was investigated at strain-rates ranging from 0.001/s to 500 k/s. Strain-rates up to 20 k/s were obtained using conventional load frames and Kolsky bar methods. The higher strain-rates were obtained using optically instrumented miniature Kolsky bars. Because these experiments require sample sizes as small as ∼30 um, a fine grain structure was desired. To achieve this, we study a Ta billet that was processed by ECAE to produce an ultrafine grain structure. The billets were subsequently annealed at 1203 K under high vacuum for 2 h to coarsen the grain size to approximately 2 um. The as-worked and annealed microstructures were investigated by electron backscatter diffraction to verify the grain structure. A strong rate dependence is observed over this range of strain-rates, although there is a discrepancy between data at similar strain-rates using different sample sizes. This discrepancy is the subject of on-going investigation.