Shear localization and recrystallization in high-strain, high-strain-rate deformation of tantalum

Tantalum was subjected to high plastic strains (global effective strains between 0 and 3) at high strain rates (>104 s−1) in an axisymmetric plane strain configuration. Tubular specimens, embedded in thick-walled cylinders made of copper, were collapsed quasi-uniformly by explosively-generated energy; this was performed by placing the explosive charge co-axially with the thick-walled cylinder. The high strains achieved generated temperatures which produced significant microstructural change in the material; these strains and temperatures were computed as a function of radial distance from the cylinder axis. The microstructural features observed were: (i) dislocations and elongated dislocation cell (εeff < 1, T < 600 K); (ii) subgrains (1 < εeff < 2, 600 K < T < 800 K); (iii) dynamically recyrstallized micrograins (2 < εeff < 2.5, 800 K < T < 900 K); and (iv) post-deformation recrystallized grains (εeff > 2.5, T > 1000 K). Whereas the post-deformation (static) recrystallization takes place by a migrational mechanism, dynamic recrystallization is the result of the gradual rotation of subgrains coupled with dislocation annihilation. A simple analysis shows that the statically recrystallized grain sizes observed are consistent with predicted values using conventional grain-growth kinetics. The same analysis shows that the deformation time is not sufficient to generate grains of a size compatible with observation (0.1–0.3 μm). A mechanism describing the evolution of the microstructure leading from elongated dislocation cells, to subgrains, and to micrograins is proposed. Grain-scale localization produced by anisotropic plastic flow and localized recovery and recrystallization was observed at the higher plastic strains (εeff > 1). Residual tensile ‘hoop’ stresses are generated near the central hole region upon unloading; this resulted in ductile fracturing along shear localization bands.