Low-symmetry plastic deformation in BCC tantalum: Experimental observations, modeling and simulations

A forged and round-rolled pure tantalum bar stock was observed to exhibit large asymmetry in bulk plastic flow response when subjected to large strain Taylor cylinder impact testing. This low-symmetry behavior was analyzed experimentally investigating both the initial stock and the impact-deformed material via x-ray crystallographic texture measurements and automated electron back-scatter diffraction scans to establish spatial microstructural uniformity. Polycrystal simulations based upon the 110 measured duplex texture and experimentally inferred deformation mechanisms were performed to project discrete yield surface shapes. Subsequent least squares fitting and eigensystem analysis of the resulting quadratic fourth-order tensors revealed strong normal/shear stress coupling in the yield surface shape. This mixed-mode coupling produces a shearing deformation in the 1-2 impact plane of a Taylor specimen whose axis is coincident with the compressive 3-axis. The resultant deformation generates an unusual rectangular-shaped impact footprint that is confirmed by finite-element calculations compared to experimental post-test geometries.