SHEAR LOCALIZATION AND RECRYSTALLIZATION IN HIGH-STRAIN, HIGH-STRAIN-RATE DEFORMATION OF TANTALUM

Tantalum was subjected to high plastic strains (global effective strai ns between 0 and 3) at high strain rates (>10(4) s(-1)) in an axisymme tric plane strain configuration. Tubular specimens, embedded in thick- walled cylinders made of copper, were collapsed quasi-uniformly by exp losively-generated energy; this was performed by placing the explosive charge co-axially with the thick-walled cylinder. The high strains ac hieved generated temperatures which produced significant microstructur al change in the material; these strains and temperatures were compute d as a function of radial distance from the cylinder axis. The microst ructural features observed were: (i) dislocations and elongated disloc ation cells (epsilon(eff) < 1, T < 600 K); (ii) subgrains (1 < epsilon (eff) < 2, 600 K < T < 800 K); (iii) dynamically recrystallized microg rains (2 < epsilon(eff) <2.5, 800 K < T < 900 K); and (iv) post-deform ation recrystallized grains (epsilon(eff) > 2.5, T> 1000 K). Whereas t he post-deformation (static) recrystallization takes place by a migrat ional mechanism, dynamic recrystallization is the result of the gradua l rotation of subgrains coupled with dislocation annihilation. A simpl e analysis shows that the statically recrystallized grain sizes observ ed are consistent with predicted values using conventional grain-growt h kinetics. The same analysis shows that the deformation time is not s ufficient to generate grains of a size compatible with observation (0. 1-0.3 mu m). A mechanism describing the evolution of the microstructur e leading from elongated dislocation cells, to subgrains, and to micro grains is proposed. Grain-scale localization produced by anisotropic p lastic flow and localized recovery and recrystallization was observed at the higher plastic strains (epsilon(eff) > 1). Residual tensile 'ho op' stresses are generated near the central hole region upon unloading ; this resulted in ductile fracturing along shear localization bands. (C) 1997 Elsevier Science S.A.